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Sensitive Magnetic Switch (ART468E)

This article shows you how to build a circuit that fires by the approximation of a magnet, but does not use reed-switches. The circuit can be used in opening doors, triggering lamps and other similar applications.

Reed switches, or blade switches, consist of components that can be used to turn a circuit on or off by the action of the magnetic field of a magnet, as shown in Figure 1.


Figure 1 - The reed-switch
Figure 1 - The reed-switch


However, these switches have little control capability, typically a few hundred milliamps, which limits their application. Auxiliary circuits must be used to control larger loads. However, the reed-switch is not the only option we have to connect a circuit from the magnetic field of a magnet.

An interesting option is to use a pickup coil attached to an SCR and this is what we will do in this project. Approaching a rapidly moving magnet near a pickup coil induces a pulse of sufficient intensity to trigger an SCR and thereby control a relay.

In our case, as the circuit is powered by DC voltage, once tripped the circuit remains in conduction. To turn it off, the power must be interrupted for a moment.



In figure 2 we have the complete diagram of the apparatus.


Figure 2 - Complete diagram of the magnetic switch
Figure 2 - Complete diagram of the magnetic switch


Since this is a very simple assembly, we chose the terminal strip version of the terminals, shown in figure 3.


Figure 3 – Terminal strip mounting
Figure 3 – Terminal strip mounting


The SCR can have any suffix, and does not need a heatsink. Observe its position and the polarity of the diode. The coil is the primary winding of any small transformer from which the core has been withdrawn and a ferrite rod inserted. For power supply, it may be necessary to use 9 V to compensate for the 2 V voltage drop in the SCR.

To trigger, simply lean on the stick or pass a permanent magnet close to it.

In figure 4 we have a suggestion of a complete circuit of an electronic lock with the rearmament system.


Figure 4 - An electronic lock
Figure 4 - An electronic lock



The rectifier diodes are 1N4002 or equivalent and the trimpot is used to adjust the sensitivity. To test, just go near the magnet of the sensor and adjust the trimpot or potentiometer at the same time.


SCR -TIC106 - see text

D1 - 1N4148 - general purpose diode

P1 - 100k ohm at 1M ohm - trimpot or potentiometer

K1 - 6 V Sensitive Relay

S1 – On/Off switch

B1 - 6 V - 4 batteries or power supply

X1 - Sensor - see text

R1 - 1k ohm x 1/8 W- resistor - brown, black, red



Terminal strip, battery holder, wires, solder, etc.