Can someone please visualize this schematic?

[Michael2]

Hi all,

I'm not good at recognising all the symbols in a schematic, can someone please help me with a drawing of the first schematic on this page? I would be grateful.

http://www.geofex.com/circuits/ldrlfo.htm

[Rob Strand]

The problem is if we re-draw it you might not get that either!
What exactly can't you read?  That's is a much easier question to answer.

Try comparing it to this schematic. 
http://2.bp.blogspot.com/-gMXh0hrPsvQ/T_WQhRjERYI/AAAAAAAABoY/KMrT1GXMKec/s1600/univibe.gif

or this

http://classicamplification.net/fv/pix/OEM_schematic.gif

[antonis]

I'm not good at recognising all the symbols in a schematic

Maybe this will help...
(sorry R.G.)



edit: Darlington Collectors junction is accidentally erased..

[Michael2]

Wow guys, thanks a lot. I have build some pedals up to now but most of them came with simple wiring diagrams and/or instructions so that was easy to follow. I am at the beginning of understanding schematics and this one was above my head. Thanks for the great explanation of the symbols Antonis, that is very helpful!

I will see how far I come! Thanks again!

[Michael2]

I feel lame, I suddenly understand that I do not have to build the whole diagram but only the part in the right corner below since everything else is already present in the Univibe 

Few questions though: what type of transistor do you recommend for Q1 and what value would D1 be?

[Rob Strand]

Few questions though: what type of transistor do you recommend for Q1 and what value would D1 be?

Any NPN will work but it is not intended for a Darlington.  A 2N3904 would be fine.
The diode can be small signal silicon type like 1N914, 1N914, 1N4148.

The speed pot should be around 10 to 20 times the resistance of Re.  You could go outside that range a bit, it's not that critical.

[Michael2]

Thanks for your help Rob! Again!

BTW: VTL5C3/2 are hard to find here in Europe and they seem pricey. Maybe a naive question but is it possible to replace them with two regular photocells and a led?

[Rob Strand]

Maybe a naive question but is it possible to replace them with two regular photocells and a led?

It can be made to work but there's a lot of variables.   Like the range of resistance of the LDR you choose and how well the light from the LED couples to the LDR.

Ideally you want to match the span of resistance of the VTL5C3/2 with the same amount of LED current.    However you can increase the LED current a bit to compensate for poor light coupling.

There's many ways to go about it.  My suggestion is to do a web search and search some of the threads on this forum for ideas.   Because your LDR(s) might be different to what others use you will have to do some experimenting.

[Michael2]

Okay, thank you, I will!

[Michael2]

Finally had time to test this out and it works exactly as described. I swapped resistors until the brightness of the led and the taper of the pot worked optimal. Ended up with a 5k1 resistor with a 100k log pot. Thanks for the help guys! 👍

[Rob Strand]

Thanks for the feedback.  It gives other people the confidence to try it out.

[Michael2]

I cheered a bit too soon. The pot works in reverse now, rocker down means slower vibe. Before I swap the pot for a normal log, is there an easy way to change something in the schematic to invert the way the led works?

[Rob Strand]

Before I swap the pot for a normal log, is there an easy way to change something in the schematic to invert the way the led works?

Probably more trouble than it's worth. 

The only way to change the direction is to reverse the two outer terminals of the pot. 

If the behaviour of the control is too bunched-up at one end then you will need to change taper.

[Michael2]

I have tried swapping the outer lugs of the pot but It doesn't feel right. I will look out for a normal pot then. Thanks again Rob!

[Rob Strand]

It's tricky to know if you need a linear pot or a reverse-log pot.

If the current pot feels *exactly* right then you might need a reverse-log pot (which are harder to get).

You might be able to massage the situation a bit by adding a resistor.
Keep in mind adding  these resistors doesn't really make one pot like another, it's more of
an approximation.

See here,
http://www.geofex.com/Article_Folders/potsecrets/potscret.htm

My guess is to start by putting a 22k across the top two terminals (wiper and clock-wise).
Tweak the resistor to get more what you want.

[Rob Strand]

It occurred to me reversing the taper by changing the electronics might not be that hard after all.

Remove junk just see ckt below:
Replace the NPN Q1 with a PNP so it's like:
+ rail -->  Re  --> PNP.e
and
PNP.c  --> LED anode  --> LED cathode --> gnd

The base circuit with the pots and the diodes stays the same.

Grrr.  Sorry there's one detail there that changes things:

+ rail --> normal diode --> pot --> LED1

This change does in fact flip the whole behaviour upside down.

[Michael2]

Awesome! But meanwhile I had an idea...what if I just wire the led in the normal way: voltage resistor->led->pot wiper->left lug pot->ground? The two LDR's are taped around the led and go to the three leads out of the univibe as in the schematic above.
I have tried it out and it works!

Do you see any complications or is your (RG's) layout preferred above this one?

Thanks!

[Rob Strand]

hat if I just wire the led in the normal way: voltage resistor->led->pot wiper->left lug pot->ground?

It should work fine.  The only shortcoming is you have to tune the value of the pot.  The original circuit you can use any old pot then tune Re.  The resistor is easier to tune than changing pots.  It also lets you do finer tuning (as you can't get some pot values easily, eg if it turned out to be 3.3k.)

[Michael2]

Ha, I see! That makes a lot of sense. I only tested it shortly but it felt that the 100k pot does a great sweep on its own, from no movement to very fast throbs. I guess a lucky shot! But have to do some more testing. If it turns out it needs improvement I definitely will go for RG's schem. Thanks a lot for your help and advice!