Car Turn Signal Flasher Circuit with Lamp Malfunction Indicator

The post discusses an automotive/car turn signal indicator circuit idea with a built-in load malfunction indicator, meaning an additional LED lamp fitted on the dashboard which would warn if any of the side indicators went bad or fused. The idea was researched and requested by Mr. Abu-Hafss.

The Circuit Concept

Hi Swagatam

I have found a transistor-based flasher including a relay. I have transformed the schematic into LT Spice. You can find the files below:


Physically, the circuit oscillates when a Load is connected across the pin "C" of the relay and earth. If the load is less than 100 Ohms the oscillation is significantly fast. And if the load is say about 1K the frequency is normal.

However, I could not understand the circuit nor it is simulating in LT Spice. Could you please explain me how the circuit is sensing the change in resistance of the load. And could you please make the simulation work.

Please follow the links below for the photos.



To make a 555 based automotive turn signal flasher circuit which can detect lamp outage. If not possible with 555 then transistors may be considered.


Above are the photos and schematic of the circuit. I have physically tested it. It also works on BC558/BC546.

With bench power supply of 12V-2A and load connected at pin (L) and (-), following is the behavior:

· No load, no oscillation

· Pin (L) and (-) shorted, very fast oscillation (relay sounds)

· Load 1 – 1.5K, normal oscillation (about 1.4Hz)

· Load increases, oscillation slows

· Load decreases, oscillation decreases

When using a car battery and 3 x 27W bulbs in parallel the circuit oscillates normally. Even, if a bulb is removed the frequency is unchanged. This means, this circuit does not detect lamp outage. However, the behavior of the circuit with a bench setup may give you some clue to my objective.


Yesterday, I came across this circuit which has a relay to detect load. I have not yet tested it. I hope this circuit might also help us. Maybe we can replace the relay with a transistor.

Thanks for your help.

Best regards, Abu-Hafss


Car Turn Signal Circuit with Bulb Malfunction Indicator


IC 555 based car turn signal indicator with pilot warning lamp:

The required circuit design which is shown below may be understood with the following points:

The IC 555 section is wired in the normal astable multivibrator whose  pulse rate at pin## can be adjusted with the help of the given 100K pot.

The two NPN transistors function as current sensor, and IC 555 reset controller.

When power is switched ON, the N/C contacts of the relay connect the turn signal lamps with the 12V supply.

The lamps light up and in the process draw the required amount of current via the Rx resistor.

The above current develops some voltage across Rx which triggers the left BC547 transistor and switches OFF the right BC547 transistor.

The above action resets the IC 555 so that it starts pulsing the relay and the connected lamps.

The LED at its N/O contacts also starts blinking in accordance indicating the correct presence of the lamps at the front and the rear side of the car.

Rx value must be selected such that it develops just sufficient voltage across it to trigger the transistor in response to current consumed by two lamps at the specified rates.

In case even one of the lamps malfunction, the voltage across Rx then wouldn't be high enough to trigger the transistor.

This situation would keep the entire circuit frozen, also keeping the warning LED shut off, providing the necessary malfunction indications to the driver.



Car Turn Signal Flasher Circuit with Lamp Malfunction Indicator

13 thoughts on “Car Turn Signal Flasher Circuit with Lamp Malfunction Indicator

  1. Have questions? Please feel free to post them through comments! Comments will be moderated and solved ASAP.
  2. Hi Swagatam

    Thanks for your help. However, due to malfunction of just one bulb the entire circuit should not be frozen otherwise the driver can be deceived that the flasher is out of order.

  3. Hi Swagatam

    In a 555 astable circuit, if a same valued resistor is connected in parallel with R2 the frequency is doubled.

    In the attached circuit, I want to connect RTL NOT gate across A and B such that:

    a) when there is pulse at the base, A and B remains open hence value of R2 = 5M

    b) when there is no pulse at the base, A and B gets short hence value of R2 becomes 2.5M

    Could you please help me how to manage the connections.

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