The post explains a simple fused or blown car brake light indicator circuit using a single IC 555 and a few other passive components. The idea was requested by Mr. Joel Bayongasan

Technical Specifications

You innovations are great! Thanks for this blog. May I request something please? I was searching for a circuit that will detect a blown brake lamp.

A car normally, there two connected in parallel or sometimes four.

I am looking for a circuit that would light an led indicator if one of the bulbs is busted. I hope you can help.

Thank you.

The Design

By introducing the above shown circuit between the lamp and the supply, the intended blown brake light bulb indicator can be easily built and implemented in any vehicle.

The functioning is quite straightforward:

The IC 555 is configured as a simple voltage comparator, where its pin2 becomes the sensing input.

The BC557 along with the associated R1, R2 resistors forms a current to voltage converter stage.

Using Resistor Sensor

As long as a working bulb lamp stays connected across the shown points, a small negative potential corresponding to the bulb current consumption is developed across Rx.

This potential becomes sufficient to keep the BC557 triggered and conducting which in turn keeps pin2 of the IC high.

With the above conditions, pin3 of the IC stays low and the LED remains shut off.

However in an event the car bulb fuses or stops illuminating, the potential across Rx vanishes or reduces to an extent where the BC557 just stops conducting.

This instantly renders pin2 of the IC high and the LED begins glowing indicating the blown brake light bulb situation to the user.

The above design can be also effectively used in many different applications which require some kind of current (amp) monitoring such as an over-current or over-load cut off etc.

R1 may be calculated as follows:

R1 = 0.7/bulb current rating

The above explained circuit can be much simplified through the following configuration:

Simplified Schematic

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