Alarm Circuits Circards 13 Service Manual

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Wireless World Circard Series

Flame, smoke and gas detectors

13:

Alarm Circuits-l

tea

bY“W

’

Flame detector

A flame offers a low conductance path to ground. In series

R1, R2,

with the gate of Tr,, that leaves the emitter of Tr, at a high enough potential to keep bring Tr, into conduction via R,. Hence Tr,, holding on the relay-interlocked with the supply for fail-safe operation. If the flame is extinguished Trl gate goes high, driving Tr5

and the relay. A short circuit to ground at the input reduces the base potential of Tr, bringing D1 into conduction and cutting of

The mid-section of the circuit offers a window action with the relay being held on for a restricted range of flame resistances, higher and lower values giving drop-out. The resistance

that conductance defines a range of potentials on

Tr4

out of conduction, but not so high as to

on via

Trz, R7.

Tr3

and hence the output.

This removes the drive from Tr,,

Tr5

conduct

Component values

Tr,: TIS34 Tr,: BC126 Tr,: BC125 Tr,: BC125 Tr,: BC126

lN914

D,: D,:

lN4002

C,:

1nF

R1,

R,:

15M

R,:

2.2k

_ R,:

12k

is high requiring a high input resistance buffer; the output is conventional.

Smoke detector

When detecting the interruption of light by smoke, to avoid the effects of ambient illumination etc., the light beam may be chopped at source and the resulting a.c. from Tr1 (see over) used via buffer Tr, to trigger the monostable

Tr3,

circuit around R10,

from rising sufficiently to fire the thyristor. If the load is a horn having an interrupter switch in series with its coil, the thyristor can cease conduction on removal of the gate drive (alternatively a.c. drive to the load would be required).

Tr,. This prevents the potential applied to

R,:

6.8k

R,:

l.5k

R9: R8: 820R R10: Rll: Rl2: 4.7k Rl3: Semiconductors not critical but TIS34 may need selection because of

parameter spread.

12k

15k

2.7k 22k

sensor

buffer ;

Tr1:

LS400 Tr2-4: 2N712 Tr5:

C106F R1:

1k R2, R8: R3: R4: R5-7,

1OOk

15k

47Ok

R9:

1Ok

monostable

R10,

3.9k

C1,

16µF

C2,

C4: 22nF C3: 0.lµF C5:

50µF C6: 4.7nF Transistor types not critical.

Gas detector

A particular gas-sensor (TGS from Figaro Engineering, Shannon, Ireland) has two fine wires embedded in a conductor. One is used to heat the material, with the ance

between it and the second being reduced on the absorption

of deoxidizing gas or smoke. The sensor is sensitive to

<O.l %,

concentrations of

with resistance falling from many tens of kilohms to as low as 1k at high gas concentrations. Response is non-linear and with a recovery time in excess of one minute. Bridge unbalance is detected on M1 and though repeatable has to be interpreted qualitatively unless special calibration procedures are available. When the unbalance

semi-

resist-

1974 IPC Business Press Ltd.

Tr,: BC126 Tr2:

TIS43

C1:

0.22µF LS: 8 to 80 R R1: 470K R2:

3.3k

R3,

R5:

IOOk

R7:

lOk 1k

R4: R6,

brings Tr, into conduction, C1 charges until the unijunction Tr2

fires and the cycle recommences. The audible note in the loudspeaker rises from a succession of clicks to a continuous tone as the gas concentration increases. A Schmitt trigger would allow relay drive, while the audible alarm could be transferred to the flame-detector circuit, for example.

Further reading

Transducer detects gas, Electronic Components, 6 Nov. 1973, p.18. Wolfram, R., Fail-safe flame sensor provides control functions, Electronics, 31 Aug. 1970, p.68.

Markus, J. (ed.), Smoke detector receiver, in Electronics Circuits Manual, McGraw Hill, 1971, p.568. Bollen,

D., Electronic nose, Practical Electronics, 1973,

pp.574-8.

Cross references

Series 2, cards 2, 3, 6

Series 9, cards 7, 10 & 11.

11. Series 8, cards 1, 3 & 8.

Wireless World Circard Series

Bridge circuits

Components

ICs: 741, Vs R1

to R4:

Bridge voltage:

Circuit description

Three bridge configurations are shown. In each case the bridge is composed of four resistors, R1 to basically Wheatstone bridges with balance occurring for

R1/R2=R3/R4. Substitution of impedances

leave the balance requirements unchanged, and other variants such as the Wien bridge can be produced. For resistive elements it may be possible to supply the bridge and amplifier from a common d.c. supply and a high-gain op-amp detects departure from balance. A small amount of positive feedback

helps reduce jitter in the output when close to balance,

via but gives hysteresis to the balance sensing.

If a separate supply is required for the bridge, one bridge balance point may be grounded, removing the need for high common-mode rejection for the amplifier. The errors in all these circuits include voltage offset of the amplifier, for untrimmed general-purpose op-amps, and input currents/ offset,

1OnA

1µA

for conditions as before. For balance

detection to within 0.1% this implies bridge voltages in excess

±15v

IOk,

R5:

1.5V

(Fig. 2)

R4,

and the circuits are

to Z4 would

l-5mV

of 1V and currents of up to

By opening the bridge and embedding the amplifier in the network as shown, balance is achieved for the same relationship between the resistances, but with input and output both with respect to ground. This circuit has an output that is a linear function of the departure of condition For d.c. applications the input may be one or other of the supply voltages. In all cases best sensitivity is achieved for

R1/R+l.

have a low resistance, power wastage is avoided by keeping the other pairs of resistances high.

