# My New Year gift to TW: A new theremin circuit

Posted: 2/27/2017 10:10:17 PM

From: Colmar, France

Joined: 12/31/2007

"Still got a question though: would it be possible to extend the range of the antenna so it would be like 5 meters instead of a few feet? Or am I once more asking too much of such a small circuit? Thanks!"

But... this circuit has a range of 5 meters. The only problem is that at that distance, your hand waving will de-tune the oscillator by only 0.01Hz, these are the laws of physics which might roughly be graphed as 1/e^x relationship between distance and frequency deviation...

Posted: 2/27/2017 10:56:00 PM

From: Northern NJ, USA

Joined: 2/17/2012

Yes.  To see actions over ~1m or so you have to completely decouple the oscillators (or not have the second oscillator present at all, as in some digital detection cases) and use a larger surface area antenna.  Even with that, stuff gets quickly lost in the noise beyond ~1m so you have to do everything you can to kill noise (low noise highly stable oscillators, hum filters, low pass filters, etc.) or make the signal larger (move your body rather than just your fingers or hand) - preferably both.  My stuff obviously starts "seeing" my body somewhere around 2m or so.

Posted: 3/15/2017 11:13:36 PM

Joined: 3/15/2017

Hi! I'm fairly new to electronics, but I decided to build this theremin as it seemed really easy. Unfortunately, it's not working, and I don't know why . I read somewhere in this thread that it was important to check the voltages across the output of the regulator, R12, R42, R22 and R23, so this is what I measured:

Output of 78L06: 6.02 V

R12: 0.670 V

R42: 0.452 V

R22: 1.112 V

R23: 1.102 V

I don't know what are the supposed voltages are (except for the output of the regulator, I think that's fine haha), because I'm not really familiar with circuits' analysis yet; are these okay?

I tried to follow the recommended layout as much as I could. Here's some pictures of the circuit:

http://imgur.com/a/BKzae

I put all this in a carton box and connected it to a guitar amplifier, but nothing happened. I tried turning the variable capacitor and modify the lenght of the antenna, but still nothing. I checked the value of capacitors and resistors and I think they are okay, as well as continuity across the circuit.

Thanks in advance for your help, and I'm sorry about my english, it's not my native language .

Posted: 3/16/2017 1:42:56 AM

From: Northern NJ, USA

Joined: 2/17/2012

Lift the circuit off of the workbench a few inches with a plastic box or similar.  Mount the antenna vertically on something that is insulated (plastic is best) and connect it with a short wire rather than a jumper.  Ground your DMM negative lead (black) on the (-) battery and probe all of the transistor leads with the positive DMM lead (red) and tell us what those are (voltage).  Do you have access to an oscilloscope?

Posted: 3/19/2017 10:14:33 PM

Joined: 3/15/2017

Hi dewster, thanks for the quick reply! I did everything you suggested but still got nothing. Here are the measurements of the transistor leads:

Q11 =

Drain: 5.88 V

Gate: 0 V

Source: 0.67 V

Q21 =

Drain: 4.91 V

Gate: 0 V

Source: 1.11 V

Q41 =

Drain: 5.93 V

Gate: 0 V

Source: 0.45 V

Are these values okay?

Posted: 3/20/2017 11:49:58 AM

From: Colmar, France

Joined: 12/31/2007

These DC values look ok. But a deeper diagnostic would really require an oscilloscope which is an elementary tool when it comes to building or fixing theremin circuits. The reasons why you don't get sound might (among others) be:

• One of both oscillators isn't working for a yet unknown reason, for example a wiring error
• Both oscillators aren't working for a yet unknown reason (are you sure about the values of C12/C13, C42/C43, L11/L41 ?)
• Both oscillators are working but on so widely different frequencies that the heterodyning product is above the audible range and is thus filtered out in the mixer

Your profile does not tell where you are living, but I can suggest that you send me your circuit in for debugging if you can really not put your hands onto an oscilloscope although those needed for analog theremin circuits are very cheap. Here is the one I use most times.

Posted: 3/22/2017 11:53:01 PM

Joined: 3/15/2017

Thanks for the offering Thierry! I would love to send you the circuit but I'm from Argentina, so it would be complicated haha. Anyway, I start university next week, and I think I can borrow an oscilloscope from the laboratory.

What should I look for when I have the oscilloscope at my disposal? The output of both oscillators?

Posted: 7/14/2017 6:02:47 PM

Joined: 7/14/2017

Built this design, it's working, thanks Thierry!  I used substitute FETs after the 2N5484s I ordered on Ebay from China turned out to be counterfeit (based on my measurements, they are NPN bipolar transistors which have been relabeled 2N5484! Not even close!) I used some BF245A Fets I had on hand, which seem to be working OK.

Posted: 8/3/2017 2:21:31 PM

From: Colmar, France

Joined: 12/31/2007

Since BJ-FETs have always a wide tolerance range for Ugsoff and Idss, other FET types might work in that circuit, but there is no warranty. Best is to use a FET tester and to hand-select exemplars with a Ugsoff of -1.5 to -2V and a Idss of 8 to 12mA, independent of the marking.

Posted: 9/14/2017 4:30:20 AM

Joined: 9/14/2017

Hey!

i really need the parts list, it will help a LOT!! thank you so much!