# Can I make a simple LED FADE OUT circuit ?

Just want my Car's Room lights to fade out as slowly as in 6 seconds.
12V Power Supply available.I guess this is possible by having a suitable cap in parallel with the Bulb.What should the cap be?

3 years ago
Depends on the wattage of the bulb what size cap you need -- You are right a parallel capacitor will cause the led to turn on quickly (if the power supply can handle the spike in current as it charges the cap), and cause the led to fade out as the voltage level drops. Along those lines you may need a small series resistor to prevent the capacitor from drawing too much current when it's first hooked up (it looks like a high-current short circuit at first).

Energy in a capacitor:
E = 1/2 * V^2 * C

Where
E = Energy in capacitor in joules
V = Voltage in capacitor in volts
C = Capacitor capacitance in farads

and a Joule is a Watt-Second

It'll be a 12 volt system, in a car probably closer to 13 or 14 volts, lets plan for that. Say for example it's a 1 watt led (substitute your actual power draw if this is different).
The led will only light after a certain threshold voltage, so 10 watt seconds or so will probably light the led fully for a good 6 seconds...that means we need to store about 10 joules of electricity at 14 volts.

again, E = 1/2 * V^2 * C

so C = 2 * E / V^2
C = 2 * 10 / 14^2 = 20/196 = 0.10 Farads = 100mF = 100,000uF.
http://cgi.ebay.ca/25pcs-100uF-Electrolytic-Capacitor-16V-100-MF-100mf-/150518048140?pt=LH_DefaultDomain_0&hash=item230b93258c
would cut it - but you need to buy lots to make it make sense.

Lastly, do not use this type of circuit with any actively current limited led driver circuits. Dropping out their source voltage while they're trying to regulate will make them very angry as they try to boost the current despite the lacking power source.

Absolute best option would be to design a pwm circuit with a cheap tiny microcontroller that dims the led with constant power applied and a signal that tells it 'the light should be on/off'.
lemonie3 years ago
An LED -big cap', what can you get?

L