simple p-mosfet switch

Thread Starter

buster05

Joined Apr 23, 2020
3
I'm trying to add DRL to an old car. The idea is to have DRL on when ACC is on. When the head light is on, DRL would be off.

I've been learning about p-mosfet on this board and come up with the follow circuitry (modified from other designs in this forum):

Questions: why do I need R1? if so, is 1K enough? Is there something else I miss?

Appreciate all the input.

BTW, the DRL spec is 12V, 8W.

p_mosfet_drl_circuit.png
 

dl324

Joined Mar 30, 2015
12,798
Welcome to AAC!

It would be helpful if you described the complete functionality you're trying to implement (e.g. DRL off if headlights are on).

Are the lights you're planning to use legal in your locale?
 

iimagine

Joined Dec 20, 2010
483
That circuit will not work and you do not need any mosfet for this if your head lights are bulbs, not LED.

DeleteMe.PNG

The 470 ohm resistor can be omitted if your DRL LED comes with a built in resistor
 

Thread Starter

buster05

Joined Apr 23, 2020
3
That circuit will not work and you do not need any mosfet for this if your head lights are bulbs, not LED.

View attachment 205295

The 470 ohm resistor can be omitted if your DRL LED comes with a built in resistor
When both Head Light switch and ACC are on, would this circuit turns on both DRL and the headlight? I really can't see the logic that turn off DRL when head light is ON. Thanks.
Welcome to AAC!

It would be helpful if you described the complete functionality you're trying to implement (e.g. DRL off if headlights are on).

Are the lights you're planning to use legal in your locale?
Hi Dl324,
Ideally, DRL would be on when ACC is on. DRL would be off when I turn on headlight.
I'm thinking of getting one of these from Ebay, did not really think about legality aspect of it.
1587766687694.png
iimagine,
Can you point out why my original circuit would not work?

Looking at your circuit, is it true that when ACC is on, both DRL and head light will be on as well?

My knowledge of analog circuit is limited to an intro college course back in the mid 80s, so please excuse my ignorance.
 

iimagine

Joined Dec 20, 2010
483
I really can't see the logic that turn off DRL when head light is ON
When both, ACC and Headlight switch is closed, the DRL will not be on because both of its anode and cathode is now at Vcc. It just like when you connect both of the LED pins (anode and cathode) to the + terminal of the battery, it wont lights up.
Can you point out why my original circuit would not work?
I misread your circuit, it should work. Apology
 
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