The circuit is designed to turn the relay on and
hold it ON for one second after seeing a positive going
input.
To explain that a little better, imagine that the relay
is connected so it feeds power to a car headlamp
washer.
Suppose that the input is connected to the windscreen
washer switch.
Each time the windscreen washers are turned on, the
circuit receives a 12 volt input.
The timer I.C. will turn the headlamp washer on for just
one second then turn off, regardless of whether the
windscreen washer is still on or not.
The coupling capacitor, C4, requires a certain amount
of time to discharge after seeing an input. So the relay
can not be operated again by the headlamp washer switch
until a short time has elapsed. This system removes the
possibility of having the relay continually clacking
on/off as the windscreen washer switch is operated.
The circuit has other uses, too.
The relay can be made to stay ON longer by increasing the
value of R2 and/or C2 (which should be a Tantalum
type).
A kit of parts is available from SatCure.
Note that not all "555" timer I.C.s will operate
satisfactorily in this circuit. The TLC555CP is
recommended.
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