reed current monitor figure 1

Reed Switch as a Current Monitor

In this current monitoring circuit we use a Reed switch with a LED and a resistor to indicate if is current through a circuit. A Reed switch usually requires between 10 and 100 AT (Ampere-turns = corresponding to the current in a coil multiplied by the number of turns). The lower the AT, the more sensitive the reed switch.

For example a car headlight (both light bulbs) has a current consumption of 7 A or 8 A at 12V so a 50 AT Reed switch requires 7 or 8 turns in order to monitor the current.
As soon as one bulb is malfunctioning the current is dropping below half and the switch opens its contact.

Current Monitor Circuit Schematic

reed current monitor figure 1

In figure 1 the LED lights up as long as the Reed switch is closed signaling that there is current.

reed current monitor figure 2

In figure 2 the LED lights up only when the switch is open signaling that there is no current.

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