Alarm Circuits Circards 13 Service Manual

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.

Alarm Circuits Circards 13 Service Manual

Specifications and Main Features

Frequently Asked Questions

User Manual