Another method of achieving input and output as referred signals, is to use an amplifier with push-pull outputs and single-ended input. A simple case is the single transistor

as shown where the power supply, if properly by-passed, closes the bridge when used for a.c. measurement/sensing.

13:

(R1, R3, R4

If the resistor whose value is being sensed has to

assumed constant as reference resistors).

Alarm Circuits-2

1mA.

from the balance

ground-

-l-_. I

The example shown would pass all frequencies except the notch frequency defined by l/RC, though with appreciable attenuation near the notch.

For many purposes, the availability of a centre-tapped supply provides a “phantom-bridge” action. If the ratio of positive to negative supplies remains constant then taking one input of the sense amplifier to the centre-tap leaves only

I___-_-_---

voltage via a resistor chain with very good stability to the ratio of their values; the absolute values are not important for such an application. The lower-threshold detector (“trigger”) when held high prevents any output change (input

1 is assumed high) regardless of the status of the reset terminal. The reset terminal regains control only when the trigger input falls below the level accurately defined by the potential

il.

---

a half-bridge externally. Used for example with photodiodes, divider. With the trigger taken from an external potential the output voltage is proportional to the unbalance currents in the diodes i.e. to the degree of unbalance in the illumination

divider containing the required sensing element the balance sensing can be obtained.

bridge-

of the diodes. Because the diodes act as constant-current devices the circuit is much more tolerant of drift in the centre-tap than for purely resistive elements. The negative feedback gives a linear output-unbalance characteristic. Manual, McGraw-Hill, 1971, Reversal of the amplifier input terminals would give positive feedback, introducing a switching action and hysteresis as

Further reading

Markus, J. (ed.), Bridge circuits, in Electronics Circuits

pp.84-9.

Graeme, Tobey

Huelsman, Operational Amplifiers,

McGraw-Hill, 1971.

in the first diagram.

l Some i.cs have internal potential dividers which can Cross references

effectively form part of a bridge. The 555 timer, for example, Series 1, cards 9 & 10, series 9, cards 1 & 11.

has its two comparators tapped at

and 8 of the supply

1974 IPC Business Press Ltd.

Series 13, cards 1 & 3.

Wireless World Circard

Time delay and generator circuits

SeriesIs:

Alarm Circuits-3

Circuit description

An i.c. such as the 555, with internal comparators driving a set-reset flip-flop offers great flexibility in the design of alarm systems. With pin 2 high, the capacitor is held low via pin 7. A negative-going edge on 2 allows potential on 6 passes 2

Linking the inputs of the two comparators (2 and 6) to the discharge path (7) causes the potential at the common point to cycle between divider. For both circuits the output has switching characteristics comparable to a output stage. An audible alarm is available by connecting a loudspeaker the on/off condition of the alarm may be controlled by

(3-2552)

VJ3,

when the original state is restored.

VS/3

and 2

VS/3,

t.t.1.

gate because of a similar totem-pole

between Vs and pin 3. If V, is +5V,

to charge Cz until the

set by an internal potential-

t.t.1.

driving pin 4 from the output of a

astable

output to the paralleled comparator inputs. When the output is high, threshold is passed; the output switches low and C1 is discharged until the trigger value set by pin 2 is passed. Timing is set by the less well-defined output amplitude, and the

frequency is less stable than the basic circuit. Addition of varies mark-space ratio.

If the reset terminal 4 is coupled to an RC network as shown, then a time-delay can be introduced at switch-on, before which firing of the circuit as a monostable can be

achieved.

can also be constructed by feedback from the

is charged positively through R1 until the upper

gate.

l A monostable using c.m.o.s. inverters can use very

value resistors, giving time delays of

<l,uF.

As shown, a short-duration excursion of the input

>Is

with capacitors of

high-

from + to ground sets the output to zero for the monostable period (about 3s) because the output of the first inverter is high, as is the input of the second until Ra can pull the gate down by charging

C1.

The high impedance makes such

monostables useful as touch-operated circuits.

A related

astable

circuit shows an additional resistor

which isolates C1 from the rapid charge/discharge imposed by the gate protection diodes in both these circuits. The resistor improves the timing stability.

Farther reading

Three articles, by 21 June, 1973, Application note for XR-2556 timing circuit, Exar, 1973.

Cross

references

Series 3, card 9. Series 13, card 5.

Robbins,

pp.128-32.

1974 IPC Business Press Ltd.

l The output stage of an astable/monostable circuit is

important where high voltage/current/power is required. For the 555 timer, the output stage is similar to the typical output (as shown above) but with a Darlington-connected top section. The positive output is thus at least 1V below supply while the low output can be to within at low currents. Above 2V and 1V respectively.

For some applications the open-collector output of devices such as SN7401 gives convenient driving of loads, while other devices such as emitter voltages of up to 30V.

Orrel and De Kold, in Electronics,

O.lV

50mA

the voltage drops may reach

SN7406

will withstand

t.t.1.

of ground

t.t.1.

collector-

Wireless World Circard

Level sensing and load driving

Circuit description The basic level-sensing circuits shown may be used with or

without positive feedback, to obtain an output change as the input passes a defined level or levels. For amplifier gain determines the range of input voltages for which the output is not switched hard to one or other extreme. (Typically 1 to 20mV for comparators, required to operate at high speeds ; 0.1 to sensing makes their slower operation an acceptable penalty.) Hysteresis introduced by positive feedback allows the circuit to latch into a final state after the first excursion through a given level, provided the input cannot reverse its sense

sufficiently to pass back through the other switching level. These circuits can thus perform the combined functions of level-sensing and set-reset action required in many alarms if for example the signal is a positive-going voltage initiating the set action, while the reset action is a negative-going pulse over-riding the former e.g. a resistor taken from the non- inverting input to the negative rail.

