Relay Timer Circuit

The following circuit is a simple timer circuit for operating a 12V relay. With the parts specified it will have an on time duration of 0 to about 60 seconds. Using a different capacitor for C1 will change the maximum on time. The timer can be triggered by a simple switch across the trigger points or connect an NPN transistor to the trigger points to allow a motion detector or logic voltage or another circuit to trigger the timer (see Fig.1 for an example of a motion detector trigger).

Part Name Jameco Part # Quantity 100K Variable Resistor 286301 1 1K Resistor (1/4W) 29663 2 1N914 Diode 36311 2 220 Resistor (1/4W) 30470 1 3 pin Terminal Block 152354 1 470µF Electrolytic Capacitor 93809PS 1 LED (Green) 253770 1 NE555 Bipolar Timer 27422 1 SPDT Relay (12V) 129349CH 1

Fig. 1 Motion Detector Trigger PIR - KC7783 PIR Detector Module or equivalent Q1 - 2N3904 General Purpose NPN transistor or equivalent