What's got me befuddled is that this circuit doesn't require a fading voltage (which is easy to produce), it requires a fading resistance. That's what the phototransistors are doing - changing resistance in response to light. There is such a thing as a voltage-controlled resistor. Maybe that would do the trick.

In South Africa, specifically Cape Town, there's a great shortage of parts, especially exotic ones, but I'll check my suppliers web sites and edit this post

PS fading voltage is not easy to produce if you know next to nothing about circuits

PPS do you have a part number for a voltage controlled resistor? Couldn't find a voltage controlled resistor from any of the suppliers

 

Let's start simple. Just use an optical coupler - an LED to light the phototransistor. So as you vary the brightness of the LED, it will simulate passing your finger over the phototransistor. All you need is that circuit as posted and 1 or 2 LEDs. If you don't already have the parts, you can get a device call an optoisolator. It contains an emitter LED and a phototransistor in one package. You never even see the light. It's possible there may be optoisolators with two pairs in one package.

The next step will be to produce the desired brightness profile at the emitter LEDs.

 

Let's start simple. Just use an optical coupler - an LED to light the phototransistor. So as you vary the brightness of the LED, it will simulate passing your finger over the phototransistor. All you need is that circuit as posted and 1 or 2 LEDs. If you don't already have the parts, you can get a device call an optoisolator. It contains an emitter LED and a phototransistor in one package. You never even see the light. It's possible there may be optoisolators with two pairs in one package.

The next step will be to produce the desired brightness profile at the emitter LEDs.

http://www.mantech.co.za/ProductInfo.aspx?Item=35M1307
http://www.mantech.co.za/ProductInfo.aspx?Item=35M2857
http://www.mantech.co.za/ProductInfo.aspx?Item=35M5986

Which one? dunno how to determine their difference, they all 3 seem the same? Price also roughly the same. (all 3 dual channel)

 

I think for this application they are nearly the same. The MCT6 one claims a higher power rating, so I'd probably get that one.

 

Ok, I have no idea when I'll be able to get stock of those parts, 'cause the supplier is about 20min drive away... and I don't have transport of my own. But assume we have the optical coupler, what next. theres 2 pins anode and cathode for each of the 2 leds, and emittor, collector for each of the transistors, I still don't know how to make it fade from right led to left led

 

For learning the basics of controlling LEDs, and some effects, take a look at Bill's Blog on this site.

I think an effect you want to look for is known as a "throbbing" LED, a soft fade in and out like the power-on indicator LED on a Macintosh.

 

wayneh, I asked in PM another member who helped me with my LM2576 project, studiot, he gave me these:

Here is the first article (of a series) of a simple to build modular synthesiser from the past.
There is lots of useful detail about whaich modules do what and how they work.

I don't know of a generator module for the sounds you want, but the series might help if you are interested.

Since wayne is helping and interested in this thread, you may wish to copy him in on this conversation.

Eric

 

wayneh: I'm getting the parts for the fader (throbbing) on Tuesday, and then hopefully the optoisolators on Friday... my question is, how do I delay the cycle's start of the second led?

 

I think I'd start with independent control of the two LEDs until you manually find the profile that you like. Then you can figure out how to produce that sequence. A microcontroller would be helpful for that part but it's not the only way.

If you can (if you have enough supply voltage), you might want to put a regular LED in series with your optoisolator LED, so that you can see what the latter is doing. This feedback will give you a much better idea of what you need to duplicate once you find a combination that gives a good result. A regular LED can only handle ~20mA. This is less than the optoisolator can handle but it's not necessary to run it that high. You can try running it at 15mA and that should be enough.

 

I think I'd start with independent control of the two LEDs until you manually find the profile that you like. Then you can figure out how to produce that sequence. A microcontroller would be helpful for that part but it's not the only way.

If you can (if you have enough supply voltage), you might want to put a regular LED in series with your optoisolator LED, so that you can see what the latter is doing. This feedback will give you a much better idea of what you need to duplicate once you find a combination that gives a good result. A regular LED can only handle ~20mA. This is less than the optoisolator can handle but it's not necessary to run it that high. You can try running it at 15mA and that should be enough.

Thx! I'll see what I can do and report back after I built it hopefully Saturday

 

Thx! I'll see what I can do and report back after I built it hopefully Saturday

the throbbing cicuit doesn't throb

The LED turns on, but doesn't fade out and in like advertised, am I missing somethiing obvious here?

 

Hello,

What is the supply voltage you are using?

You might also want to have a look at the weird sound generator from the music from outer space website:
http://www.musicfromouterspace.com/analogsynth_new/WSG2010/wsg_page1.html

Bertus

 

9V PPP3 battery, as recommended by the cookbook, will check that site just now

 

Hello,

What color led do you use?
If it is a red or green one, it will not go off.
If it is a white or blue one, the led will be turned off.

Bertus

 

S*t I only have red and green and multicolour, one that repeats the same pattern over and over again. I've bought 10 MCT6 optocouplers, will they turn off? How would I measure that easily?

 

Hello,

Put the led of the MCT in series with the red or green led.

Bertus

 

Hello,

Put the led of the MCT in series with the red or green led.

Bertus

I'm pretty sure I did not get confused with the anode and the cathode, when I connect it like I think is should be connected then nothing on the green led, and when I swop it on the optocoupler, to my very noob mind the wrong way round, then the green led is on. whaaut? maybe step me through what is positive and what is negative on the MCT

EDIT: I KNOW I didn't confuse anode and cathode

edit 2: but when I put a red led in series with a green led it also just switches off

 

Hello,

The MCT6 contains 2 optocouplers.

Bertus

 

Hello,

The MCT6 contains 2 optocouplers.

Bertus

got all that, why does the circuit cut out with a second led in serie (thats 2 wires of 2 leds per anode and cathode connected on the same path (LED 1 cathode and LED 2 cathode on the same track) to electricity)

 

Hello,

I do not get what you are saying.
Can you make a drawing or photo how things are connected?

Bertus