Elenco 130-in-1 Electronics Playground - Manual (Page 119)

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This circuit is a delayed timer that uses an
operational amplifier and the RC time constant. RC
stands for resistor/capacitor. A circuit that delays an
operation is a time constant.
Through resistors RA and RB the negative (–)
terminal of the operational amplifier receives a
voltage of about 4.5V. This is the comparator’s
reference voltage. Connected to capacitor C1 is the
positive (+) terminal of the comparator. This capacitor
receives its charge through the series resistance of
R2 and the control. The charging speed is slower
when the resistance is large, and faster when the
resistance is small. This charging speed set the delay
time for the timer circuit.
Now turn the control fully clockwise to position 10.
Set the switch to position A to turn on the power. LED
1 lights first; LED 2 lights about 5 to 7 seconds later.
This 5 to 7 second time difference is the delay time
that is set by the CR time constant.
Now, turn off the power, set the control fully counter-
clockwise to position 1, and see what happens when
you turn on the power again. LED 2 lights later than
LED 1 again, but how many seconds later?
Notes:
EXPERIMENT #99: RC DELAY TIMER
Schematic
Wiring Sequence:
81-31-63-27-131
28-87
83-33-36-70-116-135-121
34-67
68-82-84
88-69-115-136
119-124
122-132