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