Build an Analog Theremin that Rivals a Modern Day Classic for about $100

Posted: 4/9/2019 5:31:42 AM
JPascal

From: Berlin Germany

Joined: 4/27/2016

Nice to meet a physisist here.

Posted: 4/26/2019 12:28:35 AM
oldtemecula

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014


Hey all I am still breathing,

I have never worked in electronics as it is just learn as I go. This allows me to view things from a different perspective. Often I encounter something with no one to ask an opinion. In the video below the narrow pulse is downward; I wonder what characteristic of the two RF oscillator circuits being mixed decides this. In my way of thinking one oscillator might drag in phase slightly behind the other. I realize building the circuit with identical parts again the narrow pulse could go in an unwanted up direction. In all the times I have built this circuit it does favor a narrow pulse downward.


If my thinking of phase is correct, I would like to explore the circuit phase shift using component adjustments. The direction of the mixer diode may play into this but not completely if any?

Anyone...what do you think? TIA

Answer: An idea from the witching hour last night said the direction is dependant on which side of the modulated carrier from the heterodyne mix is detected. Then the trick is creating a dynamic wave shape most beneficial to PWM.  what..

If you like my results... I am first to say it is not easy, it has taken 15 to 20 years to get where I am at. Everything is easy once you have the answer.

Christopher  


Posted: 4/27/2019 1:58:47 PM
JPascal

From: Berlin Germany

Joined: 4/27/2016

Hi Christopher. You know I am not an electronic engineer. The idea with the pulse width variation is interesting. What I can see is, after the 555 the asymmetric pulses are directly filtered by a deep pass R=1 kOhm and C=470 uF to get the DC for the optocoupler diode. So a time constant R*C results of about half a second. That should result in a too slow change in loudness when your hand moves fast. If so there could enhancements be done.

Your detector seems to be unusually connected? Either R9 or D1 should go to mass. (But may by this smoothing effect if used in the pitch control unit is a good trick).

My experience with an optocoupler is, the audio distortions (THD) are strong increased for low volume pegel.

Posted: 4/27/2019 4:06:04 PM
oldtemecula

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

The schematic is old and the detector section is incorrect, I rely on students who really help me out a lot this area.

You force me to rethink my concepts from 10 yrs ago.

>What I can see is, after the 555 the asymmetric pulses are directly filtered by a deep pass R=1 kOhm and C=470 uF to get the DC for the optocoupler diode. So a time constant R*C results of about half a second. That should result in a too slow change in loudness when your hand moves fast.

The +V is 9 volts, the output of the 555 is 7.9v p-p.

In my configuration the +V at the Vactrol is held at +3.61v from the 470uf cap charged state or zero beat.

Edit: The +3.61v is from the 470uf cap across two LED's to ground. They are On until a negative pulse (hand proximity) turns them off.

Hand proximity starts the 555 discharge pulse on the 470 uf capacitor. When the capacitor charge is below 3 volts the Vactrol LED is no longer forward biased so sound is off. The control window is .7 volts, why there is speed in the volume response. Also I would say pulse width discharge per second varies and so not the same as a constant frequency in a calculation.


>If so there could enhancements be done.

Using the pulse approach opens up to several other control enhancement possibilities.


The sound passes through a Cds LDR which is a variable resistor so no distortion is introduced. I will use a better sound sample next time, just wanted something loud and obnoxious.  The Cds LDR do have their own on/off cell recovery time. In the Vactrol the VTL5C1 is the fastest, something to keep in mind when substituting.

Visit: http://www.oldtemecula.com/theremin2020/   You will be amazed

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