Secret Warning Signal (ART437E)

In an emergency situation in your living room, how do you warn your secretary in the front room? A good feature for those who wish to avoid dangerous situations is a secret switch, triggered by a magnet placed on any piece of decoration on your desk.

This current article was originally produced for one of my spy books of the 1980s.

A reed-switch is hidden under the table and in the piece a small magnet. Under normal conditions, the magnet is far from reed, but in an emergency situation, for example by pretending to pick up a pen, you move the object to the position where it aligns with the reed-switch and the signal is sent to your desk by means of hidden wires, as shown in figure 1.


Figure 1 - Secret operation
Figure 1 - Secret operation


The complete diagram is shown in figure 2.


Figure 2 - Alarm diagram
Figure 2 - Alarm diagram


The hidden receiver wire may be common.

The food is made with ordinary batteries that are in the emergency room in the adjoining room next to a desk. The volume of the alarm system is not the biggest to not catch the attention of a possible intruder who is making threats in the main room.



CI-1 - 555 - integrated circuit

Q1 - BC547 or equivalent - NPN transistor of general use

Q2 - BC557 or equivalent - general purpose PNP transistor



P1 - 100 k - potentiometer

R1 - 47 k - resistor (yellow, violet, orange)

R2 - 10 k - resistor (brown, black, orange)

R3 - 56 k - resistor (green, blue, orange)

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



C1 - 100 uF x 12V - electrolytic capacitor

C2 - 47 nF - ceramic or polyester capacitor

C3 - 100 uF x 12 V - electrolytic capacitor



B1 - 4 small batteries

X1 - Reed-switch

FTE - 4 or 8 ohm speaker

S1 - Single switch

Printed circuit board, battery holder, mounting box, small permanent magnet attached to an object on the table, wires, solder, etc.