March 2018
I’m trying to emulate the chasing lights that used to be popular under the awnings at the cinema. I have a chasing LED circuit that works well, but I want to turn the LEDs off slowly to leave the comet trail effect. I’ve tried putting a 2,200µF cap in parallel with each LED with limited success.
There must be a simple way to have the LEDs turn on quickly and then fade out.
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To provide a good answer your question, more information is needed. With the configuration of a LED and limiting resistor in series, with a capacitor in parallel, a time constant can be calculated. with a 2200μF capacitor and a 100Ω resistor, the time constant would be 0.22 sec. This would light the LED for about 1 second. I don’t know what your problem could be.
LEDs are pretty much digital (binary) devices, meaning they turn on and they turn off. They do not fade out when the voltage gets too low - they turn off.
The only way to make them fade out is to use an oscillating circuit to change/increase the time the LED is off vs the time it is on.
A joule thief voltage increasing circuit can be used to do this but so can a digital circuit.
The joule thief circuit could be made to do this if you use another circuit to control the voltage going to the joule thief circuit. Start with 1.0V for full brightness then decrease the voltage down slowly to ~ 0.20 V to dim & then turn it off.
The turn-off voltage will be higher (than 0.20V) if you use a simple joule thief where the number of windings/loops is the same for both sections. The efficient joule thief where the # of windings/loops is on a ratio of 2:5 and the 2-loop section goes to the base of the NPN transistor should have a lower shut-off voltage. I’ve had this circuit, with loops of 8:20 (8 to 20, 8 going to the base of the NPN transistor) still giving light down to 0.25 volts, so I don’t really know when it will turn off but you can simply turn off the voltage to the circuit when it gets to ~ 0.2V and that should look pretty good.
See my web site article on the Joule Thief Information Page at: http://cs.yrex.com/ke3fl/htm/JouleThief/JouleThief.htm - for all the experiments I did to find the best (efficient) joule thief circuit.
