Infrared LDR for optical theremin?

[D Wagner]

Hey Pedalheads,

Do you know if there is such a thing as an infrared light dependent resistor?  I am building an optical theremin for my son, and it seems that using an IR LED and an IR LDR would yield better results overall than just using an LDR and the ambient light from a lamp or overhead source.

I have searched on google and mouser, but maybe was looking with the wrong key word.  I kept turning up info on lasers and similar things.  

Any ideas?

Thanks in advance.

Derek

[Paul Perry (Frostwave)]

I doubt that there would be any advantage, because there is plenty of infrared in 'normal' lighting anyway.
I use a modulated infrared diode (about 50KHz) and an infrared photodiode for pickup.
The simplest thing is to use such an intense light source that any ambient is swamped out. Using a car headlight or similar from a regulated source has been used successfully, in one Australian magazine design from a few years back.

[smoguzbenjamin]

I have something like that lying in my part bin. It came out of a discman with a remote (wierd idea but hell) and it has 3 legs, and some photoreactive element to it... I THINK it's an IR phototransistor anyway :mrgreen:

[zachary vex]

you can certainly do some research to see if there's a version of LDR that responds to infrared light, and like any other device that is made infra-red sensitive, you'll have to put a filter in front of it.

[mattv]

As another option there are transistors that react to infrared light.

[D Wagner]

Hey guys, thanks for the responses.

you can certainly do some research to see if there's a version of LDR that responds to infrared light, and like any other device that is made infra-red sensitive, you'll have to put a filter in front of it.

I am on that now.  I found this site in my searches.
http://optoelectronics.perkinelmer.com/content/RelatedLinks/photocell%20application%20notes.pdf
http://optoelectronics.perkinelmer.com/content/RelatedLinks/photocell%20analog%20optoisolator%20data%20book.pdf

Thanks for the tip about the filter.  I would have overlooked that, even though I use a remote control for the TV a few times a week.

I am going to experiment with the garden variety photoresistor on the breadboard.  I just wanted to explore other options, because where my son will "play" the theremin isn't very well lit.  

BTW, he is 16 months old and has a noticeably good sense of rhythm and coordination.  And he loves electronic toys.  He would be happy with a remote control, a phone, cell phone, camera......  Anything that is not meant for a small child.

Anyhow, the schematic that I am working from, though it has no values posted, is very easy to work from.  It is simply a 555 astable multivibrator and an opamp.  I am going to add an LM386 amp and a small speaker for portability.  http://www-personal.engin.umich.edu/~jglettle/class/phys489/Theremaniac.pdf

I'll try to post a pic of him playing with his new toy when I finish.

Thanks again,

Derek

[niftydog]

I have something like that lying in my part bin. It came out of a discman with a remote (wierd idea but hell) and it has 3 legs, and some photoreactive element to it... I THINK it's an IR phototransistor anyway

it's probably something like this.  Useful if you've got something like a PICAXE that can make sence of the codes that a remote control sends.

[Paul Perry (Frostwave)]

A non-theremanic alternative with that circuit, is to replace the LDR by two metal contacts, and then as you grasp it across the 2 contacts, body resistance changes & note sweeps.