Posted: 4/2/2009 8:04:34 PM

Joined: 4/2/2009

I hope I'm not about to ask a question that hasn't been answered many times before but I was wondering if anyone on this site has any knowledge about infrared theramins.

I'm a student (sorry) and I've recently built a theramin that worked 100% by using an antenna to control pitch and an infrared LED/phototransistor combo to act as a means of controlling the volume. Souds a bit rough but does what I want.

I think I have a good grasp on how it works but one thing confuses me. Having put the fully working PCB inside an aluminium box I find that the volume controls response is very limited.

I've tried several things to eliminate this:

1) The reflective surface of the aluminium box could have affected the way the IR light is picked up by the phototransistor - corrected by covering the aluminium around the volume control with insulation tape. Didn't work.

2) Ensured that there was no way that the IR LED could activate the phototransistor through direct line of sight. All direct paths for the IR light to the phototransistor were covered. Didn't work.

3) Normal daylight was minimised by drawing the curtains. Didn't work

4)Any possible way that the conductors of the circuit may have been connected to the metal casing were eliminated. Didn't work.

5)The IR LED and phototransistor was positioned no more than 13mm apart and various positioning angles were tried. No joy.

Yet for some strange reason when I remove the transmitter/reciever IR components from the case and shield one from the other with some rolled up paper the unit as a whole works perfectly.

I'd be grateful if anyone on here knows how to solve my problem. It's a bit of a mystery to me.

Thanks in advance
Posted: 4/3/2009 10:31:06 AM

From: Eastleigh, Hampshire, U.K. ................................... Fred Mundell. ................................... Electronics Engineer. (Primarily Analogue) .. CV Synths 1974-1980 .. Theremin developer 2007 to present .. soon to be Developing / Trading as . ...................................

Joined: 12/7/2007

This doesnt really make a lot of sense - The IR circuit should not be affected by electrical / RF shielding.. I suspect there is some instability in your IR circuit which is causing the problem .. some capacitance 'damping' or similar... There is no particular reason for high impedence circuitry in the IR sensor.

You say: "built a theramin that worked 100% by using an antenna to control pitch and an infrared LED/phototransistor combo to act as a means of controlling the volume" -

This is a long shot - but try turning the antenna circuit off, and then try the IR volume circuit using an externally input audio signal in the configuration that works - it MAY just be that your IR detector is, in fact, picking up RF from the antenna and rectifying this - if this is the case, this RF signal's attenuation when the circuit is 'screened' may cause it to 'fail' under these conditions.
Posted: 4/6/2009 1:16:08 PM

From: Escondido, CA

Joined: 2/6/2008

The "aluminum box" is probably not helping much. Most theremins are encased in wood boxes ... and the occasional plastic box.

Posted: 4/6/2009 1:18:14 PM

From: Escondido, CA

Joined: 2/6/2008

What does your IR volume control do? In other words, are you using the IR LED/photocell combination to affect the frequency of an oscillator? Or does the audio go straight through the IR photocell?

Posted: 4/10/2009 1:14:46 PM

Joined: 4/2/2009

I disconnected the generator stage and tried using a function generator to no avail.

My electronics tutor suggested that I connect the aluminium casing to the zero volt rail and it seens to be working fine now (although I find that I have to use a sheet of white paper to replect the IR light)

From looking at the diagram a 555 astable timer is used to turn the IR LED on and off which is then picked up by the phototransistor and the resultant change of strengt from the reflected light is used to amplify the output stage of the tone generator circuit.

What confuses me is how the aluminium box affected the circuit in the first place since it wasn't electrically connected to any other part of the circuit.

Many thanks for your help and suggestions

Posted: 4/10/2009 1:38:04 PM

From: Escondido, CA

Joined: 2/6/2008

A couple of possibilities:

1. If the aluminum box is affecting the IR part, maybe it is because the box is colder or hotter than ambient air. The IR photocell is going to be sensitive to temperature changes.

2. The box adds stray capacitance. So it if is not grounded, it might be capacitively coupling circuits in an undesirable way (you said it didn't affect things if the box was grounded).


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