Hi people, I wanted to know if anyone would help me with a small project. Im currently not very experienced in electronics but thinking of going into that area. My idea is that I want to put LED's under my doorhandles on my car but I want them to trigger when the unlock button is pressed on the remote. I'm thinking it would need some sort of relay that provides a 12v supply to the LED's when the alarm is triggerd to unlock by the remote. Also the circuit would need a timer that would turn the LED's off after 60secs or so. I've seen it on some latest BMW's and thought it would be a good project. Hope you can help, kind Regards Graham

 

hi Graham,
As the timing of the 60secs is not going to be critical I would suggest the following.

Connect a diode from the unlock mechanism supply input line to a large value capacitor. When the unlock is energised the capacitor will get charged upto near 12v.
Across the capacitor connect a resistor and LED in series, the LED will be lit while the capacitor discharges. The LED intensity will fall off as the cap discharges.
If thats a problem use a small transistor connected as a constant current source in the LED drive circuit.

Do you follow this OK?

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Thanks for the response Eric, I was testing some capacitors yesterday in small circuits with an LED and a 6v power supply but couldn't get the capacitor to charge. Maybe they weren't the best capacitors. Any ideas what sort of capacitor I would need? Also the unlock pulse only energises 12v for roughly 1.5secs, so would this be enough time to charge the capacitor to power the Led's for 10 - 60 secs? Thanks, Kind Regards Graham

 

Something like this will work. You'll have to experiment a little with the capacitor value (And OR the 470K) to get the timing right. As for the FET; one that can handle the total LED current is all that is needed. You can add more LEDs in series/parallel depending on LED voltage drop:

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hi,
The circuit 'kchriste' has posted would be a good choice, as he says, just tweak the RC to get the on time you want.

A N MOSFET, a 2N7000 would be OK

__________________
Eric
"Good enough is Perfect"

PIC tutorials:
Gramo's: www.digital-diy.net/
Bill's: www.blueroomelectronics.com/

 

Many thanks Guys, ill give it a go and post up the result.