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Passage Alarm (ART434E)

The following circuit serves to detect the passage of a person by a place, activating a warning at a distance or a recorder of time mode.

This article is part of a spy book that we published in the 1980s and was updated in 2015.

The principle of operation is based on a LDR photocell (photo-resistor) that must be permanently illuminated by a small lamp. When the light beam incident on this cell is interrupted, circuit tripping occurs.

In figure 1 we have the complete circuit of this interesting automatism that also functions as an alarm.


Figure 1 - Passage alarm circuit.
Figure 1 - Passage alarm circuit.


The printed circuit board is shown in figure 2.


Figure 2 – Printed circuit board
Figure 2 – Printed circuit board



P1 adjusts the sensitivity as a function of ambient light, while P2 adjusts the timing from a few seconds to about 1 hour, depending on the value of the electrolytic capacitor connected to pins 6 and 7 of the integrated circuit.

The maximum recommended value of this capacitor is 2 200 uF since larger values can destabilize the circuit due to leakage. The relay terminals are connected to the microphone key of a recorder or to a transmitter for clandestine listening, and at the same time to a remote alert system to know what is happening.

To use the device is simple:

Position the LDR so that it receives light from the small lamp hidden in the passage (2 a15watts); The LDR and lamp can be fitted with tubes with lens to direct the light.

Set P2 to the shortest timing position and then open P1 until the relay trips. Come back a little and wait. If the relay does not open, turn it back a little until the trigger threshold is reached. Try moving your hand in front of the LDR to interrupt the light beam.

Checking the operation is just to do the installation. In figure 3 we have the way to do this installation, activating a recorder and at the same time m remote warning.


Figure 3 - Activating an alarm
Figure 3 - Activating an alarm




CI-1 - 555 - integrated circuit

Q1 and Q2 - BC547 or equivalent - general purpose NPN transistors

D1 - 1N914 - silicon diode



P1 - 100 k - potentiometer

P2 - 1M or 2.2M - potentiometer

R1 -10 k - resistor (brown, black, orange)

R2 - 47 k - resistor (yellow, violet, orange)

R3 and R4 - 1 k - resistor (brown, black, red)



C1 - 1000 uF at 2 200 x 12V pF - electrolytic capacitor

C2 - 100 uF x 12V - electrolytic capacitor



K1 - 6 V x 100 mA Relay

B1 - 6 V - 4 medium batteries (alkaline)

S1 - single switch

Printed circuit board, mounting box, wires, solder, etc.