Supervisor Circuit

It is often needed to check if a unit is operating, and if that unit is working, another devices can be operated.

On the other hand, if the unit is switched off, it can still operate for a while, even if its power supply is switched off. For safety reasons, it is a must to prove that none of the unit’s parts are moving, so nobody can hurt.

The examples above illustrate the necessity of the supervising circuits.

A simple but very useful circuit is to be presented in the followings.


This circuit accepts frequecy type input, and provides a relay contact output.

The first example shows its operating principle. There is a motor, which has a rotation sensor connected to its shaft. This sensor provides voltage pulses when the motor is operating (rotating).

These pulses are the input for the supervisor circuit. If the shaft is rotating, the relay output is closed, thus provides a signal to the operator people, that working on the motor is not allowed (dangerous).


In the second example, there is a unit (called CPU on the picture) that is doing some function. This unit can be a PLC (for eg.). This CPU has some digital outputs (DO), one of which acts as a “Lifesign”. This digital output provides 1 pulse per second, which signals that the unit is operating corretly, it has no internal failure. When the CPU is working, another units can be switched on, or operated.

In this case the supervising circuit is checking if the CPU is OK or not. If the CPU fails, the supervising circuit’s relay output disables other units’ operation.


The third example was taken from the automotive industry. This solution is found (for eg.) in Mercedes type cars. Mercedes calls it “Fuel pump controller relay”.

The supervisor circuit enables the fuel pump’s operation if the ignition key is switched on, or the crankshaft is moving.


It is easly understood that the engine needs pressurized fuel. This pressure is present if the fuel pump is operating. To start the engine the ignition switch is contacted, which triggers the supervisor circuit’s output relay. When the engine is running, the crankshaft’s rotation sensor outputs pulses to the second input.

The inside of such a supervisor unit is surpisingly simple.

Basically it only needs a monostable multivibrator and a relay. The input signal triggers the monostable IC, which gives a pulse (for a given time) at its output.

In the picture below HEF4538 type monostable multivibrator is shown. With the components used, it provides an approximately 2,2 sec output pulse if triggered. If another pulse arrives at the intput while the output is ON, it retriggers the monostable IC and lengthens the relay  ON state.


To use this circuit in harsh industrial environment, some additional elements must be added to the circuit: an optocoupler for the galvanic isolation and a voltage regulator for handling “universal supply voltages”.


This circuit is as superb as simple. It gives reliable operation for a long time, without failure. That’s why it is often used for “safety function”. No microcontroller, no softwarem just simple, proven analogue techniques.

Easy to understand, easy to check, easy to repair.


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