This circuit is quite simple and if the LDR is mounted in an opaque tube with a lens, its sensitivity allows a separation of up to more than 1 meter between the sensor and the source. The light source is a 5 to 12 V lamp with at least 100 mA of current, depending on the separation distance. A 5 to 40 W incandescent lamp can also be used for connection to the power grid. The sensitivity is adjusted on the 1 M Ω trimpot and the relay must have a maximum coil current (activation) of 50 mA, with voltage according to the power supply used. The circuit is shown in the figure. Remember that, when an object passes in front of the relay, it will close its contacts for a brief moment, thus sending a control pulse to the external circuit. It is, therefore, a momentary action circuit, and not monostable or bistable. We can use the relay contacts to trigger a monostable with the 555 integrated circuit, for example, if the application requires it.
