The circuit we present causes a relay to close its contacts when the sensor, a common LDR is darkened, i.e., in the case of shadow or even at dusk, allowing the use of the project as a twilight switch or automatic light.
The circuit doesn’t have a lock, which means that the contacts of the relay will only remain closed during the time which there is shadow on the sensor.
We can power the circuit with voltages from 5 to 12 V depending on the relay. In the standby condition the consumption is low, which allows the circuit to be powered by batteries. But if the relay is energized for a long time, it will be convenient to use the power supply.
In Figure 1, we have the complete diagram of the device.
In Figure 2, we have a suggestion for the printed circuit board.
Material list
Q1 - BC548 - NPN general purpose transistor
D1- 1N4148 - general purpose diode
LDR - Common LDR
K1 - ML2RC1 (6 V) or ML2RC2 (12 V) - Metaltex miniature relay
R1 - 4k7 x 1/8 W- resistor - yellow, violet, red
P1 - 10k to 47k - trimpot or potentiometer
Miscellaneous:
Printed circuit board, batteries or power supply, wires, welding, etc.
