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.
Simple Switch On Time Delay Circuit
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.
