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THEORY OF OPERATION
Theory Intro:
The UT5A is made up of many small circuits used to control the 555 timer IC,
which is the heart of the kit. First we’ll cover these circuits individually, then
we’ll place them into the two most basic modes of operation (monostable
and astable) and analyze their operation.
Power Entry:
The UT5A requires an external 5 to 15 VDC DC power supply to operate the
timer circuits. This external DC supply is connected through J1, positive at
pin 1 and negative (GND) at pin 2. J1 pin 3 allows for a separate 3 to 24
VDC supply to be used to set the output signal amplitude parameters or
Output Power. JMP1 allows the Timer Power to be used for Output Power.
Input Circuits:
Triggering the 555 timer IC requires a negative going signal applied to its
input. We have three ways to apply this signal to the IC.
1) NEGATIVE INPUT - If the source signal is negative going it can be
applied to J2 pin3 the boards (-) Input which is attached to C1. For this
option JMP2 (Pull-up Select) may or may not be installed depending on the
source. If installed, it supplies VCC through R1 to C1’s input. J2 pin3 allows
us to bring a negative signal into the timer at C1. With a negative input, D1
is turned off, thus preventing Q1 from effecting the input signal.
2) POSITIVE INPUT - This option requires JMP2 (the Pull-up Select) to be
installed to provide power for Q1. The positive pulse enters throug h J2 pin 1
(+ Input) current limiting resistor R2 to the base of Q1. The positive signal
turns on Q1. It amplifies and inverts the signal, and now we have a negative
going pulse passed by the steering diode D1 to C1’s input.
INTRODUCTION
The UT5A is most versatile 555 timer kit on the market. The input circuit options allow for connecting to almost any kind of digital signal. The output drivers allow for connection to digital circuits, relays, indictors, etc... It is designed
to provide the easiest way to apply accurate timing signals to almost any application.
The UT5A is also an excellent teaching tool. The detailed theory of operation
will arm you with the knowledge to effectively set up and use the 555 timer.
You will have the chance to experiment with the 555 timer IC, transistor
switches, pull-up resistors, limiter, and differentiator circuits.
In addition, experienced 555 IC users will welcome the practical and truly universal setup of the PC board.
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3) START SWITCH INPUT - We can also generate a negative signal using
Start switch S1. This final option also requires JMP2 (Pull-Up Select) to be
installed. Pull-up resistor R1 keeps a High (+VCC) at S1/C1 junction with no
input signal applied to J2 pin3. When S1 is pressed it pulls S1/C1 junction
low, giving us a negative pulse at C1’s input.
Differentiator Circuit and Limiter:
If the input pulse has a long negative duration, it can override the 555 timer’s
output. In most cases this is not desirable, however if it is then you can install
JMP3 (Differentiator Select). When JMP3 is not installed, C1 and R3 act as a
differentiator circuit. This circuit will turn an input square wave or long
duration pulse into a negative and positive short duration pulse. Pulse width is
determined by C1’s charge or discharge path. Since R3 is tied to VCC the
junction of C1/R3 will rest at VCC. The short duration pulses will be imposed
on that DC level. The negative short duration pulse will have an amplitude
slightly less than the original pulse. The positive short duration pulse will have
its amplitude limited by D2 to a maximum of VCC + 0.7V. This is to prevent
false operation of the timer IC. JMP2 (Input Select) must be installed for any
circuit requiring an external trigger input.
Charging Network:
R4, R5, R6, R7, and C2 make up the RC network responsible for all timing in
the UT5A. The component values determine the rate at which the capacitor
(C2) charges and discharges. JMP6 controls the way the charging network is
connected to the timer IC. JMP6 also determines the mode of operation,
which we will discuss later.
Timer Pin-out and Function:
The 555 timer IC has 8 pins.
Pin1 provides Ground to all timer functions.
Pin8 is +VCC and provides power for all IC functions.
Pin3 is the Output pin; it will be either High (near +VCC) or a Low
(near 0 VDC) depending on the status of the input pins.
