Spark Plug Monitor Circuit Diagram


Spark plug monitor circuit is an electronic device used to diagnose the status of spark plugs installed in the engine of a vehicle. The circuit is designed to detect whether the spark plugs are functioning efficiently or not. It is an essential tool for preventing engine misfires, reducing fuel consumption, and improving the overall performance of the engine.

As the name indicates, the monitor is intended for checking whether the spark plugs of a car actually produce a good ignition spark. There are commercial monitors for this purpose on the market but these cost rather more than the one described here.

Circuit Diagram

of Spark Plug Monitor Circuit

This amplifier can be designed using a few basic components. The circuit diagram of this project is shown below.

Spark Plug Monitor Circuit Diagram
Spark Plug Monitor Circuit Diagram

More Circuit Layouts

Circuit Explanation of Spark Plug Monitor Circuit

The circuit consists of two anti-parallel connected LEDS that are placed in series with the spark plugs. Two diodes are used since the direction of the current is not always evident and also because oscillations occur in the ignition circuit. High-efficiency diodes that light brightly at a current of about 1 mA should be used.

The brightness of the diodes depends on the level of the current drawn by the spark plugs and this, of course, is a measure of the strength of the spark. If the spark plug is short circuited, which is highly unlikely (and could be ascertained with a ohmmeter), neither of the diodes lights.
The monitor must be provided at one side with asmall piece of tin-plate that is inserted into the spark plug socket and at the other with a short length of tough wire that is wound a couple of times around the spark plug lead.

Working Explanation of Spark Plug Monitor Circuit

Normally, only one of the LEDs will light, but, owing to oscillations in the LC circuit formed by the coil and the capacitor in the ignition circuit, it may happen that both light.
When the engine is started, or when it runs at low constant speed, the diode flickers, but at higher engine speeds it lights uniformly.

In the case of an engine with point contacts as in the diagram, the brightness of the diode remains virtually constant when the engine speed increases. This is because although the ignition frequency rises, the pulse duration shortens.

If at higher engine speeds the brightness of the diode diminishes, either the gap in the contact breaker is set incorrectly or the contacts ‘float’. Both of these events result in a shortening of the time that the contacts are closed. As a consequence, there is insufficient time for the ignition coil to recover its electric field. When the contacts are then opened, only a weak spark will be produced.
In multi-cylinder engines it is advisable to give each spark plug its own monitor, which makes it possible for the four or six spark plug currents to be compared.

Warning! Make sure that the ignition switch is off when connecting the monitor to the spark plug socket since random ignition pulses may be lethal.

Conclusion of Spark Plug Monitor Circuit

The spark plug monitor circuit is a useful tool for vehicle maintenance. It helps in diagnosing problems with the spark plugs and enables timely repairs, thereby ensuring smooth functioning of the engine. Regular use of this circuit can prolong the life of the engine and improve its performance. Therefore, it is recommended to include the spark plug monitor circuit in your vehicle maintenance routine.

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Spark Plug Monitor Circuit Diagram
Spark Plug Monitor Circuit Diagram
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