A light operated switch is presented here. A light operated switch turns on or off a relay according to ambient light. The circuit is quite simple and has only 15 components. The key components are the two transistors, the LDR (Light Dependent Resistors - Photoresistor) and the relay. There is also an adjustable resistor (trimmer-pot) which is used to adjust the light sensitivity.
The relay is activated when the LDR is exposed to light. Apart from relay, the circuit can operate from a single power supply source in the range of 5 to 12V. However, the selected supply voltage is limited by the relay type and since a relay is usually designed to operate at a limited voltage range the circuit must operate at the same limited voltage range, too.
It is recommended to use a photo resistor which has a value of about 100-600 Ohm when exposed to light and above 10K at absolute darkness and a relay which can be energized with less than 50mA of current. Remember also to place the LED away from the photo-resistor, in order to avoid oscillations due to feedback.
Theory of operation
R5 (photoresistor) and R6 form a voltage divider (also known as a potential divider) which produces an output voltage that is a fraction of the power supply voltage. That voltage depends on R5 value and rises as the ambient light intensity rises.
The output voltage which is across R6, remains high as long the photoresistor is exposed to light. C1 is charged threw R4 and as the base voltage rises; the emitter and collector currents of Q1 rise exponentially. Q1 goes almost completely on (becomes saturated) and its collector’s voltage drops at an almost zero value. That way, Q2 remains completely off because there is not enough voltage at its base (with respect to its emitter) to bring it in the on-state and the relay stays off.
At darkness, the voltage across R6 remains low and Q1 goes to a completely-off state. Since no current flows threw R3, the voltage at Q1’s collector becomes high enough to activate Q2. Q2 becomes saturated and current flows threw the LED and the relay. That way, the LED emits light and the relay is activated.
C1 is used as an integrator in order to produce some delay and filters out any instantaneous luminous changes (flashes).