5mV

for op-amps where accuracy of

R*-+~o, RI-d,

level-

Series 13:

Tr,:

BFR41,

Vs f6V,

to R,:

An adaptation of the output stage shown in Fig. 5 gives an

output when the p.d. across either R1 or Rz exceeds about

0.6V.

voltage defining sufficient positive supply current via i.e. VI~RJR, the output via Trl. The switching action is not particularly sharp as it uses only the gains of the transistors.

A standard window comparator gives sharper switching but requires two amplifiers/comparators and still requires an additional transistor is an output swing comparable to supply voltage is required e.g. for efficient switching of lamps relays

etc., particularly at higher currents.

Tr,: BFRSl

RL: 20052

10052,

R,:

1.2kQ

In the former case this corresponds to a positive input

-0.6V.

Similarlyanegativeinput voltage switches

Alarm Circuits-4

1kSL’

IC: 711

Trr: BFR41, Tr, BFR81

D,, D,:

Rr,

R,:

lN4001,

18OQ,

IC: 741

R,: 68052

Ve f6V

A previously-described output stage (series 2) gives

Tr,, Tr,, Tr,, Tr,, Tr,: BFR41

R1,

R,:

Vs +6V

L up to 300mA

push-pull drive using one op-amp as driver. Resistors

1OOQ

Tr,:

BFR81

Rr, Rz

are selected to keep Tr,, Tr, out of conduction in quiescent state. The op-amp is used in any of the sensing/oscillating modes that result in p.ds across Rs sufficient to drive Trl, T, into conduction. Either may be used alone for driving lamps, relays, or the circuit as shown may be capacitively coupled to a loudspeaker for a.c. power drive.

An output stage using a bridge configuration requires antiphase switching at the inputs, but gives a load voltage whose peak-peak value is twice the supply voltage. This is equally applicable to audio alarms or to driving of servo systems for which it was designed.

l Complementary m.o.s. buffers may be used to drive

complementary output transistors as shown and with the aid

of an additional inverter a similar stage provides a bridge output. The transistor base current is limited to a few

milli-

1974 IPC Business Press Ltd.

IC: f CD4049

or CD4050

Tr,: BFR41

Trr, TrS: Trz,

Tr, : MJE371

Tr,: BFR8l Tr,:

BFR41

Tr,:

2N3819

2N3055

amperes but in all these output stages, short-duration current

spikes may occur during the output transitions. Diode protection against inductive voltage spikes as in Fig. 5 should be used for loudspeaker, relay and solenoid loads.

Any of the output transistors may in principle be replaced by the compound transistor pairs if higher peak currents are needed. To reduce the above requirements it is worth considering the use of f.e.t. devices as the input transistor of the pair.

Further reading

Electronic Circuits Manual (Markus, McGraw-Hill 1971): Main circuits-pp.l-6; lamp control circuits-pp.344-9; trigger

circuits-pp.889~907.

Linear Integrated Circuits Handbook, Marconi-Elliot,

pp.165-170.

Industrial Circuits Handbook, SGS-Fairchild,

pp.6-13.

Wireless World Circard Series 13:

Applications of 555 timer

Alarm Circuits-5

Circuit description

The 555, designed as a timing circuit with either monostable or

astable

it to be used for many other purposes. In alarm systems, the power output stage that permits currents of either polarity of up to 200mA (though 50mA minimizes voltage losses)

means that lamps and relays can be driven quite readily. When used as an be applied to a loudspeaker to give an audible alarm, while a voltage fed to the control terminal modulates the frequency for warble or two-tone effects. As a monostable circuit it can be used to provide delays from microseconds to minutes, allowing, for example, a warning alarm to be held for a defined period of time after the appearance of the condition being detected. In such cases the condition (closure of a switch in a burglar alarm for example) is converted into a negative-going pulse, applied to the trigger input. A further application for the device involves the controlled hysteresis

operation, has internal circuit functions that allow

astable

circuit the output square wave can

provided by the two comparators biased from an internal potential divider. With via the flip-flop which ignores any further excursions of

about

Vrerl

flip-flop is reset, the output going positive. In the astable circuit

is charged and discharged between

in either sense. When Va falls below

VI=

V,, V,,r,=2Vs/3,

VI> V,,f,

V,,r,=

the output is driven negative

Vrerz

the

Vs/3

and the capacitor

Vs/3

and

2Vs/3.

Typical performance

IC:

NE555V

R,:

2.2kQ,

k=0.6,

Upper set point: Lower set point: Output swing: 9V for If

R1, Dz

2V and V/k.

(Signetics), Vs +

R,:

D,: 5.6-V Zener diode

1OkD

5.7V

(

4.75V

RL >

omitted, VM,= 2 V, Vrer,= V and set points become

VZ) (Vz/2k)

25052

1OV

Component changes

IC:

Vs:

RI, D,:

R,:

Motorola used with independent reference voltages or a single comparator with hysteresis defined by

see Series 2.

4.5 to 18V. At low voltages the saturation voltages at the output may not allow adequate drive to electromechanical/filament lamp loads. Any network to provide constant voltage at control

input. Voltage may be to within 1V of common line or positive supply, but for optimum performance

should be close to

lk to source; at high values inaccuracies due to threshold

current of up to

MC1455

Separate comparators could be

2Vs/3.

1MQ.

At low values, excessive loading of

0.25,uA.

Circuit modifications

Use as battery charger illustrates method well (above).

