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Electronic Stethoscope (ART436E)

The stethoscope is a sensitive device that allows you to hear very weak sounds, being therefore a useful device for the detective. With it we can detect the noise of mechanisms of a clock bomb (if it is of the mechanical type) or even find hiding places and even listen through walls.

We originally published this article in one of our espionage books of the 1980s. The article was updated in 2015.

In figure 1 we give an idea of how this device can be used in listening through walls.

 

Figure 1 - Listening through walls
Figure 1 - Listening through walls

 

The very simple proposed circuit is shown in figure 2.

 

Figure 2 - The stethoscope circuit
Figure 2 - The stethoscope circuit

 

The arrangement of the components on a printed circuit board is shown in figure 3.

 

Figure 3 – Printed circuit board
Figure 3 – Printed circuit board

 

For listening through floors and walls the microphone needs to be equipped with a special acoustic system. This system consists of a heavy metal disc with the padded edges as shown in figure 4.

 

Figure 4 - The special sensor
Figure 4 - The special sensor

 

The disc shall weigh at least 500 grams so that side sounds are damped and only the front sound of the wall or floor are picked up and amplified.

 

CI-1 - TBA820 - integrated circuit

 

RESISTORS

P1 - 100 k - potentiometer

R1 - 33 ohm - resistor (orange, orange, black)

R2 - 56 ohm - resistor (green, blue, black)

R3 - 10 k - resistor (brown, black, orange)

 

Capacitors

C1 - 100 nF - ceramic capacitor

C2 - 100 pF - ceramic capacitor

C3. C7 and C8 - 100 uF - electrolytic capacitor

C4 - 47 uF - electrolytic capacitor

C5 - 220 nF - ceramic capacitor

C6 - 100 nF- - ceramic or polyester capacitor

C9 - 470 uF - electrolytic capacitor

C10 - 68 uF - electrolytic capacitor

 

SEVERAL

MIC - electret microphone

B1 - 6 V - 4 small batteries

F - handset or speaker from 8 to 40 ohms

S1 - Single switch

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