• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Cool Counterpoint LED Fadeout

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
So I'm looking for the cool tricolor led fade circuit that was on my SA 5.1 preamp. I have seen the circuit on the schematic but cant find it anymore. This used the combo green/red LED and faded from red thru orange to green for the warmup period. Or a circuit that would perform just like it.
Joined 2011
Something like this? Use an input RC filter for a gradual transition, by adding a 100uF/16V capacitor
from the 2N2222A base to ground. Make R2 around 5k, and R1 around 1k.


  • LED-switching.png
    14 KB · Views: 289
Last edited:
Joined 2011
By the look of it, no. The red will be on all the time, and the green will fade
in to make an ugly orange/brown colour :)

This circuit was in a commercial product for years, and worked flawlessly.
I used a red/amber LED, which goes well with visible tubes.

When the transistor is off (0V in), the left LED has no path current and is off.
The right LED is then driven by current from the supply through the two diodes.

When the transistor is on (5V in), the transistor is saturated and its CE voltage is small.
The voltage at the top of the string is now about 2V since the left LED is on, but the string
needs over 3V to conduct, so the string of 2 diodes and LED is off. That's the reason
for the two diodes.
Last edited:
Joined 2011
I see now. Thanks for the explanation, rayma. Also, what is the value of R1?

Just saw this. Choose R1 so the transistor is fully saturated under worst case conditions,
like minimum beta and lowest supply voltage. Then it will always have a low saturation
voltage drop.

The 2 regular diodes have a larger drop than Vcesat, so the left LED is on when the transistor is on,
since the 2 diodes don't conduct (not enough voltage available). and so the right LED is off.

When the transistor is off, the 2 regular diodes conduct and the right LED is on.
Last edited:
You've got a golden tongue :)


Like in Apple's PCs? With digital control I'd guess, modulating the switching pulse width at a fractional Hz rate.

I've used this circuit in the past, works well for the usual high-brightness blue and green LEDs that you buy by the sack from eBay :)

LED throbber

or the simple 555 timer version-

Simple 555 LED throbber

You would just need to figure out how to bypass it to full brightness at full power mode, shouldn't be too difficult :)
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.