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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 Output
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).
UT5A 5
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