Make tubelights automatically switch on at night and switch off in daytime using this automatic control system. Automatic darkness-controlled lighting system means that whenever there is darkness, light source like bulb or tubelight glows automatically.
The circuit works off regulated 5V and uses triac BT136, NOT gate 7404 and light-dependent resistor (LDR).
Operation of the circuit is simple. During daytime, low resistance of LDR1 makes pin 1 of gate N1 low and its output pin 2 goes high. This high output is applied to input pin 3 of gate N2. As a result, the output of gate N2 goes low. Hence no gate signal is applied to triac BT136 (triac 1) and it acts as an open circuit and the bulb does not glow.
At night, the high resistance of LDR1 makes pin 1 of gate N1 high and its output pin 2 goes low. This low output is applied to input pin 3 of gate N2. As a result, the output of gate N2 goes high, which is applied to the gate of triac BT136 (triac 1) and it acts as a short circuit and the bulb start glowing.
Assemble the circuit on a general-purpose PCB and keep at a suitable place. Keep LDR1 at such a place that enough light falls on it in day-time. You can also use this circuit as a street light controller.