Sound Modulated LED Project Help

The resistor does NOT have polarity, but the Capacitor DOES, and should be indicated on the capacitor itself, as small ++++++markings. can u take a picture, or scan what u have and send it to me to take a look at|?

No

Usually it's the negative lead that marked with a - .

Actually the audio simply 'modulates' the led's output at audio frequencies. There won't be only visible change in brightness. You can only tell if it's working by 'listening' to the receiver part. Also I'm not too impressed by the receiver end ! Not least they don't say what kind of photocell to use.

Graham

The resistor doesn't have a positive or negative side, that is, it isn't polarized. A capacitor of that size likely is, and it probably has a mark on one side or the other. There's usually a stripe on the negative side. Look for a plus or minus sign. Unless the audio source is so strong that it changes the average level of operation of the LED, you won't be able to see a change in brightness of the LED, because audio sources change too fast to be seen by the eye. Try connecting the rest of it up to see if it's working.

-- john

Thank you. So I won't be able to 'see' any change in the LED brightness..only 'listen' to it with the receiver..that explains it...thanks!

I used these photocells from radio shack,

.( * 5 x Cadmium Sulphide(CdS) photo cells - various styles and values) will these work?

Thank again...

No. They have too slow a response time which won't respond to an audio signal.

You'll need a silicon photodiode ( probably a bit more expensive ).

Graham

Probably not. CdS cells are super-slow, not fast enough to follow the audio. And besides, they don't generate voltage, which this arrangement requires. You need a silicon "solar cell", just a small one. You might connect, say, a 5k or 10k ohm resistor across it... that may help, depending on the amplifier input circuit.

To check the receiver, aim its detector at an incandescent lamp (hummmmm) or a fluorescent lamp (louder, raspier hum) or a TV or computer monitor, fun noises.

I did this a long time ago as a school science project. I used a flashlight bulb as the transmitter, driven from a radio speaker output in series with a flashlight battery. The receiver was a selenium solar cell, homemade transistor amp, and headphones. It worked pretty well, sending intelligible voice and music.

Hang in there.

John

** Where the article says "photo cell" substitute the term "solar cell".

The fools who wrote the article mixed the names up.

A CdS cell is virtually useless.

....... Phil

The only thing I found was a infrared pair, which the detector will probably work with a red LED, since you want to see the light, if not the infrared emitter will do. Model: 276-142 Radio Shack

greg

No this is going to be trouble also sine its not just a photodiode, but a phototransistor. Perhaps someone can send you something.

greg

I say build the receiver first, so that you can use it to test the transmitter while the transmitter is in development. The web page you quoted points out how to test the receiver with other sources.

Try the Radio Shack Phototransistor Model: 276-145 perhaps if you just want a receiver and want to make separate arrangements for the transmitter.

Now don't let the name phototransistor upset you, seeing that it only has two wires. Normally, the base current in a transistor is the "input" and the current flowing between the other two terminals, the emitter and the collector, is the effect. In a phototransistor, the light stimulates the device instead, and there is no base connection: you just wire up the other two terminals. Put the phototransistor in series with a resistor accross a battery (possibly the battery in the audio amp?). The resistor limits the current through the phototransistor, and gives something for the phototransistor to work against. Now route the signal fromt the phototransistor to the audio amp: connect one side of the phototransistor to the audio amp ground, and the other side of the phototransistor , via a capacitor, to the signal input of the audio amp. Try a resistor value of 20k to start with, and swap values until you can measure one or two volts across the phototransistor: now you know it is biased OK. Try the capacitor you've already bought, or maybe a 100nF capacitor because it is big enough to drive the audio amp input and will not have you guessing which way round to connect it (it will probably be non-polarised).

Wander around listening to light sources ;-)

Now setup the transmitter similarly, i.e. bias the light source so that it is "on" but not working flat out. The point is that something working flat out does not respond to a relatively small audio signal asking it to do a little more. I understand that setting the light source to dim gives you too little range, so you need to experiment to find the best level. (But you should have a receiver on hand to discover the first glimmer of success and help you to improve your transmitter!) To get round this problem, you should use a transistor to drive the light source.

