# EL Dimmer Circuit Questions

Hey Michael,

I had a couple questions about your EL dimmer circuit where you released the Eagle project files (V1.0).

It looks like your microphone is biased to 5V through R11, which is also the upper rail for your opamps.  Aren’t you clipping your usable mic signal in half by doing this since the output of the microphone wants to swing in both directions around the bias point?

From there, its AC coupled to a noninverting, adjustable gain stage, and then goes through a 1o LPF, and then a voltage follower that chops off the negative-going part of the waveform with D1.  I’m actually not sure how this works since your opamps are operating from 0-5V.  Am I missing something?  The signal then goes through another 1o LPF with a leaky capacitor array to produce the low frequency signal envelope.  Why does the leakyness help?

I’m pretty lost on how Q2 (npn) and Q3 (pnp) function.  I can see that the U6G4 opamp will drive its output until the voltage at R10-R8 divider node equals the voltage of the low frequency envelope, but I’m not really sure whats going on with Q2 and Q3.  Any hints?

Thanks,
Alex

D’oh, just saw your explanation of the feedback block here: http://ch00ftech.com/2012/12/14/sound-reactive-el-panel-dimmer-pretty-much-done/

My others questions still stand, though.

Thanks,
Alex

Hi Alex,

Electret microphones have internal amplifiers that are actually powered by the biasing signal. The datasheets states that you just need to provide a bias of somewhere between 3 and 9V (not 100% sure on these numbers). I’m not sure where the signal coming straight out of the microphone is biased, but it doesn’t really matter because R4 re-biases the AC component around 0. I don’t have a high enough impedance oscilloscope to measure this signal because it’s so weak, but again, it doesn’t matter.

D1 isn’t there to chop off the negative portion of the waveform. That task is already achieved with U6G2. Take a look at the power gate of the opamp. It has no negative supply, so it cannot output anything lower than 0V.

D1 is there to prevent the opamp from sucking any current out of the capacitor array. As a high magnitude audio pulse comes in, current will quickly fill the capacitor array through R19, but it will leave more slowly through R5. This means that the device will more or less directly follow any rising audio waveform, but take more time to follow decreasing signal. The effect is an output waveform that rises quickly to any loud noise (base drum), but dies down slowly. This provides the pulsing effect you can see in the video. Typically with this design, I set R19 to 0 ohms (see kanye glasses for details), but I found that slightly slowing down the rising waveform produced a more pleasing pulsing effect at the cost of an imperceptible time lag.

Q3 is a current control for the resonant supply. The more current you suck out of its base, the more current the resonant supply will receive, and the brighter your panel will get.

Q2 is just an inverter. When the envelope waveform is at a higher voltage, more current will flow through the base of Q2 which will increase the current pulled down through R14 and in turn the current through Q3’s base. Higher voltage out of U6G4 -> brighter output of EL.

This circuit controls current, but the output brightness is proportional to the voltage provided to the resonant driver. The feedback loop through R10 and R8 is provided as a way to make sure that Q3 supplies enough current to get the resonant driver’s voltage at the desired level. Ideally, the output of the opamp would drive the resonant driver directly (assuming it could supply enough current), but because the audio signal is operating at 0-5V and the resonant driver needs 0-9V, Q3 and Q2 are required to step up the voltage.

Hope that helps. Feel free to bug me with more questions if you get stuck!

~ch00f

Hi Michael,

Electret microphone: got it. The signal output of the microphone is biased at the biasing voltage, but, like you said, it doesn’t matter since you’re re-biasing it to GND with R4.

D1: got it. You don’t want the envelope to follow the audio waveform directly, and instead have it be controlled by the values of R19 and R5. Larger values of R5 result in a slower decay and larger values of R19 increase the ramp-up time for aesthetics. Without R5, it would just be pseudo-peak detector.

Q2 and Q3… So after hearing your explanation, I get the purpose of Q2 and Q3, but am having trouble fitting that into the context of U6G4’s feedback loop.
Lets say we power up the device and apply 1V to the (+) input of U6G4.
It will drive its output to try and make its (-) input 1V as well, which is at the junction of R10 and R8.
For this to happen, the collector of Q3 needs to be at 151V, but since we’re powered with a 9V bat, it can only be 9V at most, which in turn means that V_R10R8 can only be 0.060V at most.
Does this mean the output of U6G4 will hit the rail for inputs greater than 0.060V?

You know, your question is totally valid. A lot of the values on this schematic were empirically determined (i.e. guess and check), and I’m not entirely certain that the schematic even reflects what I ended up using. Judging by the response shown in the plots under the “feedback” section of the post, I’m guessing I ended up using something else. maybe something like 10k instead of 100?

Your math is right. I’ll take a look at it when I get home.

~ch00f

Just took another look at the circuit. The values are as I specified on the schematic.

As it turns out, the entire audio rail for this circuit is incredibly low voltage. Just took a measurement of the signal coming out of U6G3, and never gets above 100mV, so 60mV seems right on the money. Apparently the entire audio system runs at about that voltage. Yay analog electronics amirite?

This might help explain why this circuit is so succeptible to feedback.

If I designed it again, I’d accomodate a larger audio rail, but it appears to be working regardless.

~ch00f

Ahh, that makes sense. Dem got dern electrons are at it again! I definitely need more practice with analog. When I last needed a mic preamp, I just threw a MAX4062 at it so I could have something working on the first go. The hoodie videos looks pretty awesome, and I agree that it appears to be working well. Have you had the chance to test the circuit in a noisier, concert-like environment?

I brought it to a pretty loud club a few weeks ago. Definitely worked just as advertised, just had to turn the gain down some. Got some positive attention.

That is really cool. So many sound-reactive projects are possible.

## 2 thoughts on “EL Dimmer Circuit Questions”

• It’s an analog circuit… so I guess you could feed it with the DAC of an arduino if you really wanted to.