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Basic circuitry help? [not yet resolved] Answered

(I wasn't sure if this should be a comment on the Instructable or not, as it is a bit long, and I have some pictures..)

I am following a sound-reactive LE 1.D tutorial and having a bit of trouble. The circuit consists of a SPDT switch, resistor, LED, transistor, and a 3.5mm stereo jack. When I play music using this stereo jack, I get no response from the LED.

I'm trying to make something on a breadboard for the first time, so I'm thinking maybe I'm missing something because of that. Here's how the machine is supposed to work:

- Positive charge flows to SPDT switch, and if it's in the correct state, continues to the 150 ohm resistor
- Anode of LED is connected to the resistor, and the cathode is connected to the "emitter" lead of a TIP31 transistor ("base" is wired to the power bus)
- The "collector" lead of the transistor is attached to one of the outputs on the stereo jack
- The ground wire on the stereo jack returns to the common lead of the SPDT switch.

Perhaps I wired something wrong? I've included pictures if the description is not sufficient.

EDIT: I've modified the circuit a bit - maybe I can take pictures / draw a diagram tomorrow. Now, here's how it works:

- Positive charge flows from positive side of the power bus to the "base" lead of the transistor
- Collector lead of the transistor is connected to the left-channel lead of the stereo jack (also, the ground pin of the stereo jack is connected to the negative side of the power bus)
- Emitter lead of the transistor is connected to a 150-ohm resistor
- Resistor is connected in series with two LEDs
- The cathode lead of the second LED returns to the negative lead of the power bus

The LEDs do turn on now, but that's the problem - they are always at full brightness no matter the volume of the music. Even if no music is playing, the LEDs are fully lit. I've used a multimeter to check what's happening, and after pressing the negative lead of the multimeter to the cathode lead, and the positive lead to the anode lead, it shows a constant voltage of 1.986 V.

I don't understand how this isn't working - I've verified that the collector lead is correctly connected to the left-channel wire... maybe I should try the right channel? Or a different cable?

tl;dr: Rewired and the LEDs are always at full brightness. Transistor doesn't seem to be affecting the emitter output, for some reason..



8 years ago

Alright, so I tried this out. It didn't work for me... at first.
I sent the signal from the computer at full volume to an additional amplifier at almost full volume, and it flickered. It seems that a very sensitive transistor would be needed without the use of an amplifier.


Reply 8 years ago

Here it is with a 2n4401 as a transistor. Sorry about the blurry image and lack of sound... I had to use a webcam from the '90s.<br /> <object height="344" width="425"> <param name="movie" value="http://www.youtube.com/v/N7Gpk4p6BqA&hl=en&fs=1" /> <param name="allowFullScreen" value="true" /> <param name="allowscriptaccess" value="always" /><embed allowfullscreen="true" allowscriptaccess="always" height="344" src="http://www.youtube.com/v/N7Gpk4p6BqA&hl=en&fs=1" type="application/x-shockwave-flash" width="425" /></object><br />


8 years ago

How did you test which side was the anode and cathode on the LED?Remember that a diode will only allow the power to flow one direction,so if you flipped the LED the wrong direction, even on accident, it willstop the cuicuit from functioning at all.

Personally I like drawing the cuircuit on a piece of paper beforeattempting the breadboard

Alos, it's hard to tell by looking at the pictures, but it looks likeone of your jumpers next the the regulator isn't inline with anything.


8 years ago

Looking at the original ible, the circuit calls for 4 leds.  Thereis another circuit in the comments.  Just reading through it seemsthat this circuit is sensitive to the level of the audio input going inso that splitter jack you use may affect the audio source goingin.  I don't know if this is driven off of the headphone output orline out from the computer.  Also, do you have the correctpower/voltages  going in at the right places?  I only know thebasics so maybe others can really help.  Good luck.


8 years ago

Without being there to play around with the circuit and prod and pokethe wires it's difficult to troubleshoot your circuit from the pictures.That and the ible you're following isn't the clearest.<br /><br />Here are some things you could do to debug your circuit to find out whatis wrong.<br />* Check your npn transistor works. Your multimeter should have a slotfor npn and pnp transistors. You can plug it into there to check itworks. Also check you're sure that the collector, base and emitter arethe right way around. From the pictures it looks as if they are.<br />* Check that your LED is working. Run 5v across the resistor and LED tocheck that it isn't a broken LED.<br />* Eliminate the switch. Single Pole Double Throw switches can beconfusing, if you get rid of that just for now to debug the circuit youmight find you can get it working. When you have the LED flashing to themusic without it, check in what configuration it's on or off by usingthe ohmmeter on your multimeter.<br /><br />Check all that then beep me on IRC if you're still having problems :)<br />