In this post you will learn about a very simple light activated relay circuit in which a relay is toggled On/OFF in response to the light or darkness over a photo sensitive device, the circuit utilizes only a Darlington transistor for activating the relay
The light sensitive relay circuit presented here has been configured such that it increases the sensitivity of an attached transistor many folds.
A couple of transistors can be identified connected in Darlington configuration and the relay can be any type rated between 12V to 6V supply. The diagram also depicts a photo sensitive device TIL78 but one may practically incorporate any similar phototransistor to serve the purpose, even a stripped 2N3055 power transistor having its junction exposed to light will work, to do this take a good 2N3055 transistor and very carefully grind of the cap surface such that the internal junction gets exposed, but make sure you do not damage the entire cap or the junction while scraping the device.
The 2M2 preset may be appropriately adjusted for increasing or decreasing the light sensitivity of the photo transistor device