Pin7 is called Discharge, it will provide ground to the charging
network when the Output (Pin3) is Low and an open when the Out put
is High.
Pin2 is the Trigger input. When the voltage on this pin falls below the
trigger level (usually 1/3 VCC) it will cause the Output (Pin3) to go
High and remove the ground from Discharge (Pin7).
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Pin6 is called the Threshold input. When the voltage on this pin rises
above the Threshold Level (usually 2/3 VCC) it cause, the Output to
go Low and place, a ground in the charging network through the
Discharge (Pin7).
Pin4 is the Reset. When this pin goes low it will override the other
inputs and force the Output to go Low and the Discharge to provide a
ground to the charging network.
Pin5 is the Control Voltage. This pin allows the Trigger and
Threshold levels to be independent of VCC. When this pin is
connected the Threshold level will be equal to the Control Voltage
pin (J3 pin1) and the Trigger level will be ½ Control Voltage pin.
Reset Circuit:
The Reset circuit consists of R11, C6, J3 pin3 (Reset Input) S2 and U1 pin4.
R11 is a pull-up resistor that keeps the timer’s RESET pin normally High. C6
is a filter capacitor to ensure that noise doesn’t accidently reset the timer IC.
S2 is a normally open push button switch that when closed pulls U3 pin4 low,
resetting the timer IC. J3 allows for an external reset of the timer IC.
Control Voltage Circuit:
The control voltage circuit is used to change the trigger levels of the timer as
mentioned in Timer Pin-out and Function. It also can be used for pulse width
and pulse position modulation. C4 is a filter capacitor that ensures noise does
not effect timer operation. R8 allows the Control Voltage to be set to any
value between 0 and +VCC. JMP5 allows R8’s wiper voltage to be applied to
the timer IC. J3 pin1 allows for an external Control Voltage. This voltage can
be any voltage up to +VCC AC or DC. JMP5 should not be installed if an
external Control Voltage is being used. With JMP5 removed and no input on
J3 pin1, the 555 timers Trigger and Threshold voltages are at their default 1/3
VCC and 2/3 VCC values respectively.
Timer Output Circuits:
The Inverted Timer Output circuit consists of current limiting resistor R9, pullup resistor R10, protection diode D3, and transistor Q2. The Non-inverted
Timer Output consists of JMP7 the Non-Invert jumper, R12 pull-up resistor,
protection diode D4, and transistor Q3.
The output of the timer chip U1 pin3 is connected to Q2 through current
limiting resistor R9. If U1’s pin3 output is Low the base of Q2 is Low, which
turns it off. This allows pull-up resistor R10 to pull Q2’s collector High. If
JMP7 is removed this High (approximately +V) is applied to J5 pin2 as the
Inverted Timer Output. If JMP7 is installed this high (approximately +0.7VDC)
is applied to the base of Q3. This causes Q3 to turn on pulling, its collector
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low (approximately 0VDC). The Low appears at J4 pin 1 as the Non-inverted
Timer Output.
If U1’s pin3 output is High, the base of Q2 is High, causing it to be turned on.
This pulls Q2’s collector Low, and this Low (approximately 0VDC) is appli ed to
J5 pin2 as the Inverted Timer Output. If JMP7 is installed, this Low
(approximately 0VDC) is applied to the base of Q3, turning it off. This allows
pull-up resistor R12 to pull Q3’s collector High (approximately +V). The High
appears at J4 pin 1 as the Non-inverted Timer Output.
The Inverted Timer Output at J5 pin2 and Non-Inverted Timer Output at J4
pin1 can be used as logic level outputs. A Low is less than 200mV and a
High is from +3 to +24V, depending on the supply voltage used for V+. They
also can be used to drive a load of up to 200mA if the load is connected
between +V pin at J4 pin2 and either of the Timer Output pins. To have the
load active when the timer’s output at U1 pin3 is High, connect the load
between J4 pin2 and J5 pin2. To have the load active when the timer’s output
at U1 pin3 is Low connect the load between J4 pin2 and J4 pin1. D3 and D4
protect Q2 and Q3 respectively from inductive kick if an inductive load is used.