Upper threshold when ~,VL=

k,VL= Vz/2.

When upper threshold is exceeded output at

V,;

lower threshold when

feedback-

1974 IPC Business Press Ltd.

pin 3 reverse-biases diode Dz and battery discharges into load when present. As voltage VL falls below lower threshold, voltage at pin 3 rises and charges battery through limiting resistor

Vzlk,+ V&k,.

l To increase hysteresis, the potential at pin

Rz.

Hysteresis may be reduced towards zero for

may be reduced following a transition through the upper threshold. This may be done as in Fig. 2 by using output pin 3 via a diode-both thresholds are varied if the diode is replaced by a resistor.

The increased swing similifies the triggering of a following 555 used as a Schmitt trigger, as the capacitor voltage in Fig. 2 can approach zero. Complete alarm systems can be based on such circuits combining level sensing, time delays and waveform generation, as well as audible alarms.

Further reading

Four articles, by De Kold, McGowan, Harvey & Pate, in

Electronics, 21

June 1973,

pp.128-32.

Wireless World Circard Series

Frequency sensing alarm

at a lower frequency (f/20), and hold on during signal failure or for temporary interruptions of the signal.

The upper frequency in the band mode or the datum in the datum mode is set by

tl+ tr.

to comparators Schmitt-triggers and window-comparators.

Typical performance

IC: FXlOl (Consumer Microcircuits) [OBSOLETE PART]

Circuit description The circuit is a monolithic m.o.s. i.c. which uses external

RC elements to fix the frequencies at which the circuit

orovides

switching times defin;d by monostable circuits with the second time interval being initiated at the end of the first. The input may be a repetitive

signal of arbitrary waveform, provided the amplitude is in excess of pk-pk). Internally this is presumably squared by a Schmitt type of circuit to trigger the monostables. Three distinct conditions may exist; if the period of the received signal is

t=l/f

t<tl, tl<t<tl+tz, t>tl+tz.

guished by additional internal circuitry that allows sensing of frequencies above a given datum or within a given band with a switched output that can be made to latch on or off, toggle

a switching action. If does so via two senarate

1OOmV

pk-pk (though it should not exceed 20V

and the two delays are

CIR,

and

CaRa,

as from a pair of

t,=k/C,R, t,=kjC,R,,

These conditions are distin-

then

vi*: R1,

R,: c,: Ground pins: 2, 3, 9.

Gutout on for:

3 5

13:

The circuit provides frequency-sensing function similar

12V supply, 250mV pk-pk to pin 1

47OkQ 22nF, C,: lOnF,

f>lSOHz (f% 1/0.6C,R,).

signal input. grounded, holds switch state during signal loss. open, switch off. ground via ‘c’, switch off after signal break of for

‘c’.

ground, circuit automatically resets on change off. open, switch latches when turned off. link to 8, switch latches when turned on. Ground 3. link to 8 via ‘C’, hysteresis in datum point of

100%. ground, datum mode, switches on for open, band mode, on for link to pin 5, output toggles at f/20 when in band.

Alarm Circuits-6

and the lower band-frequency by

-3mA+

C,:

load current

O.l.uF

fi>f >fi.

2OOms/pF

‘C’/C,x

>fi.

0”

-4

(b)

1MB

1pF

SOkHz.

1pF

(not critical)

C,= C,=

(&+Rn+Rc)C

C, variation i.e. it is the

1974 IPC Business Press Ltd.

(a)

Component changes

Vs:

-12 to -22V some samples operate with reduced

Vin:

0.1 to 20V pk-pk freq. set points: response time :within 5 to 10 cycles of receipt of correct

Circuit modifications

As the lower frequency in the band mode is affected by time constant frequency is not, variation of R, increases the band by variation of its lower bound only. For in the tapping point of RC in (a) at left leaves the sum of the time constants unchanged at lower frequency that remains constant while the upper frequency is charged.

accuracy down to -8V.

O.OlHz

frequency.

1OOk

250pF

1OnF

CaRa

in the original circuit while the upper

Variation in both frequencies while retaining a reasonably constant ratio of be achieved by varying the common bias applied to the resistors. If strong dependence on supply voltage is to be avoided the bias voltage should be supply-proportional as in (b).

Constant-current sources allow linear control of period against a separate reference voltage, which may be proportional.

Filament lamps may be driven via an additional transistor, currents up to on right. Direct drive of reed relays, l.e.ds is possible though current is marginal.

Further reading

Volk, A. M. Two i.c. digital filter varies

Electronics,

McKinley, R. J., Versatile digital circuit filters highs, lows or bands.

FXlOl : Consumer Microcircuits data sheet D/026.

Cross references

Series 1, cards 6 & 7.

Electronics, 21

fi:fi

(the equivalent of a constant

lOOmA

15 Feb. 1973,

June 1971, p.66.

Q),

supply-

or so being provided by circuit

passband

p.106.

easily,

can

Wireless World Circard

Digital alarm annunciators

ICs:

1-3, 5, 7-15

)

SN7400

SN7410

4,6,

r: 3.3k52,

Circuit function

It is assumed that a fault condition is the opening of relay contact RLr, though any other sensor that maintains the NAND-gate input terminal at a low A fault will turn off a “safe” green light and illuminate a

“danger” red light, and operate an audible alarm. When the

“recognise” push-button is depressed, the red light stays on, but the alarm is silenced. When the fault clears, the alarm is restarted, the green light comes on and the red light goes off.

(‘0’)

level is adequate.

R,:

68Q

Series 13:

The “recognise” button is again pushed to reset circuit to its normal state.

