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Retriggerable 555 timer/Marker light interrupter

May 2023

My car has a running (marker) light in the same light group as the front indicator. I have noticed that it can be hard for other drivers to tell if the indicator is on at night, as the orange of the indicator is overpowered by the adjacent white light, making it difficult to see. Some modern cars turn off the running or tail light on one side while the indicator is active, similar to the animation below:

I decided to replicate this on my car, by modifying an astable 555 circuit to become a monostable circuit with an active low output. The trigger (input) would tap off of the indicator signal, and the output from the circuit would then drive a relay circuit to switch off the (white) marker light. Below is the circuit I used to achieve this. Note that this excludes the relay and circuitry necessary to make the 555 output drive it. Also the output is active low (i.e. goes to GND when active, otherwise is at Vcc). Two copies of this circuit are made (one for left, one for right).

Vcc is the positive supply. For this application it is 12v, but this same circuit could be used in any application with a voltage between 4.5v and 16v, as per the 555 timer's datasheet.

Timing of this circuit is based on the relationship between R1 and C2. Output pulse width (time at GND) in seconds is $0.693\; \times \; R1\; \times \; C2$. So, for this circuit, the output pulse width is given by $0.693\; \times \; 18k\Omega \; \times \; 100\mu F\; =\; 0.693\; \times \; (18\times 103)\; \times \; (100\times 10-6)\; =\; 1.2474\; sec$. If R1 remains fixed at 18kΩ, C2 can be adjusted to fine tune timing, with 1.2474sec per 100µF of capacitance in C2. The value of R2 isn't of much importance, as long as it is equal to or larger than R1. D1 prevents the circuit from oscillating when the input is pulled to ground (inactive). The 1N4004 was chosen as it is very common, but any small signal diode should suffice. R3 provides a weak pull-down in case the device driving the input goes to a high-impedance/disconnected state.

Two diagrams below demonstrate the behaviour of the output. The horizontal axis is time, while the vertical axis is from GND to Vcc. The blue trace shows the voltage at the input to the circuit (i.e. the indicator signal), and the red trace shows the voltage at the output (driving the relay circuit). Note that the inputs and outputs won't actually be this square, this is simply an ideal scenario. As a retrigger resets the time, $T1\; =\; T2$ in these diagrams. Remember that the input is active high (Vcc), while the output is active low (GND).

For automotive use, I coated the circuit with a few layers of a conformal coating laquer to protect the circuit from condensation. I then mounted it in a project box with some non-conductive padding before attaching the box to my car on a frame section behind the front bumper and splicing the connections in to the light wiring loom.

GND goes to the chassis (or battery negative), Vcc goes to the power supply for the running light. INPUT goes to the indicator supply without breaking the connection to the indicator. OUTPUT goes to a relay driver circuit, which has the relay C terminal connected to the marker light supply from the car, while the NC terminal provides power to the marker light. This assumes that the car has a negative chassis (almost all cars do) and that the indicator and marker lights have their 12v supply switched to turn on/off (most cars do).

Below is a demonstration of the circuit in action on a car that has the front indicator and running light (marker) grouped together: