Single speed pot Univibe question with vtl5c3

[Anthony]

Hello all. I need some help adapting my "univibe" clone to use a single control pot "ala" RG Keen's circuit.
1) LED1 is not associated directly with the photocells, correct? It's not meant to modify the existing LFO bulb?
2)If my V+ is 15 volts won't that damage a 1.5v LED used for LED1? Something I'm missing there.
3)If I use a 2N5089 for Q1 will Re calculate out to be approx 500ohms @ 20ua on the LDR.

Thanks, Anthony

[danielzink]

Anthony,

I've implemented this mod on a rehoused Chicken Salad with great results.

(1).The LED is strictly for voltage drop purposes.

(2).Why are you running 15v in ? Most pedals run on 9V - Maybe it would help to know what pedal you're trying to apply this mod to.

(3). Here's a quote from RG when I was trying to sort out the mod myself. BTW I used a 10k pot for RE to make it adjustable.

Dan,

(1) find +9V somewhere on your Chicken Salad. Start where the power comes into the board and trace it to an easy pad for powering the mod. Solder a white wire to the pad. 6" should do it.
(2) find ground somewhere on your Chicken Salad. Start where the power comes into the board and trace it to an easy pad for powering the mod. Solder a black wire to the pad. 6" should do it.
(3) Make yourself up a small perf board with the added circuit on it.

LED1 is a red LED, any kind. It will not be seen outside the box and is used only for its voltage drop.
D1 is a silicon diode, any kind, both 1N4148 and 1n400x will work fine; it's used only for its voltage drop.
Transistor Q1 is an NPN transistor with a gain over 100. 2N4400, 2N3904 will work fine, as will most other NPN silicon transistors you can find. Be sure to check the pinout on the transistor you have before soldering it in.
Re is the scaling resistor that converts the voltage on the emitter of Q1 to a current through Q1. It should be sized so that the 9V supply minus the approximately 1.5V LED voltage plus the approximately 0.6V D1 voltage gives a workable current through the opto isolator LED.

What's a "workable current"? It depends on the LED/LDR's LED. Over 20ma probably kills the led, so Re is going to be bigge than (9-1.5-0.6)/0.02A = 345 ohms. How much bigger? You need to put a pot in there and diddle it until the control range from min to max is what you like for your LED/LDR.

Some LED/LDRs are very sensitive, needing less than 1ma for full-on to essentially full-off on the LDR. That would be an Re value of 6900 ohms. So you're looking for a value between 345 and 6900 ohms.

Here's a pic of my Chicken Salad with the mod:


Good Luck ! Dan

[Anthony]

Single speed pot Univibe question with vtl5c3 Reply to Thread

Hey Dan. Thanks for the reply. I got the rig to work. I found a spot to get a little under 9v in the circuit. I was originally taking it from the voltage regulator in the circuit. Now I'm getting power from the diode bridge. I built this Univibe with on board transformer and voltage regulator..hence the high voltage.

Anthony