Alarm Circuits-7

Circuit operation

Consider the circuit in its normal state where inputs R, T and F are at zero volts (or binary zero) i.e. R=T=F=O. This makes

X=0(x= l),

(green) and

If a fault occurs,

i.e.

F=l,

remains as before. Hence Pushing the T=O, then remains on, but A=O, and alarm stops. This state will be maintained until the fault is cleared.

but A= 1, and the alarm operates,

R=l, R=l,

alarm, when started from normal state with LED, on.

causing

When the fault is cleared, R=F=T=O, Y does not change,

X=0 (Y=z=l, X=p=O).

Final recognition of the fault clearance is obtained from

which will return circuit to its normal state i.e. for F=T=O, Y=O and

Depression of the test button will check

Circuit modification

As X, R, Y, p are available, the elusive-OR function of A can be obtained as shown.

Y=O

LED2

(red) is off.

RLt

X=0 (;iz=l),

recognise

Y=l @=O>,

(P=

1) and hence

opens, F goes high (or binary one)

but the state of Y (and

A=l,

and triggers audible alarm.

button causes R= 1, and as F= 1,

but X does not change. LED,

Hence

X=0.

ex-

LED1

LEDz

is energised

is illuminated,

LED1

and the

;

Circuit description

Complementary-m.o.s. devices may be. used in the circuit above to minimize stand-by power consumption.

Normal safe condition obtains with the fault-switch closes,

X+0.

Hence L=O, E= 1, opening gate

F-t1

L=A=O, F=

1. When

and since L is already high,

ICr,.

Also since F=O,

Y+O, and hence A is forced to zero, therefore A= 1. This

transition may be used to switch an audible alarm. Simultaneously the oscillator gate is opened which will cause lamp flashing at a rate determined by the astable frequency.

If the fault is rectified, the alarm condition is maintained until the clear button is pressed causing C to be low. Hence

L-+1,

and will latch in this condition via memory circuit

and

ICI.

Also E=O, thus

tained via ICI and

x=0,

this condition being main-

IC5,

and the alarm is silenced.

IC2

Circuit description

Arrangement right allows detection of first-fault occurrence from three sensors the number of inputs available per NAND-gate.

Sr, Se, Ss,

this number being restricted by

1974 IPC Business Press Ltd.

+V,(+SV)

Outputs

Qa, Qs, QB

are set to zero when the reset button is depressed. The 0 output of each flip-flop is applied to the other two NAND gates, but not to the one associated with

itself. Hence two of the three inputs of each gate are high.

If S, closes, for example,

IC2

output goes low, and this negative-going edge being applied to IC, preset terminal sets

Q&=1

(and hence

q=O).

Therefore ICI and

ICs

are now

inhibited and cannot respond to a fault condition.

Further reading

Zissos, D., Logic design algorithms, Oxford 1972.

Wireless World Circard

Filament lamps and relays

Filament lamps are widely used as visual alarm indicators and often connected in the collector-emitter circuit of a bipolar transistor that is switched on and saturated under alarm conditions. These lamps have a positive temperature coefficient of resistance with a large difference of resistance between the cold and hot states-see Graph 1 which is typical for a 6-V, voltage source, a large current surge flows in the lamp, and switching transistor, which then decays exponentially to its normal or rated value in the hot state. This surge may be ten times the rated current, or even higher, shortens the life of the lamp, may destroy the switching transistor or blow the power supply fuse. Graph 2 shows the typical initial surge current characteristic of a 6-V, thermal time constant of about 2ms.

lO@mA

panel lamp. When switched on across a

60-mA

panel lamp having a

Series 13:

When lamps are used as flashing alarms, the initial surge current is as shown in Graph 2 but the surge current on successive pulses depends on the thermal time constant and the time between flashes. Graph 3 shows the typical variation in surge current when a 6-V, on for 5 s then off for t

If the p.d. applied to the lamp is gradually increased the current rises in a controlled manner to its normal operating value, prolonging the life of the lamp and reducing the probability of transistor damage. A simple arrangement is shown above where Tr, is normally held on and saturated with a low value of Under alarm conditions, the base drive to Tr, is removed and the capacitor charges through Rn. The base voltage of

Trl rises exponentially so that the lamp surge current is

avoided.

Vc~(sat)

Alarm Circuits-8

60-mA

panel lamp is switched

,,rr

seconds.

holding Tr, and the lamp off.

To prevent damage to

circuited, a resistor Rc could be included in

Trr

should the lamp become short-

Tr,‘s

free collector, but this would reduce the lamp voltage in normal operation. Circuit above shows a modification that ‘allows an almost normal lamp voltage and also limits the short-circuit current to the desired value by using only a small Rc value and a saturating transistor Tr,.

Relays are used to actuate alarm devices that need to be isolated from their control circuitry for various reasons such as their current, voltage or power requirements being incompatible with the electronic circuitry. Circuit right,

acommonly-

used relay drive circuit which takes into account both the resistive and inductive properties of the relay coil. When actuated, the steady-state coil current is fixed by the coil resistance and supply voltage, but when Tr, is turned off the inductance of the coil causes the collector voltage to rise towards a level greatly exceeding V CC if the protective diode

were omitted. Diode D1 allows

VCE

to rise only slightly above Vce before the diode conducts to dissipate the energy stored in the relay coil. When Tr, turns on D1 is

reverse-

biased and does not affect the operation. The diode must be able to withstand a reverse voltage slightly greater than V CC

1974

IPC

and be able to conduct the relay-coil discharge current for a brief time. Transistor Tr, must have a V CE rating exceeding

Vcc and be capable of carrying the relay operating current.

