Sound to Light Circuit  - Page 1
Forum section: electronics / general

author: Mint.
author_rank: Frequent Contributor
timestamp: July 02, 2012, 10:57:36 am
content: Today I have begun a new project that I aim to finish by the end of next week, which also happens to be the end of holidays for us, students. The circuit that I want to create is a sound to light circuit, based on a previous project that I did, except a bit more complex. Attached is the circuit which I used (sorry for the crude drawing, if anyone has any good mac based schematic creators please post a comment). It is really basic and just flashes the LED's to the music, it also changes the LED's brightness and sensitivity according to the sound coming from a 3.5mm jack which goes into an audio device. It worked on my iPad and computer, but it didn't work from my phone for some reason... The NPN transistor that I used was recommended by one of the guys at JayCar which is the MJE340. Now I was thinking about changing the circuit a bit and then using it on my disco ball so I can have a small party atmosphere. Interestingly enough in the circuit that I first used and when I try it on the breadboard, it would work even if I have the ground from the battery connected to the base of the transistor which just puzzles me completely and I now know that I don't know anything about this circuit. I have to use an splitter audio jack in order to be able to hear the music and see the LED light up and this also causes distortion plus sometimes the sound only plays from one speaker. I would like to make the following changes to the circuit:- Make the circuit less dependent on the volume of the device e.g. make it more versatile.- Minimise the distortion- To be able to use both speakers (not just one!) when using the circuit.- Have some filtering going on for each LED, e.g. have bass on one LED, treble on the second LED and so on. This means that I will have about 3 LED's (bass, mid, treble).- To be able to switch between keeping the LED's just on and having the LED's light up to the music.Any suggestions would be much appreciated!

author: Rerouter
author_rank: Super Contributor
timestamp: July 02, 2012, 12:26:36 pm
content: not really able to contribute much on the circuit this late into the night, but some thoughts, why not use similar to an audio amplifier design, where it biases the transistor half on and swings above and below with a capacitor to remove dc offset, this way volume, and would make it very easy to implement your filters, (hi pass/ low pass / band pass)and if center biased, your switch would just be a mosfet or transistor open circuiting your input, and also lower distortion (how exactly does a flashing light distort?)

author: Mint.
author_rank: Frequent Contributor
timestamp: July 02, 2012, 12:35:50 pm
content: Quote from: Rerouter on July 02, 2012, 12:26:36 pmInteresting, I am going to go experiment a bit more with the circuit tomorrow and check out the results with on scope. By distortion I mean the sound output into the speaker amp gets really distorted, the bass sounds all wobbly and horrible. However the higher frequencies seem to be just fine.

author: Gall
author_rank: Frequent Contributor
timestamp: July 02, 2012, 04:43:41 pm
content: There was a very similar circuit in one of 70's Russian books.Like that:http://radiomaster.com.ua/uploads/posts/2009-09/1253010288_r5-23.gif(using these old ugly Ge high-power transistors)Or the same using SCRs (needs high power input):http://radiomaster.com.ua/uploads/posts/2009-09/1253095711_r5-27.gifLEDs:http://shemabook.ru/samodel/103.gifSCRs and LC-filters:http://unradio.ru/wp-content/uploads/2011/08/pristavka-na-trinistorax-1.jpgA really old one using LC filters:http://prispriem.narod.ru/18.files/image001.gifOr even using a TTL IC:http://www.bestreferat.ru/images/paper/78/78/4997878.png(The IC is SN7401, open-collector legacy TTL)

author: T4P
author_rank: Super Contributor
timestamp: July 02, 2012, 06:18:20 pm
content: It won't work just like that, you need to bias it for mid point operation.Using it like that means open loop bias, meaning it goes to max gain and slams against the voltage rails when the input goes too high, and that is common with low frequenciesIn any case you should buffer the input with an unity-gain configuration opamp and take the amp input from there, taking the input before the opampor you can take the output directly before the transistorWhat you just did by swapping the base and emitter connectors is making it a common-base amplifierif you biased it common-emitter this wouldn't happenRemember, bias it mid point and draw out a DC loadline and figure out the mid point of the transistorLearn your transistors first

author: Mint.
author_rank: Frequent Contributor
timestamp: July 02, 2012, 10:08:16 pm
content: Quote from: Gall on July 02, 2012, 04:43:41 pmThanks Gall,I may use one of those circuits and I was thinking about using a similar kind of circuit like that with 3 transistors and some RC filters for this. Here is the site that I got the original circuit and idea from:http://www.kripkrap.ru/home/svetyashhijsya-korob/Quote from: DaveXRQ on July 02, 2012, 06:18:20 pmI'm trying to by building this circuit. I've already bought a few books on electronics and I re read them multiple times, yet I still don't understand transistors 100%.