The circuit shown in figure 1 is triggered when the light falling on the LDR is cut off.
It can be used as a pass sensor or even to trigger an emergency lighting system.
We can make the assembly in a protoboard or terminal strip.
For the strip version, the arrangement of the components is shown in figure 2.
To assemble, observe the position of the transistor and the arrangement of the relay terminals.
The resistor is 1/8 W with any tolerance and the LDR of the common round type.
Material list
Q1 - BC548 - NPN general purpose transistor
D1 - 1N4148 - general purpose diode
LDR - round common LDR
P1 - 100 k ohms - potentiometer
K1 - 6 or 12 V sensitive relay
B1 - 4 batteries or source - 6 or 12 V
S1 – On-Off switch
R1 - 1 k ohms - resistor - brown, black, red
C1 - 100 uF x 16 V - electrolytic capacitor
Several:
Terminal strip or protoboard, battery or source holder, wires, solder, etc.