If a relay is required to operate when an input level exceeds a certain predetermined value, it may be included in a Schmitt trigger circuit; e.g. the relay coil and protective diode could replace R1 in the basic circuit of series 2, card 2.

If the alarm indication uses a

1.e.d.

or alpha-numeric array

of l.e.ds consult series 9, cards 2, 5 & 6.

Further

reading

Shea, R. F. (Ed), Amplifier Handbook, section 3 106, McGraw- Hill 1966. Egan, F. (Ed), 400 ideas for design, vol. 2,

pp.18/9,

Hayden,

1971. Cleary, J. F. (Ed), Transistor Manual,

pp.202,

General Electric Co. of New York, 1964. Industrial Circuit Handbook, section 2, SGS-Fairchild, 1967.

Cross references

Series 2, card 2. Series 9, cards 2, 5 & 6. Series 13, cards 4 & 7.

Press Ltd.

Wireless World Circard Series

Signal domain conversion

Fig.

Fig. 1

Fig. 3

13:

Fig. 4

Alarm Circuits-9

Voltage-to-current conversion

It is often required to supply signals to relatively long transmission lines in which case the signal is more convenient in current form rather than as a voltage. Thus, current converters are useful and may be realized using operational amplifiers especially if the load is floating. Figs. 1 & 2 show the more common forms the former being an inverting type and the latter non-inverting. In both Figs.

Vin/R

and is independent of the load impedance, but the source and operational amplifier must be able to supply this load current in Fig. 1, whereas little source current is needed in Fig. 2 due to the high input impedance of the amplifier. Fig. 3 shows another floating-load V-to-I converter which requires little source current if R1 is large and allows iL to be scaled with R,, the operational amplifier supplying the whole of the load current;

iL=

Vin(l/R, +

RP/R1R3).

of Fig. 4 is suitable for V-to-I conversion when the load is

voltage-to-

The circuit

grounded. When

R, and the current source impedance seen by the load very

R1R3= R,R,

the load current is

iL= -

Vi,,/

high.

Current-to-voltage conversion

If a device is best operated when fed from a voltage source but the available signal is in the form of a current, a current- to-voltage converter will be required, one example being shown in Fig. 5. Current is fed to the summing junction of the operational amplifier which is a virtual earth so that current source sees an almost-zero load impedance. Input current flows through R1 producing an output voltage of Vout= volts/amp. The only conversion error is due to the bias

current of the operational amplifier which is algebraically

summed with

iin.

The output impedance is very low due to

the use of almost 100% feedback.

Fig. 6

Fig.

Voltage-to-frequency conversion

Many voltage-to-frequency converters exist, the circuit complexity often being a guide to the degree of linearity and maximum operating frequency. Fig. 6 shows one form of V-to-f converter (a below about differencing

LM3900

v.c.o.)

suitable for use at frequencies

lOkHz,

each amplifier being of the current-

type. Amplifier A, is connected as an

integrator with AZ acting as a Schmitt trigger which senses

the output from AI and controls the state of Tr, which either

shunts the input current through Rz to ground, making

Voutl

run down linearly, or allows it to enter AI causing

Voutl

to rise linearly with

wave and

Vout,

a square wave having a frequency that is

linearly dependent on

RI=

2R,.

Voutl

is a triangular

RI, C1

and the threshold levels selected

for the Schmitt trigger.

Frequency-to-voltage conversion

Diode-pump, transistor-pump and op-amp pump circuits are widely used for low-cost frequency to voltage conversion. Another circuit, using a single

LM3900

quad currentdifferencing amplifier package, is the phase-locked loop shown in Fig. 7 which uses the is in the

LM3900

package used as a phase comparator having

V.C.O.

of Fig. 6. Amplifier A,

a pulse-width modulated output depending on the phase

difference between

Vin

and

VOUtg

of the

V.C.O.

Resistor

and C, form a simple low-pass filter which makes the d.c.

output vary in the range +V to

+V/2

as the phase difference changes from 180” to 0”. This direct voltage controls the frequency of the

V.C.O.

and its lock range may be increased by using the fourth amplifier in the package as a d.c. amplifier between the filter and the integrator. Centre-frequency of the

p.l.1.

is about 3kHz with:

30kQ; R5, RB 1.2MQ; R, 62kSZ;

RI, R,

C, 1nF; C,

1MR; Rz

510kSZ;

1OOnF;

R,,

R8, Rg,

V= $4

to +36V.

Further reading

Graeme, J. G. & Tobey, G. E. Operational Amplifiers, chapter 6, McGraw-Hill 1971. Linear Applications-Application notes AN20 and AN72, National Semiconductor 1973.

Cross references

Series

3, cards 3,5 & 10.

Series

13. cards 1& 6.

1974 IPC Business Press Ltd.

Wireless World Circard Series

Pressure, temperature and moisture-sensitive alarms

Pressure-sensitive alarm

A pressure-sensitive alarm may be made using a modified transistor known as the Pitran. It is a planar n-p-n transistor having a diaphragm mounted in the top of its metal can which is mechanically coupled to its base-emitter junction.

When a pressure is applied to the diaphragm a reversible charge is produced in the transistor characteristics. The mechanical pressure input can be used to directly modulate the electrical output of the transistor which may be fed to the alarm circuitry which switches state when the input pressure to the Pitran either exceeds or falls below some critical level. The Pitran may be connected as a single-ended-input single-ended-output stage, as shown left or as a differential-input output stage, as shown middle. Conventional transistor circuit design techniques may be used for the Pitran stages. Linear

e.g.

via a comparator or Schmitt trigger

specially-

balanced-

output voltages of up to one-fifth of the total supply voltage are obtainable.

