easy DIY theremin?

[nieh]

Pretty much just want to make an oscillator with a photocell resister to change the pitch rather than a knob.

[multi_s]

You can do it many ways. One way is with a 555 "astable" setup

heres aschem

http://www.falstad.com/circuit/#%24+1+5 ... -5+1+-1%0A

You can either replace the resistor from V+ to "DIS" or the one in between "DIS" and "TR". I would measure the resistance of your photocell in light/dark. Then lookup teh 555 datasheet there are simple equations to calculate teh right capacitor to use to get the frequency range you want.

Also depending on which resistor you change you will get different range sweeps. Experiment to see what you prefer. If your trying to keep teh duty cycle of teh wave fairly constant then i would replace the second resistor i mentioned and use somethign fairly small (1k?) as the first resistor.

[unownunown]

weird, i was just checking out light theremins earlier today. this might be what you're looking for!

[nieh]

weird, i was just checking out light theremins earlier today. this might be what you're looking for!

I have some 555 timers laying around!

[eatyourguitar]

I built those when I was warming up to DIY. it works best with your finger half on half off the LDR. as soon as you take your hand off it stops tracking your hand. you could play it better with a flashlight than with blocking the sensors.

if you want a REAL theremin go HERE

$160 and you supply the wood case. what an awesome project at an awesome price.

[multi_s]

ya they are not "great" therimins but you can step it up by building another box thats just flashing lights driven by 555's and create pretty pulsing rhythms by moving the ldr around the face of the box. that into a looper and you are set. you can synchronize them via pin 4.

also might be nicer to have a more sinusoidal output . iirc merlin blencoe posted a pretty good sine wave generator on diystompboxes at some point that just uses a quad opamp.

[eatyourguitar]

4ms has a triangle lfo that works on one 4558 dual opamp and runs on 9v the quad op amp is probably used for wave shaping the triangle to a sine.

[multi_s]

triangle oscillators are just square waves fed into an integrator. if you then integrate the triangle you do get some sort of sinish shape but its really just a patch work of porabolas and not really sinusoidal (ie the spectrum is wide, not singular). You can use phase shift and feedback to create pure sine oscillators. Look up "phase shift oscillator" there are many. his circuit was neat because it was able to change the frequency with a single pot which is, as you will see if you look the them up, not trivial.

[eatyourguitar]

I have been checking out monolithic function generator IC's lately. you can just get a chip that makes a sin. I have seen that 13700 OTA used in tri>sin wave shapers. just two trim pots. one for tri-sin-square and one for bias. MFOS uses that OTA for everything.

[multi_s]

did you check out the ad9850? i wanted to try that out. smt only though but its soic which is pretty easy.

http://www.analog.com/en/rfif-component ... oduct.html

[eatyourguitar]

I could do soic if i buy a shield to dip on ebay or spark fun

[multi_s]

ha looks like some people have already been using this... type ad9850 into ebay , there are hundreds of results.

10 bucks for premade circuit?
http://www.ebay.ca/itm/AD9850-DDS-signa ... 35b49f5c8c

and 50 bucks for a whole generator with LCD.. not bad

http://www.ebay.ca/itm/5MHz-DDS-Functio ... 8524wt_952

edit: my bad. i dont think the second link is actually using a ad9850. but you _could_ use one i think to do something similar . (;

[eatyourguitar]

thats pretty sick but its 40mhz. do you think its better just to start with the IC and make something that tracks 1v/oct?

[multi_s]

ya i think its _up to_ 40 mHz. I have only briefly looked at the spec sheet but it has 32 bits resolution for setting the frequency. That means it can do 4,294,967,296 different frequencies (over 4 billion?) so even with a range that high you would still have pretty good resolution at low frequencies.( If i understand how it works correctly, you still have roughly, accuracy to 1/100th of a Hz over a 40 MHz span). Beyond that you should be able to just clock it slower (most of these designs im seeing 125 mhz crystals) and put all that resolution into the auidio range instead of spreading it over the MHz range. I will look more into it, seems at least worth farting around with one.

Setting it up for 1/v per octave is a good idea too.