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# Fading Leds

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#### Cyclone

##### New Member
One of the outputs i'm using on the decade counter for is to fade two leds in and out. so if LED1 is fading on LED2 will be fading off.

I was thinking on usin the "Two Transistor Leds Flasher" circuit but i don't think its going to work. I was hoping to use a simple circuit like that one.

Any one have any ideas? thanks.

On a side note. I have been posting quite a few questions here recently. If anyone is getting anoyed please let me know.

Hmmm... well if you can find a chip to do it, you could consider using stepped resistors, so if you calculated out how powerful a resistor would have to be in order to fade the LED, you could use a counter that would bounce up and down, so say you've got 10 resistors from 1Kohms to 10K ohms, you could set up the counter so it would go...

1234567890987654321234567890987654321 and so on, and each value would then tell the chip which output to use, something like a binary counter could be modified to work.

Sure with resistors only up to 10K, it wouldn't fade too much, but you could alter it, like a 1K, 10K, 25K, 50K, 100K, 250K, 500K, 1M, 1.5M, 2M (almost unlit)

Or if you wanted to go Ultra low tech, just get a 1K-1M ohm pot resistor, and a servo, hook the servo up to the pot dial, and just have it rotate back and forth.

Heh, pretty cheap huh...

How about a varistor?

https://www.sweetwater.com/insync/word.php?find=MOV

use a DAC to convert your digital voltage levels to analog levels and feed that to the varistor which in turn will raise and lower the resistance rapidly. You control the speed of the diming LED by controling the clock speed of your digital circuit.

HOw about using a DAC (digital to analog converter) ? Where you set the analog voltage and drive the LED off it.

Ivancho

the problem with the dac is that the LED requires atleast a certain voltage to turn on. Usually arround 1.7v and the current ins what control the brightness

I believe that if you use a DAC and perhaps a OPAMP you will have what you need.

Also if you use a digital pot you will be able to get the same results.

Ivancho

I could tell you several ways to fade two LEDs in complementary fashion. I don't understand how the decade counter is involved.
What control signal do you have for your fader? What supply voltage(s)?

I learned by trying that fading leds is best done with PWM

Now that is a great idea......... PWM is probably best to do that.

hmm lol whats a DAC and whats a PWM?

Ron H the Decade counter is just used to crontrol which leds will light up. I am modding my computer and i'm using the Decade counter to control which leds light up inside my case. one of the outputs for the decade counter i'm using for the fading leds that i'm asking help for.

I did a search and I found this schematic(Fading Red Eyes)

**broken link removed**

I was hoping to be able to just use the parts I have like a LM308 Op-amp, 3906,3904 transitors, 5v Relay, capacitors and resistors.

is there any way to replace the LM324 Op-amps with the LM308?

The Lm324 is designed to work with a single supply voltage. A LM308 requires conventional opamp supply (+15V/-15V). So you'll need to check if these voltages are available.

Wow I didn't think I could do it but i did. I used the LM308 op-amps instead of the LM324 and it works just fine! The don't seem to need a dual supply to run? hmm

Depends on the project

This is quite similar to something I've been thinking about. Plenty of fading LED circuits, and suggestions pointing to them, but they seem to be for a single fader.
What is needed is a circuit that fades two LEDs, one fades in as the other is fading out. For me, I don't want either LED to go completely off. I want to simulate a candle flame with different color LEDS.

https://www.redcircuits.com//Page58.htm

Just found the link, haven't built it yet, but sounds like just the circuit we are looking for... Cross-fading LEDs.

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