Temperature-sensitive alarm

Circuit above shows the input circuitry of an alarm which may be operated by the output signal from the operational amplifier when the temperature monitored by the probe transistor exceeds a pre-determined value. The temperature- sensing transistor is a low-cost n-p-n type that can produce a resolution of less than 1 deg C in a temperature range of 100 deg C. If the operating current of the probe transistor is

made proportional to temperature, the non-linearity of its

base-emitter voltage may be minimized, being less than 2mV in the temperature range -55 to +125”C. Zener diodes set the input voltage to

13:

Alarm Circuits-10

1.2V

and this is applied through RI to

fix

the operating current of the probe transistor. Resistor

may be adjusted to make amplifier’s output zero at 0°C and

is used to calibrate the output voltage to

lOOmV/deg

or any other scaling factor, independently of the Vout= 0 condition. RI, Rs

D1, Da

LM113; TrI

12ksE; Ra 3kQ; R, 5kSZ; Rs, RB 1OOkQ;

2N2222;

Al LM112; V

hl5V.

Moisture-sensitive alarm

A low-cost audible alarm which operates when the electrodes of the input sensor become damp due to increase in humidity, direct contact with water, rain or snow is shown above. The sensor is conveniently made from parallel-strip printed circuit board or commercial equivalent, so that increase in moisture at the strips produces a very small current to Trl base via RI which forms a high-gain compound pair with Tr, which switches hard on. Transistors

Trs

and Tr, form the

1974 IPC Business Press Ltd.

audible alarm multivibrator, that acts as a load on the compound pair, having a repetition rate determined by the

C,R, time constant. A piercing note at about 2SkHz is

produced with RI, R,

ZTX300; Trs

OC71; LS 8-Q loudspeaker; V +9V.

lOOkS2; RJ lkS2; C1 1OnF;

Tr,, Tr, Tr,

A flashing display with a rate of about 2Hz may be obtained by replacing the loudspeaker with a 6-V, and changing the values of Rz to

470kQ

60-mA

panel lamp

and C1 to

2.2pF.

Further reading

Tingay, E. The Pitran-a new concept in pressure measure-

ment,

International Marketing News, p.8,

Linear Applications-application notes AN3 1, AN56 and AN72, National Semiconductor, 1973. Brown, F. Rain warning alarm, Everyday Electronics,

pp.208-11,

1972.

1970.

Wireless World Circard

Security, water level and automobile alarms

-I

Circuit description

Component R, is the resistance in the search loop which if obtained using two

switches either in series with the loop or in parallel with either resistor, or both. In the latter case changing a switch condition from open to closed in the parallel case and from closed to

open in the series case can give rise to either a positive voltage

or a negative voltage being applied to the diode bridge; the

bridge is, of course, balanced initially. The diode bridge

being a full wave rectifier will apply a negative

following circuit in either case.

The bridge resistors are large valued to minimize current drain from the battery but requires that the following circuit have a large input resistance. Hence the Darlington pair Tr, and

Trz

is employed.

When vbs goes negative, Trl and with it Tr,, conducts. Transistor

TrB

lOOkS2

resistors allows one to include

then drives Tr, which is a higher power device

Vba

to the

Series 13:

capable of drawing a relay coil to produce the warning

signal. At the same time when

TrS

goes negative and hence via positive feedback through the base of zero. Hence, a latching action is obtained, which keeps the warning signal on. The warning signal will only be removed if the power supply is removed.

Capacitors C1 and Ca are required to prevent spurious pulses from triggering the alarm, in the case of prevent switching transients from triggering the alarm when the alarm is being reset, in the case of

Tra

remains negative, even if

Alarm Circuits-II

Trs

conducts, the collector of

Vba

is set back to

C1,

and to

Ct.

Component values

Tr,,

Trz:

R1, R,: 15OkQ

R,:

2OOkB

R,:

25OkQ

variable

R,:

27kQ

R,: 47052

Ci,

C!,: 0.33/tF

Tr,: BFR41 or BFY52 Diodes 0A81 V,:

vs: 9v

BC126

18V

Brake light monitor (circuit over)

Both of the identical counter-wound coils are wound round the reed relay. Hence the relay switch will only close, giving a dashboard warning, if either of the brake lights fails either with an open circuit or short circuit.

Water level alarm

This circuit is designed to produce a note from the loudspeaker when the sensor input terminals are shorted. As such it can be used for many applications apart from suggested water level/rain alarm. When the input terminals are shorted base drive to Tr, via R1 is obtained, and the supply voltage is switched to the unijunction relaxation oscillator comprising Tr,, R,, R, and C (card 4, series 3). A train of pulses of period mainly determined by the product to the base of the loudspeaker. The loudspeaker alarm note can be altered by altering the product can be obtained by selecting the note to correspond to the resonant frequency of speaker. In practice the alarm will

sound for any resistance between zero and five megohms.

The quiescent current of the unit is of the order ofnanoamps so that battery life is many months even if the unit is switched off. Provision to test the battery condition is made by switch position 2 which should cause Trl to switch on the oscillator provided the battery is in good condition.

For water level sensing two conducting rods spaced an inch, or less, apart and positioned at the required level is all that is required.

Tr,,

thereby producing pulses of current through

R&T.

Considerable effective output

R&is

then presented

Component values

R,:

lOOks2

R,:

3.3kQ

R,: 27052 C:

0.5,uF Trl, Tr,:

Tr,: LS: 8-Q loudspeaker

Supply voltage:

paper will suffice. When the blotting paper becomes wet contact between the rods is made, the alarm sounds and the washing is saved once more (provided the shopping).

