The brightness of the surrounding light controls the function of this circuit. The light sensor used in this circuit is a light dependent resistor(LDR). This LDR is connected to the
non-inverting input of the 741 opamp IC. Once the intensity of the light striking the LDR sinks below the minimum level, the output of the opamp reverses from "low" to "high" level.
The follow up transistor T1 conducts and the relay is activated thereby switching on any electrical load connected to the relay (for example: lamp). The most common application of this
circuit is as an automatic switch for street lamps.
Of course you can use it for other applications that require a circuit controlled by the surrounding light.
The supply voltage of the circuit is highly flexible. It can be between 10 and 36 volts. Potentiometer P1 adjusts the threshold level where the relay must be activated.
If you intend to use this circuit to automatically control street lamps, replace P1 with 50K and R1 with 10K.
The voltage rating of the relay must be selected to handle the voltage load that it is going to support.
Parts Placement Layout
Electronic Circuits volume 1.0 - Circuit Nr. 87
The complete data of the electronic circuit described above can be found in the following book and is available from Amazon.com.
Click on the image to view the book.