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I'm making a circuit which will change brightness level of an LED in tune with music. So far, this is producing some results, but I'm not very satisfied with it:

circuit If it matters, the forward voltage of the LED is around 2.5 V.

The light output of the LED is good enough for me, but the circuit isn't sensitive enough for my taste. The AC source is the speaker port of my radio and the 10 mF capacitor is used to provide AC coupling. I started with a 100 nF ceramic capacitor (the idea came after reading the audio coupling capacitors question), but it gave almost no response. I tried with various other capacitors I have available and when I got into the microfarad range, it got somewhat better. Now I'm at 10 mF and it seems like 20 mF would be just right, but I'm supposed to make several of this devices and the 10 mF capacitors are just too big and expensive (especially since I'd need 2 per device) for this to work.

Right now I'm considering moving to BC546C which should give me higher \$ \beta\$ (420 minimum (up to 800) compared to 300 which I have in my transistor right now), but I'm afraid that it will make too big impact at the current of the LED and change its brightness level which is now just right.

So how do I make this thing more sensitive other than by throwing bigger capacitors at it?

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  • \$\begingroup\$ How did this work out? What circuit did you finally use? Did you conclude that I was right ? :-) \$\endgroup\$ – Russell McMahon Feb 18 '12 at 5:01
  • \$\begingroup\$ @Russell McMahon I still don't have the final version, mostly because of some problems with the other party, the thing which made the most impact here was actually changing R2 and the fact I didn't have a potentiometer handy at the time. \$\endgroup\$ – AndrejaKo Mar 1 '12 at 10:01
  • \$\begingroup\$ By "10 mF" and so forth I assume you mean micro (10 uF in the more common ASCII notation lacking Greeks). \$\endgroup\$ – Fizz Sep 11 '15 at 11:45
  • \$\begingroup\$ @Respawned Fluff You assume wrong. I used appropriate SI prefix to indicate exact value of what I used at the time, which is one of the big points in the question, since 10 mF capacitors are indeed bulky and not very convenient to use. \$\endgroup\$ – AndrejaKo Sep 11 '15 at 12:23
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    \$\begingroup\$ m, mili. u, micro. n, nano. p, pico. m = 1000u. \$\endgroup\$ – Jason Han Jul 29 '17 at 0:26
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You could try a lower value for R3, for instance 470 or 220 ohm. (edit) I mentioned R2, but I meant R3!

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  • \$\begingroup\$ I'll try that and report back. \$\endgroup\$ – AndrejaKo Jan 26 '12 at 19:01
  • \$\begingroup\$ Actually it was the change of R2 that in my case made the biggest difference. \$\endgroup\$ – AndrejaKo Mar 1 '12 at 9:55
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Replace R3 with lower value - say about 100R or remove R3 (emitter to ground)
OR 10 uF to 100 uF across R3.

Get a variable resistor / potentiometer.
Connect it from base to ground.

Use a value of R1 = pot max value or more.

Adjust pot.
Observe.
Report

What this does is move bias point to an fro around transistor turn on point.
Lowering or removing R3 increases gain.
Adding capacitor across it does same thing.

Pot to find best operating point plus lower or bypassed R3 is main "gain".

Driving ... (soon)

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You are not going to get a lot of (current) gain with that setup.

Since you (probably) don't need linear gain the dodgy biasing should be okay. To increase the gain a bit I'd try changing R3 to ~200 ohms and R1/R2 to a total of ~200k.

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