2N2926

2N2646

For a rain alarm two rods separated by some blotting

(G)

‘Wit& po,i,l0”,

1 -Us.

P - tslt 3 - ott

‘f Ra( “(

missus

isn’t away

Component changes

Resistor R1 may be any value up to shorting of the sensor input terminals is obtainable. The R,C product is dictated by the pitch of the note required. Resistor Rs should be much less than R, e.g.

5M.Q

provided a true

R&O.

Further reading

Andrews, J. Security Alarm,

Moorshead, H. Rain & Water Level Alarm, Practical

Electronics,

Morum, S. W. F., Brake Light Monitor, Practical Electronics 1973, p.588.

1971,

p.820.

Practical Electronics,

1973, p.338.

1974 IPC Business Press Ltd.

Wireless World Circard Series

Electromechanical alarms

Electromechanical transducers are obtainable in a wide variety of types: they may be d.c. or or capacitive, contacting or non-contacting, analogue or digital, linear or angular, etc. Insofar as most alarm systems use a comparator (cross ref. 1) to compare the signal with a reference and as d.c. signals are easily compared we shall assume here that any a.c. systems are followed by signal conditioning equipment which includes a rectifier (cross ref. 2) of some sort so that the effective output is d.c.

a.c.,

resistive, reluctive

Displacement alarm

O”t

Circuit shows a reluctive displacement transducer, of the differential transformer type, followed by a demodulator to provide the d.c. output shown in graph. The core, which is shown in its zero output position, is attached to the member whose displacement is required. The core is generally made

from high permeability ferromagnetic material so that flux linkages with and hence the e.m.fs of the secondary coils are highly dependent on the position of the core relative to the coils.

Reluctive

span of between 0.01 and

transducers generally have a displacement

120in,

in rectilinear form, and

tin

between 0.05 and 90” in angular form. As the induced e.m.fs are proportional to frequency, very sensitive system can be. made at high frequencies.

Capacitive transducers are used in situations where very

small displacements have to be measured and/or non-

contacting measurement has to be performed. Photoelectric/

digital measurements (again non-contacting) are used when

high accuracy is required, although fairly low cost versions can be constructed if accuracy is not essential.

Velocity alarm

Linear velocity transducers are most commonly used in the

vibrations field where the displacement of the member whose

velocity is required is small. Essentially, they consist of a coil

moving in a permanent magnetic field, the coil

proportional to the speed. As a large proportion of the speed producing systems are driven by motors one can generally obtain information on linear speed from a knowledge of angular speed. This can be obtained by various types of a.c. or d.c. tachometers, but with the increasing use of digital instrumentation, toothed rotor, photoelectric and similar systems are becoming increasingly common. Diagram shows basis of operation of the toothed rotor tachometer and the

13:

Alarm Circuits-12

e.m.f.

being

corresponding output when the rotor is rotated by the shaft of a motor. The output waveform is obtained because of the changing flux pattern caused by the changing magnetic circuit. If the output signal is fed to a zero crossing comparator (cross refs. 1, 3) or to a Schmitt trigger (cross ref. 1) one will then obtain a train of pulses, each pulse representing the passage of a rotor tooth past the permanent magnet. Obviously the pulse frequency is proportional to the shaft speed. If the train of pulses is then fed to a frequency-to-voltage converter a direct voltage proportional to shaft speed is obtained and this can be fed to a comparator to give an alarm if it exceeds

a predetermined level. Because the number of teeth on the

toothed rotor can easily be varied, the range of speeds measurable by this technique is extremely large. Further, the rotor can easily be constructed in any workshop, no great precision being required for many applications. Both heads on a coupling between two shafts often suffice as the toothed

rotor.

Acceleration alarm

Acceleration transducers all have one feature in common viz the seismic mass, M. The basic acceleration transducer is

shown below. The case of the system is attached to the

body whose acceleration is required. Due to a constant

acceleration the seismic mass exerts a force MU which in the steady state will stretch or compress the spring by an amount

where Ma = Kx, K being the spring constant. The dashpot

1974 JPC Business Press Ltd.

simply provides damping whilst the mass is moving. If we know M and K then a measure of x gives a signal proportional to the acceleration. This can be done by any displacement transducer of suitable dimensions and sensitivity. Frequently, however, the spring arrangement is a leaf spring arrangement with strain gauges attached. The spring deflection gives rise to changes in resistance in the strain gauges which if connected in a Wheatstone bridge circuit gives a voltage proportional to the deflection and, hence, to the acceleration. As the Wheatstone bridge can be supplied from a d.c. source there is no need for rectifiers before feeding to a comparator. Strain gauge bridges usable up to

For higher frequencies piezoelectric crystals replace the spring. The crystal produces a charge or voltage across its terminals when subjected to the stress of the seismic mass under acceleration. However, the output impedance of the crystal is large and amplifiers with an input impedance in excess of cable between the crystal and the amplifier requires to have low capacitance and to be free from friction induced noise. On the other hand very large accelerations (> sured and they can be used over a large temperature range (570°C for a lead metaniobate crystal).

5OOMO

typically have to be. used. Furthermore, the

75OHz

have been built.

1OOg)

can be mea-

Further reading

H. N. Norton, Handbook of Transducers for Electronic

Measuring Systems, Prentice-Hall.

Considine. Encvclonedia of Instrumentation and Control.

McGraw&ill. -

Cross references

Series 2, Comparators and Series 4, A.C. Measurements.

Schmitts.

Series 13, card 4.

Alarm Circuits Circards 13 Service Manual

Specifications and Main Features

Frequently Asked Questions

User Manual