[sdiy] Voltage controlled resistor, sort of

[Harry Bissell]

Making a silk purse: part 1  (aka you needen't gild the lily... ;^)

(imho) Add a diode in series with the pot, with the cathode feeding the cap and
emitter of the UJT. Then add another diode (wire-or) in the same direction
and with a series resistor going to your voltage input.

The pot sets the initial pulse rate, excess voltage from the CV input makes
it speed up.

Its crude but effective and might even be in keeping with the style and era
of the original design.

H^) harry



----- Original Message -----
From: David Ingebretsen <dingebre at 3dphysics.net>
To: 'synth diy' <synth-diy at dropmix.xs4all.nl>
Sent: Mon, 03 May 2010 14:21:17 -0400 (EDT)
Subject: [sdiy] Voltage controlled resistor, sort of

I'm still in the throes of my Steiner-Parker resurrection. I am looking at
his Trigger generator from the Synthacon schematics. It is a very simple
oscillator using a typical UJT oscillator.

      +       +
      |       |
      |       |
    __|       | 
    |  /      /
    |  \      \
    |->/      /
       \      \
       |      |
       |      | 
       |     _|
       |    |
       |----|
       |    |_
       |      |
       |      |-------------- out to NPN transistor to create an "S-Trig"
output
     __|__    \
     _____    /
       |      \
       |      |
       |      |
      ___    ___
       _      _


I want to replace and/or supplement the pot with a voltage controlled input.


To that end, I decided the charging circuit was really dependent on the
magnitude of current, so I built a voltage controlled current source (VCIS)
from an OpAmp and the simulations work fine, but I was wondering if someone
could point me to a "standard" way to replace a pot like this with a voltage
control? I like the OpAmp solution, but in order to get the thing to work, I
need to run the VCIS off +/-15 VDC with the UJT circuit running on + 12 VDC.
I'd rather not have to re-invent the wheel especially when I'm just
re-learning all this.

I know there are much better ways to generate a series of trigger pulses,
but that defeats the resurrection mode I'm in. I'd really like something
that runs on +/- 12 VDC to keep the power supplies simple.

Thanks

David



David M. Ingebretsen M.S., M.E.
Collision Forensics & Engineering, Inc.
2469 East Fort Union Blvd. STE 114
Salt Lake City, UT 84121
www.CFandE.com

801 733-5458 Office
801 842-5451 Cell

dingebre at CFandE.com
dingebre at 3dphysics.net



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-- 
Harry Bissell & Nora Abdullah 4eva