MODES OF OPERATION
Note: For these explanations we will assume no Control Voltage input. Only
basic operation is covered here.
Monostabe Mode:
In Monostable mode the timer acts as a “one shot” pulse generator. It has
one Stable and one Unstable state. The pulse begins with a negative input
trigger. This causes the output of the 555 timer to go from low to high starting
the pulse (Unstable). The Pulse Width is determined by the charging
network’s RC time constant. When the circuit times out the output of the 555
timer goes from high to low completing the pulse (Stable). The timer will
remain in this Stable state until it is triggered again.
In Monostable mode the 555 timer requires an external negative pulse or
spike to start its operation. We will be using the pulse provided by the Input,
Differentiator, and Limiter circuit previously described. This means JMP4
(Input Select) must be installed.
JMP6 (Mode Select) set to pins 1 and 2 prepares the Chargin g Network for
Monostable operation. The C2’s charge path can be traced by starting at
+VCC down through R4, R5, to the pin7 (Discharge) of the timer IC, through
JMP6 pin1 to pin2, to C2. You may notice that R6 and R7 are bypassed in
this mode. C2 is also directly connected to pin6 (Threshold) of the timer IC.
When C2 is charging the Monostable is in its Unstable state. C2 is allowed to
charge when U1 pin3 (Output) is High and pin7 (Discharge) is providing an
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open.
The Monostable mode’s Stable State is defined as U1’s Output at pin3 is Low
and its Discharge pin at U1 pin 7 is held at ground allowing C2 to discharge or
preventing it from charging. The timer will stay in this mode until properly
triggered. The discharge path will be from C2 through JMP6 pin2 to pin 1, U1
pin7, to ground through the 555 timer IC, back to C2.
When the proper trigger appears at U1 pin2 (Trigger) input the Output at pin3
goes High and the ground at pin7 is removed (unstable state). This allows C2
to start to charge through JMP6 pins 1 to 2, R5, and R4 to VCC. When the
charge on C2 reaches the Threshold level (2/3 VCC or Control Voltage if
used) U1 pin6 (Threshold) will cause the 555 timer to change to the stable
state setting the Output pin (pin3) to a Low and applying ground at the
Discharge pin (pin7). The Discharge pin (pin7) being at ground discharges
C2.
The time C2 charges in the Unstable state is the Pulse Width (PW) of the
signal, it can be calculated with the following formula. C2’s discharge path
has no resistance so it is considered to be instantaneous. It is important to
note that the use of a control voltage will change the Threshold level of the IC;
therefore, change the RC timing and the pulse width.
PW = 1.1 * C2 * (R4 +R5)
Astable Mode:
In the Astable (no stable states) mode the timer is free running and acts like a
square wave generator. We will call the High output of the square wave from
the 555 timer Pulse Width (PW) and the Low portion of the square wave Rest
Time (RT). The Pulse width is determined by the RC charge time of the
Charging Network and Rest Time is determined by the discharge time of the
Charging Network.
In Astable mode JMP4 (Input Select) is removed disconnecting the signal
inputs. JMP6 is set connecting pin2 and pin3. This sets up C2 char ge and
discharge path and connect both U1 pin2 and pin6 to C2.
Since this mode is free running no input, other than applying power, is needed
to start the circuit. Lets start at the end of rest time with U1 pin3 (Output) Low,
U1 pin7 (Discharge) grounded, and C2 discharging slowly. When the charge
on C2 decreases below the Trigger level (1/3VCC or ½ control voltage if used)
U1 pin2 (Trigger input) causes the start of Pulse Width by setting U1 pin3
(Out) to go High and Discharge (pin7) removes grou nd. C2 is now allowed to
charge through R7, R6, R5, R4, to VCC. When C2’s charge reaches
Threshold level (2/3 VCC or Control Voltage if used) U1 pin 6 (Threshold) ends
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