This is a simple photo switch. A photoswitch is a kind of sensor that detects the presence or absence of light.
This photo switch turns - on and off a relay, as soon it detects the absence or the presence of light, respectively. Click here to download the schematic and the pcb artwork.
The circuit is based on two transistors and an LDR. A pot-trimmer is used to adjust the light sensitivity. The relay is off when an LDR is exposed to light.
To setup the circuit, switch on the power supply, cover the top of the LDR using a black paper (or your hand) and adjust R5 so that the relay gets just activated. Now, if you uncover the LDR you'll notice that the relay is turned off. If not, adjust R5 again. Ensure that the relay is off when the LDR is exposed to light. After some trial and error you will get the correct set point of R5.
This circuit has been tested and proved to be very good. For a simple circuit as this, it is normal to expect any sensitivity drift due to temperature changes. It is also normal to expect some oscillations when the light intensity crosses the threshold - region.
Author: G. Adamidis
Simple Photo Switch on CircuitLib.com by G. Adamidis is licensed under a Creative Commons Attribution 3.0 Unported License.