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Shadow Alarm with the 741 IC (ART429E)

The circuit we present is that of an alarm of lack of light, ie a circuit that produces an audible signal on a speaker when a sensor is no longer illuminated.

This alarm can be used for various purposes, such as for indicating the burning of lamps in an ambient light system, the presence of persons in a passage, as suggested in figure 1, and the very darkening of the environment at dusk.

 

Figure 1 - The alarm
Figure 1 - The alarm

 

In Figure 2 we then have the complete circuit of our alarm which is powered by a symmetrical 9 V source.

 

Figure 2 - Alarm circuit
Figure 2 - Alarm circuit

 

This source can be formed by two 9 V batteries connected as shown in figure 3.

 

Figure 3 - 9 V symmetrical power source
Figure 3 - 9 V symmetrical power source

 

Or if the reader wants, it may be the source shown in figure 4.

 

Figure 4 - another dual symmetric source
Figure 4 - another dual symmetric source

 

The transformer used in the case has a primary according to the local network and a secondary one that provides a voltage of 9 V under a current of at least 250 mA.

Cl-741 or any of its equivalents

R1 - 470k - trimpot

R2, R3 - 199 k x1 / 8 W - resistors (brown, black, yellow)

R4 - 100 ohms x 1/4 W - resistor (brown, black, brown)

C1-O, 1, uF-polyester capacitor

C2 - 100 uF x 16 V - electrolytic capacitor

LDR - Common LDR of any type

FTE - 8-ohm speaker

Miscellaneous: Printed circuit board, fountain, wires, solder, etc.