The circuit shown in figure 1 is based on a 4093 and has great sensitivity given the high input impedance of the CMOS ports.
This circuit triggers a relay when light is on the LDR and its sensitivity is set at P1.
We can reverse its action by making it a sensitive alarm about overriding the P1 and LDR positions.
The circuit may be mounted in a contact array or on a printed circuit board with the array of components shown in Figure 2.
The supply can be made with voltages of 6 or 12 V depending on the relay and if non-DIL based types are used, the design of the board must be changed.
For greater directivity and sensitivity the LDR can be installed in an opaque tube with a converging lens.
Material list
CI-1 - 4093 - integrated circuit
Q1 - BC548 - NPN general purpose transistor
D1 - 1N4148 - general purpose diode
K1 - 6 or 12 V sensitive relay
P1 - 1 M ohms - potentiometer
LDR - round common LDR
R1 - 1 k ohms - resistor - brown, black, red
Several:
Protoboard or printed circuit board, wires, solder, etc.
