Art's Minimum Theremin

I'm entirely new to theremins, and found the instructions for the Minimum Theremin on Art's Theremin page. I've put it together solidly, or at least as well as I could.

I give up, I cannot get the thing to work properly. The antenna should work well enough, I clipped a telescoping antenna off of a radio. I'm not entirely sure about my soldering work, but was careful not to have any splashes or accidental connections.

When I touch some of the solder joints or wires with my fingers, I get a buzzing static from the amp (like when you touch a bare jack, or are first plugging in an instrument). I can touch the antenna and get the same result, but it's not a tone at all. The strongest is when I put my screwdriver to the 15-turn Potentiometer.

If anyone can provide any insight, or perhaps know where I can find pictures of a completed Minimum Theremin (not just the component side, but the finished wiring too), that's would be superb.

The fact that you are getting buzzing sounds (picking up mains hum) from everything, including the antenna, makes me think you have a power supply / grounding problem..

If you dont have a multimeter, you need to get one! Check the power connections to EACH IC.. (put the -ve lead on pin 7, and the +ve lead on pin 14.. you should see +5v on pin 14 of each)..

Then check that the -Ve is solidly connected to the Ground of whatever amplifier you are using.

When you touch pins 8 or pin 10 on U1 (the output buffer) you should NOT hear mains hum.. If you do, it means that this point is at thigh impedence, which it should not be - it should be producing a low impedence square wave - or if the circuit is faulty, it should be held High (+5V) or low (0V) at low impedence, and induced mains hum should be greatly attenuated (to a level where it is inaudible or barely audible).

If you dont get hum when you touch this point, but get hum when you touch other points, it gets more complicated .. This would indicate that theb oscillator stage has a fault, and induced mains is being 'squared' by the gates.. But I do think this is unlikely.

Hopefully you put the IC's in sockets and didnt solder them directly on the board (dont know what the kit provides) - In my opinion good turned-pin sockets are essential - They allow one to remove IC's for debugging purposes, and remove the chance of over-soldering ICs which are easily cooked to oblivion by beginners.

This is a real simple circuit - but even simple circuits can be a pain top debug without equipment.. For this sort of circuit I would say you need a minimum of a multimeter and a logic probe (low cost unit with LED's showing +,0,and pulse states).. I have assumed you dont have a 'scope.

If you don't have a 'scope, you should at least invest in a cheap AM transistor radio.

Christopher has some information about using an AM radio to determine if an oscillator is working or not on his rs-theremin site http://www.oldtemecula.com/theremin/

This should probably work for a "digital" theremin (people call them that because they are built with CMOS digital gates) as well as an analog one, since it would still be radiating enough RF energy to interfere with the radio if you place it close to the theremin.

Since the theremin is only producting a single-frequency RF signal (i.e. a carrier), you will know when you have hit the right place on the dial when the radio goes dead silent. There's no modulation (audio signal riding the carrier), so the theremin's output just overpowers any external radio signal at that frequency when the radio dial is set to the frequency of the theremin.

This technique should at least tell you if you have oscillation or not. You know the audio output is working since you can get a buzz from the speaker when you touch other parts of the circuit.

Hope that helps a bit.

Don

Guys, i'm sorry about digging too old posts, but I'm having the exactly same problem.

I've bought every single component from Art's Minimum Theremin ( http://www.theremin.us/101/101.html ) and put everthing together, but not much goes out but static when touching the antenna or some solder joints.

 I can also hear some very low oscillative sound when I set the volume to maximum, which I can change by moving the potenciometer.

Any idea instead of giving up the project ?

Have you tried emailing directly Art?!

 

"Have you tried emailing directly Art?!"

 I did, but there's no answer yet.

"Any idea instead of giving up the project ?" - panthergois

It's an RC oscillator Theremin - I hate to say it but I think you'll probably give up on it even if you do manage to get it going.

Do you have an oscilloscope or access to one?  If so, check for oscillation on pin 6 of U1 and U2.  You should be able to adjust the frequencies at these two pins to be pretty much the same via RV1 (coarse) and RV2 (fine).

You are hooking this to an amplifier, and not just a raw speaker, correct?

An oscilloscope is absolutely needed during the build and debug process of whatever theremin circuit. It is possible that your circuit works perfectly - but with a beat frequency above the audible range. That can never be diagnosed by ear. The faint sound you hear at maximum volume might be a side band interference or as dewster wrote, there is not enough AF amplification behind the 101 circuit to make the signal audible. No 40xx CMOS IC can drive a speaker directly. It is important, too, to not use standard 4069 CMOS ICs but the rare and difficult to find unbuffered version which has an "U" as a first letter behind the "4069".

"It is important, too, to not use standard 4069 CMOS ICs but the rare and difficult to find unbuffered version which has an "U" as a first letter behind the "4069"."  - Thierry

Yes, unbuffered inverters don't have internal node, so they are better for linear applications like oscillators.

Looking at the CD4069UBC (Fairchild) datasheet, I see the following:

 - Average input capacitance : 6pF typical, 15pF max

 - Propagation delay @ 10V supply : 30ns typical, 60ns max

 - Output transition time (slew) @ 10V supply : 50ns typical, 100ns max

You can find better (faster) devices that work at much lower voltages these days, like the 74LVU04.  CD4xxxU may have been the bee's knees back in the day, but IMO they are too slow for precision oscillators operating in excess of 300kHz, where the period is less than 3000ns.  Any semiconductor delays in the loop will be influenced by temperature, and delays in general throw the tank off the resonance point.  They have a fair amount of input capacitance which might also be a problem.

I'm entirely new to theremins, and found the instructions for the Minimum Theremin on Art's Theremin page.  I've put it together solidly, or at least as well as I could.I give up, I cannot get the thing to work properly.  The antenna should work well enough, I clipped a telescoping antenna off of a radio.  I'm not entirely sure about my soldering work, but was careful not to have any splashes or accidental connections.When I touch some of the solder joints or wires with my fingers, I get a buzzing static from the amp (like when you touch a bare jack, or are first plugging in an instrument).  I can touch the antenna and get the same result, but it's not a tone at all.  The strongest is when I put my screwdriver to the 15-turn Potentiometer.If anyone can provide any insight, or perhaps know where I can find pictures of a completed Minimum Theremin (not just the component side, but the finished wiring too), that's would be superb.

alexciam, panthergois,

Did you resolve your issues with your Minimum Theremin circuits? I'd be happy to offer assistance if you email me.

Art Harrison
arthur@harrisoninstruments.com