This Switch On Time Delay
circuit has been designed to create a lamp switch operated
electronically with an option of setting a delay in the time of
execution of operation to reduce one or more lamps in a stairwell or
any other places where this circuit may be useful. The circuit can be
useful to control various lamp or appliances that can be connected in
relay contacts.
The circuit that takes advantage of the emitter/base breakdown voltage
of an ordinary bi-polar transistor. The reverse connected emitter/base
junction of a 2N3904 transistor is used as an 8 volt zener diode which
creates a higher turn-on voltage for the Darlington connected
transistor pair. Most any bi-polar transistor may be used, but the
zener voltage will vary from about 6 to 9 volts depending on the
particular transistor used. Time delay is roughly 7 seconds using a 47K
resistor and 100uF capacitor and can be reduced by reducing the R or C
values. Longer delays can be obtained with a larger capacitor, the
timing resistor probably shouldn't be increased past 47K. This Switch
On Time Delay circuit should work with most any 12 volt DC relay that
has a coil resistance of 75 ohms or more. The 10K resistor connected
across the supply provides a discharge path for the capacitor when
power is turned off and is not needed if the power supply already has a
bleeder resistor.
