Need a simple courtesy light circuit

Printed From: the12volt.com
Forum Name: Car Security and Convenience
Forum Discription: Car Alarms, Keyless Entries, Remote Starters, Immobilizer Bypasses, Sensors, Door Locks, Window Modules, Heated Mirrors, Heated Seats, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=25401
Printed Date: July 12, 2025 at 6:29 AM

Topic: Need a simple courtesy light circuit

Posted By: yahd2000
Subject: Need a simple courtesy light circuit
Date Posted: January 28, 2004 at 12:22 AM

Hello, I've searched but can't find a diagram to a simple courtesy light circuit, the only ones I find involve an IC chip and multiple components... Is there a way to just add a small capacitor in parallel with the domelight and once the door is closed the light will act sort of resistor and drain the charge off..??

I'm sure I saw that somewhere but I can't find it now...

Thanks.

Replies:

Posted By: sroth140
Date Posted: January 28, 2004 at 1:59 AM

the IC chip will control the timming, other than that, you can use a timer relay to shut off the signal for the dome light circuit.

Posted By: xetmes
Date Posted: January 28, 2004 at 7:22 AM

the problem is the low resistance of the dome light...

lets assume it is 30 ohms, lets say the light will still be lit until about 6 V (unlikely though).

If we place a capacitor across the dome light we will have an exponential decay when power is removed....

So the equation of the dome light voltage becomes: V(t) = 12e^(-t / tau), where t is time and tau is the time constant (R*C)

if we assume resistance is 30 ohms.... what capacitance would we need for say 30 seconds of light...?

so after 30seconds we need 6 V or higher...

6 = 12e^(-30 / (R * C)).... R = 30 so...

6 = 12e^(-1/C)

.5 = e^(-1/C)

ln(.5) = -1/C

therfore...

C = -1/ln(.5)

C = 1.443 F

thats why people use active components, would you really want to buy a 1.4 F cap just for that?

Posted By: yahd2000
Date Posted: January 28, 2004 at 8:17 PM

Thanks sroth140 and xetmes... I would only need to output for 5 secs after all the doors are closed, but after looking over everything, I got the idea to integrate the front map lights into the domelight circuit with a switch, so then it would be better to use a controller...

but now that its involving more, I think I may just scrap the idea and just use the output from the alarm with a relay...

Thanks again...