2CV Daytime Running Light design (DRL)

I decided to develop a circuit that automatically could turn the Daytime Running Light (DRL) on when you started the engine.

The idea is that when the generator charges the battery and reaches 12.6v it turns on the DRL. It keeps the light on down to 12.0v as the voltage will drop when the battery is loaded (low beam 12.5v). With ignition ‘off the voltage will drop below 12.5v and the light will switch off.

As I need +5v for the electronic to a later gas control system I included a +5v 1amp. Also include is a 12v 10 amp with 0 to 5 v control input for an electric fuel pump.

The circuit diagram is shown in the pic. The FET’s are overkill as they can take 50 amp. The control box the electronic was build into is also shown.

The ‘light switch’ on a 2CV is a bit tricky. This means there must be separated output for ‘position light’ and ‘low beam’ and input for ‘high beam’ to prevent low and high beam are not run at the same time. The circuit should not interfere with the light switch in any positions.

Is the automatic DRL design a Good or Bad idea?

The automatic Daylight Running Light electrical design was working ok, but was it a good or bad idea for a 2CV? Both Yes and No.

The 2CV position light takes about 1.8amp but give not sufficient light as DRL.

The requirement for DRL in DK is two lamps giving approx. 460mL each. A bulb like BA15s at 21W can do this.

The 2CV needs to run the low beam together with the position light. That’s what I designed for. Both takes together about 12amp. The 2CV generator and battery system is not designed to always run this. Battery charging at idle is low (<4amp). The position bulb is a BA9s socket on a bulb 12v 3W dia. 10 mm mounted in the headlight. I decide to find an LED that could give >460Lm and replace this bulb.

Her was the two candidates

BA9s 12W 0.7W 600Lm (Not performing well. All four I got only took 10mA eq. 120mW)


BA9s LED 12W 2W 700Lm https://www.aliexpress.com/item/1005002158859116.html?spm=a2g0o.order_list.0.0.de221802L6QWFJ

The 12v 2W 700Lm was very good and give very strong light. Check the photo.

With this solution the low beam is not necessary, and the position light current gets down from 1.8amp to 1.5amp.

The simple solution for an automatic DRL is to have a relay (or a P channel FET switch) that tuns the position light ON when the ignition key is turned. The circuit I designed was a little overkill. I might take some of the electronic out of the small box 😊