Switching 12V vehicle headlights with Arduino / Mosfet combination

Question

Ground level noob here, started to tinker with electronics at the beginning of this year after I couldn't get any electronics shop in my town replace my bike's gauge cluster SMD LEDs.

I learned few things last few weeks and now I'm at the mosfet stage. I want to control the headlight with mosfet and I managed to assemble a code for Arduino IDE that controls the LED the way I want, tested the 12V 25W LED bulb on my desk with bench power supply and Arduino hooked to buck converter, with the LED bulb controlled with a breadboard size pushbutton and a mosfet. I bought the wrong (high Rds, not logic level) nMOS at first, but learned to read some specs and got IRL2203 that operates the light and keeps cool with no heatsink.

The challenge is how to install it in the vehicle while keeping the original controls intact and without adding anything extra. I am going thru YouTube vids on schematics and trying to figure out where in the "headlight circuit" should I tap in with the Arduino and mosfet. One thing that kind of worries me now, I'm starting to think I planned this wrong going with the nMOS and all, I highlighted all the headlight-relay wires and it seems that pMOS would be the correct choice to tap into these wires, and accessing ground wires for nMOS would be too challenging?

As I'm not well versed in these, please let me know what can I do here.

One idea I had was completely remove headlights from the circuit, from the relay and everything, and run a separate circuit from headlight fuse? Just thinking out loud :D Let me know how would I go about solving this, cheers

Answer 1

I would say yes to your last thought.

Just keep those two separated from the whole circuit and hook your arduino and mosfet there. Keep the connections from the rest of the circuitry as little as possible.

For example you can take the output from this "LIGHNING SWITCH" and use it as an input to your arduino. When you press the switch, the arduino will sense it, the lights will turn on.

Something like this will do the trick:

^{simulate this circuit – Schematic created using CircuitLab}

EDIT

Regarding the N-mos, P-mos, this is a picture of my noted regarding those:

N-mosfets have lower internal resistance (Rin) (look in mouser.com for the mosfets, play with the N/P- chanel filters and check the Rin differences between the N channel and P-channel (Ignore the NPN, PNP etc options)

N-channel mosfets are easier to use when put on the low side (since you control Vgs, and you have a steady GND at source, you can play with the V at the gate. (This is why i have an "L" box on my notes, its where i put the "Load").

Also, enhancement type means it turns on when the Vgs is (lets say for logic mosfets) 5[V] or more. Depletion type means the fet turns on when Vgs is 0[V] or less.

Having said that, if you check on mouser, P-channels have easily 10 ohms of Rin (huge huge!) N-ch can easily have 0.009Ohms of internal R.

Also, if you check on Mouser the "Depletion type", you will notice that there are no P-channel mosfets with Depletion type. That is, becouse they are expensive to manufacture, they have higher Rin, no one wants them.

This leaves you with enhancement type P-channel mosfet. So, If you want to power your bulb with 12V, you put it on the low side, how will you turn it on? you need Vgs = 5V, you will need another supply of 12+5=17V, like this:

^{simulate this circuit}

That is why, it ususally is a bad idea to use a P-ch. Only when you really need a device (load) to be connected to the GND directly, you would put a P-ch there (like if you wanted to power a computer, motherboard, MCU etc)