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Question about placing a cap on car lights

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New Member

I have a question and found this site and figured if anyone had an answer it would be found here.

I have a stock alarm on my car and when activated it sends a signal to the lights to flash, now I have located the wire that the signal is sent from and tried installing this

4700µF 35V 20% Axial-Lead Electrolytic Capacitor - RadioShack.com

in line to extend the period of time the lights is on hoping that the cap will become charged and then slowly fade out after about 10 seconds or so but wehn I installed it nothing changed, the lights flashed on as they normally do no prolonged period, am I missing somthing?

Thanks in advance.

Nigel Goodwin

Super Moderator
Most Helpful Member
Yes, you are missing how much current car lights take - you would need an impossibly huge capacitor to do it that way.

A 555 timer and a bit of electronics could do it though.


New Member
you are missing some more farads... :D

the capacitor cannot hold that amount of energy needed for the lamp to light even seconds more. You need a delay circuit, a 555 as nigel said connected as monostable multivibrator. something near to this for example. it would be a delay circuit.


Well-Known Member
Most Helpful Member
The cap will decay to 37% of its initial voltage in one time-constant, which is R x C. If the lamps took 1A, giving a resistance of 83Ω, and assuming their resistance doesn't change with voltage (although is will reduce some as the voltage drops), then for a 10 second time constant you would need a capacitance of 10/83 = .12 farad or 120,000µF (26 of the 4700µF capacitors you used in parallel).

And of course, your lamps probably take more than 1A so the required capacitance would go up accordingly.

So just connecting a capacitor to get your desired delay is theoretically possible, but rather impractical with available capacitors. You could conceivably use a super capacitor, since they come is farad sizes, but they are usually low voltage devices (typically about 2.5V), so you would have to put about six in series to get the 15V rating required for your car's maximum battery charging voltage.
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