Automotive lighting control

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Thread Starter


Joined Jun 12, 2013
Hello all. First post, need a little advice on a project.

I'm putting relays in a '71 Ford F-100 to take the current load off the switches. I'm also relaying the blinker's which requires a silly amount of relays or a little logic circuit and 5 relays. This is due to the fact that the blinkers and brakes share the same filament in the tail lights.

So I've got my logic circuit all planned out. I'm using 4000 series CMOS ICs. I'll then use a layer of MOSFETs to trigger the relays.
*See crude abstract illustration*

*I missed a couple nodes on the rails*

First question is what voltage to run the logic at. At first I wanted to use a 10v regulator. The battery should never drain near 10v. If it does, I've got bigger issues. Unfortunately, if I'm understanding this correctly, the standard MOSFETs aren't fully on/closed until Vgs is 10v. Since I'd be using the absolute minimum voltage to trigger the MOSFETs, I fear there'd be some unreliability. To avoid this I could use a 12v regulator *pictured* for the logic's Vdd, but I don't know how the VR will react to the battery draining near or below 12v. I could just forgo a VR all together and rely on a cap to smooth the voltage between when the alternator is and isn't spun up. The CMOS chips can handle up to 20v, but I don't know how changing their Vdd during operation effects their operation.

Second is the flyback diodes for the relays. What kind of attributes am I looking for when it comes to the diodes? What kind of currents and voltages should I expect from the back EMF?

Lastly, if I'm running the CMOS at a Vdd of 12v or 10v and the inputs are at the higher alternator/battery potential, will this cause issues? Or will the CMOS just take it and interpret it as a high input?

Any observations/critiques encouraged!

Thanks in advance!:)


Joined Mar 24, 2008
The owners of All About Circuits has elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

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