This circuit consists of a simple but effective light-activated alarm which uses only a half dozen components. A light falling on the LDR turns on the SCR. The lamp will remain on until the power is of or 51 is used to reset the circuit. p1 adjust sensitivity according to the application.
To get better performance the LDR can be housed in an opaque tube with a convergent lens. A 6-volt lamp can be used without problems since there is a voltage drop of about 2 volts when in the ON state.
The schematic diagram is shown ln circuit 1. 51 is optional, as you can reset the circuit by turning off the power supply.
Components layout, using a terminal strip, is shown in Figure 2.The lamp can be replaced by an LED in series with a 470 ohm resistor or a buzzer. You also an control an audio oscillator, replacing the lamp with it.
The lamp isn't a critical component. Any 6V lamp ranging from 50 to 250 mA can be used.
The LDR is a common Gds photoresistor. The SCR needn't be mounted on a heatsink as the current is very low.
Parts List - Experimental Light-Alarm
SCR -TlC106 Silicon Controlled Rectifier
LDR - Common LDR - see text
P1 - 1,000,000 ohm Potentiometer
Xl - 6V incandescent lamp
Bl - 6-9V - four AA cells, D cells, battery or power supply
S1 - SPST momentary switch
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