This add-on circuit automatically turns on your porchlight when your doorbell rings, so you can see the person ringing the doorbell at the doorstep. This also helps to guard against burglars, who usually press the doorbell switch to confirm that there is no one at home. By turning on the porchlight, the circuit will trick them into believing that someone is inside the home.
You can easily connect the circuit to your doorbell. The light remains on for around 20 seconds and then turns off. This duration is enough for you to find your way in the dark to open the door. However, duration can be varied by changing the RC components (R1 and C2).
When you momentarily press push-to-on DPST switch (S1), the AC mains is supplied to:
1. The doorbell via S1(b) and it rings.
2. Stepdown transformer X1 via S1(a) and it delivers 12V AC at its secondary. The secondary output is rectified by diode D1 and filtered by capacitor C1 to provide the required DC. The DC voltage triggers timer 555 (IC1) and its output at pin 3 goes high for the preset time. Simultaneosly, the relay energises and AC mains flows via its N/O contacts to switch on the porchlight bulb.
Triac 1 is wired as an automatic light controller to switch on the porchlight at night and switch it off during day. The conduction angle of triac 1 depends on the bias provided to the gate of the triac through diac 1, which, in turn, is controlled by preset VR1 and the light falling on LDR1.