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Re: Uni-vibe LFO questions

2/2/1999 10:56 AM
Dave B.
Re: Uni-vibe LFO questions
Hey, That sounds like a good idea!  
So, what exactly detrmines the waveform of the LFO??? How do I change it?? I'm gonna be doing the LED version of it. Also, I'm gonna be using some 30 ohm to 10 meg light to dark photcells. Will these be adequate??  
2/2/1999 12:10 PM

Phase shift oscillators, whether current or voltage mode, always produce near-sine waves, with various degrees of distortion. This is true of the LFO in the 'vibe. You change it by any number of waveshaping tricks.  
Driving a sine into an exponential-response converter is one means of waveshaping, as the positive peaks cause a LOT more current in the LED than the negative ones.  
On the photocells, sure, 30 ohm to 10megohm (where did you get those, by the way...) will be a fine range. They also have to be fast enough to follow the highest end of the speed in the LFO. Wide range LDR's tend to be slow, and getting both wide range and 10Hz response can be tough.
2/2/1999 4:25 PM
David Britton

I got the photoresistors from Hosfelt electronics at 50 cents each, part number 24-136. Hosfelts web address is  
2/2/1999 6:02 PM

Wow, great! If they're fast enough, it's a great resource for 'vibe builders. Thanks!
2/2/1999 11:00 AM
Dave B.

oh yeah, forgot to mention something.  
I was thinking of using the VTL2C2 which is the same thing as the VTL5C3/2 just w/ one photo resistor in it instead of one. Will two of these work in place of the other one??
2/2/1999 12:14 PM

The problem there is that the LDR's are not necessarily very similar. While phase shift oscillators will work with non-matched time constants per section (the two pot sections form two of the R's in RC time constants) the results can be disappointing in terms of range and startup being reliable.  
If you select for resistances no more than 20% apart at the same LED currents, you'll be OK. The VTL54C3/2 has fairly matched sections, as they are two halves of the same LDR.
2/2/1999 6:02 PM
Mike Irwin

A simple way to make a wide-range sine wave LFO (if you don't mind using op amps) is to make the "classic" 2-op amp triangle-square wave generator and feed the triangle output into a very simple triangle to sine shaper. For this I use a back to back pair of silicon diodes (approximately matched) with one end of the pair grounded and the other end connected to the triangle output through a 4K7 resistor. The sine wave is present at the diode side of the 4k7 resistor, where it may be used as is (only 1.4 v p-p) or buffered and amplified with an additional op amp. For this to work properly, the triangle must be symmetrical about ground, and the amplitude is adjusted until a good sine wave is formed. With this method you can have a choice of sine, triangle or square LFO waveforms.

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