This circuit can be used to trigger a lighting system when it darkens or even as an alarm when shadow appears on the sensor. The circuit is extremely sensitive because it uses two transistors in the amplification of the LDR signal.
Optical features can help you achieve directivity and sensitivity depending on the application. The LDR can be installed in an opaque tube with a converging lens.
The circuit operates with relays of 5 to 12 V, depending on the power and its consumption is very low in the standby condition. Relays of the RL2 series for voltage in the indicated range of Metaltex are recommended.
In figure 1 we have the complete diagram of the apparatus.
In figure 2 we have the assembly on a small printed circuit board that can be installed in a plastic box.
Material list
Q1, Q2-BC548 or equivalent - general-purpose NPN transistors
D1- 1N4148 - silicon diode
LDR - round common LDR
P1- 100 k ohms - trimpot or potentiometer
K1 – Sensitive 6 or 12 V relay
R1- 10 k ohms x 1/8 W- resistor - brown, black, orange
R2 - 22k ohms x 1/8 W - resistor - red, red, orange
R3 - 100 ohms x 1/8 W - resistor - brown, black, brown
Several:
Printed circuit board, power supply, wires, mounting box, solder, etc.