T T | | | v LED | - | | | | | | / / R1 \\ \\ R3 / / \\ | C1 | |/ --||--+---| T1 audio | |\\ input | v | | | | / / \\ \\ / / R2 \\ \\ R4 | | - -

I've not shown the battery connections: + to the top, - to the bottom. The "other wire" for the audio input can go to the bottom. C1 can be

100nF, but try anything. R1 and R2 can be about the same, and about 10k: that lets enough current flow to hold the base at a reasonable voltage, but leaves enough flexibility for the audio source to be able to have some effect. R3 and R4 can be say 1k: use these to set the brightness of the LED. Too high a value and the LED will be too dim, to low a value and the audio signal will have less effect, but you should get a much better compromise than with your original circuit.

HTH

Jean

thank you for your suggestions... I will give some of this a try.

Thanks again!!!!

Would this be an adequate solar cell to use?

I wouldn't use the Model: 276-124. It feels all wrong: you want something delicate and sensitive. The Model: 276-145 is the kind of thing. Actually, if you get the Model: 276-142, then you get a part for the transmitter too: up to you. Probably not the Model: 276-142 because you've already bought your LED.

BTW, I should like to correct my circuit in post 11: add a capacitor in parallel to R4. This stops the emitter voltage changing, which would otherwise give less gain. You want gain: this means that there is plenty of effect on the LED for a small audio input signal. The other way to correct the circuit is to have R4 as zero ohms and miss out R2, but the adjustment of R1 and R3 becomes more critical, and you could end up with a circuit that only works at one temperature. Do try it, though! Get a feel for these things!

If you want help adjusting a circuit having built it, turn it on, measure all the voltages and post them for us all to ponder.

Have fun!

Jean

276-142 gives you a *photo-transistor* which is entirely unsuitable for this task.

Will you please stop posting nonsense.

Graham

Not with you there. The task I had in mind was turning light into a signal, prompted by the suggestion of Model: 276-124, which turns light into electrical energy, a kind of signal of course. But the man is trying to listen to light, and I think that requires a more delicate component, like a phototransistor. It will have better sensitivity (he wants to listen to an LED, not bright sunlight) and better frequency response (sound bandwidth of kHz, not sunlight varying sub 1Hz).

Do you see what I have in mind now?

Jean

I have a set of headphones (Sony TMR-IF330R) that receive light "signals" from an LED transmitter plugged into my radio.

It's all digital.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
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I love to cook with wine.      Sometimes I even put it in the food.

OK, OK: I'll take the bait ;-) He doesn't want a fancy digital design: he's in 8th grade, and wants something he can play with. (How old *is* 8th grade, anyway?) And you try explaining how the AGC on the photoreceiver in your headphones is digital ;-)

Hello,

I think I will add to your confusion.

*********

This is the approach you should use on your receiver side:

The square solar cells mounted on the top of battery operated yard lights will work excellent for your purpose.

Using a solar cell requires the amplifier input to be sensitive enough for a microphone to work. This is a direct connection plug in no extra parts needed. Try it on your computer mic. sound card for instant success. You will fine that room lighting hums at 100/120 hz because of 50/60 hz sine wave.

********

I have experimented with all of the suggested light sensors.

One that "will not work" for your application is the Radio Shack solar cell 276-124. jean.easter also reasoned this as a bad choice.

Phototransistors and CdS photocells both work excellent for light modulated sound, adding their own unique enhancement, but need a bias current which makes their use more complicated than your immediate use requires.

Good Luck,

• * * Christopher

Temecula CA.USA

Have you actually looked a the 'design' ?

It needs a photo-voltaic *source* as a detector. Photo-transistors are not such an animal.

Please stop giving bad advice.

Graham