Another sound-to-light circuit

[merlinb]

I found this in a magazine from the 1980s, and thought it might interest folks. It's shown with a 12V supply, but I suspect it would still work equally well at 9V.

[Bill Mountain]

Awesome this looks great!

[garcho]

Great, thanks Merlin. Could help people deal with PT2399 input clipping? Or preamp output, say on the buffer at the end of your chain?

[StephenGiles]

From Elektor I'd wager!

[PRR]

Cute.

> suspect it would still work equally well at 9V.

It collapses. The middle LED hardly comes-on at all.

The input levels are a function including both 12V and Vbe. Change to 9V, you need to change the R1-R4 string.

First-crack: change 4.7K to 2.2K and you get reasonable thresholds on 9V supply.



Cons: Complex and easy to wire wrong. LOW input impedance. Does have sharp current-drain spikes "in the cracks".
_________________________

While I am often "down" on modern tricks, I think the LM339 is a heap easier to use.

Something *like* this:





This was proposed for a dual-color LED where Red+Green appears Yellow. I probably cobbled it from an actual 3-LED plan which I've misplaced.

The "-15V" rail may as well be the ground end of a +9V supply. The LM339 is quite happy at 9V. The LED resistors could be under 1K at 9V, though that might be quite bright. The several little "batteries" are of course taps on a resistor-string, which may be trimmable. The LM339 inputs are very high impedance, though not infinite; keep input resistors somewhat below 1Meg.