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Registered Member #3781
Joined: Sat Mar 26 2011, 02:25AM
Location:
Posts: 701
I was wondering if it would be possible to fade 32 Led's using one or two 555 timers? The circuits I have come across are not able to fade more than five
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
I'm guessing that you're doing something like putting the LEDs on the timing capacitor in a 555 circuit which has a sawtooth-like waveform, and the LEDs are drawing too much current past 5 LEDs or so and the capacitor isn't charging. Decreasing the charging resistor value should allow some increase in the # of LEDs, but a more extendable method would be to just buffer that voltage with an opamp voltage follower or another type of buffer circuit so that the LEDs don't affect the 555.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
i presume hes not power the leds throught 555? he should be using a pass transistor for power, the 555 for signal, thus the number of LEDs wont matter, so long as you keep up with the power transistors.
Registered Member #3781
Joined: Sat Mar 26 2011, 02:25AM
Location:
Posts: 701
Ah yes, I was using a transistor but I couldn't get the LEDs to fade. They either constantly stay on or won't turn on at all. I think it's just a silly error on my part, I was going to troubleshoot it tomorrow.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
My answer to the original question is, Yes. The fading I assume is achieved by pwm using the 555 in astable mode. e.g. LM555 @ 15Vdc supply, load (leds+resistors) between +Vcc and pin3 Datasheet Values ..ON output = 12.5V @ 200 mA = 2.5W e.g. red led @ 20mA = 1v8 approx. ... 4 series led = 7v2 ... resistor=(12v5-7v2)/20mA = 5.3/0.02 = 265 R .. use 270 R 8 parallel sets of (four series led with resistor) = 8x 20mA = 160 mA = well within spec. and 4 x 8 = 32 leds. (plus 8 resistors)
on the other hand, 32x 3W led would need a power transistor between load and 555 pin3 if using a power transistor, other drive options are available e.g. switched constant-current (or voltage) from the higher un-regulated dc supply. ...etc.
Registered Member #3781
Joined: Sat Mar 26 2011, 02:25AM
Location:
Posts: 701
Yeah I'm trying to go the transistor route because I want to use a 5v usb port as my source voltage. The LEDs are rated for 2v at 10ma so I have 16 groups of 2 LEDs (The LEDs in each group are in series). With each group in series with a 33ohm resistor that is connected to the emitter pin of the transistor. Pin 3 from the 555 timer is wired to the base pin of the transistor through a 33k resistor and of course the collector pin of the transistor is connected to my source voltage. But for some reason my transistor is only oscillating between 1 and 2.5v. There's a 2.5v drop through the transistor and I don't know why.
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Fading 32 Led's with 555 timer's
