The use of the SCR causes the alarm described to have the effect of the latch, that is, remains fired even after the light no longer impinges on the sensor.
To reset, the circuit must be turned off or on for a moment or S1 pressed for a moment.
The circuit for this alarm is given in figure 1.
In figure 2 we have the assembly of this circuit in a terminal strip, being the load of 12 V connected directly in its exit.
For high current loads up to 3 A, the SCR must be mounted on a heatsink.
The sensitivity adjustment is made in the pot and the LDR is of the common round type.
Material list
SCR - SCR1 - TIC106 - SCR with any suffix
LDR-LDR common round
P1 - 100 k ohms - potentiometer
S1 – Push button
Several:
Terminal strip, 12 V supply, wires, solder, etc.
