The short story:
I'm looking for a schematic of a simple circuit that runs on 12VDC, outputs 0 to 12 volts by using a 65K POT, and handles up to 1A.

The long story:
I have an old man lift (80's model), that has a bad joystick. It actually has 3 of these joysticks, so I am able to determine how the defective one is suppose to work. And I've taken one of the good ones apart to get some good photos of the board. It's old technology, so I'm thinking it should be either easy to fix or easy to build a replacement board. I've found a used replacement joystick for $700, but it's really only a $10 pub if I can make one. It looks like this photo1. It's function is not complicated at all. It has 4 wires connecting to it, 2 of which give it power, 12VDC & GND. The 3rd terminal goes high (12VDC) when the joystick is moved out of center position, by a microswitch in the joystick.
This PCB is mainly all about the 4th terminal, which tells it how fast to move forward and backward. It put's out 6VDC at center. It goes from 6 to 12 VDC in one direction, and 6 down to 0 volts in the other direction. It's basically just like these handicapped wheelchair joysticks that I've worked with, except they are 5 volts, with a 2.5 volt center.
The defective one puts out the full voltage as soon as I move it in either direction, even the least little bit. I found one burnt diode on it and replaced it, but it still doesn't work. It looks like one of the power transistors (2N4918) is burnt but not sure. Everything is so epoxied that you can't really remove anything to test it. So I'm thinking it'd be easiest to just make a new board. I've done a lot of work with Arduinos, and custom PCB, so this should be easy to make. I just need a schematic of a variable voltage supply. Well, not the power supply, because I already have the 12VDC, but need to output it as 0 to 12 volts. Would someone know of a little circuit that puts out 0-12VDC, controlled by a 65K POT. That's the original POT on the joystick. The power transistors on the old board are rated at 1A, so that's what my new board should be rated at.
Thanks.

 

It puts out 6VDC at center. It goes from 6 to 12 VDC in one direction, and 6 down to 0 volts in the other direction.

Are you sure? I can't trace the circuit fully, but it looks to me as though the 741 is wired as an oscillator for generating a PWM output. I would expect a PWM output rather than an analogue one, since the power transistors apparently have no heatsink.
Did you measure the output with a DMM or an oscilloscope? A DMM would probably display 6V for a 50% duty-cycle PWM output. A 'scope would give a better indication of what the output really is.

Edit: Here's how a 741 could be wired as a PWM oscilltor

 

I'm not sure about that but I'm pretty sure it's just voltage driven. I measured it with a fluke measure on lowZ. The out out wire goes down to the two hydraulic solenoids, with each of their second lead connected to 12 CDC and gnd. So the voltage going up or down from 6 volts causes one valve to open more and more and the other to close.
There's nothing in there that could be pwm controlled. I don't think. It dates from early 80's.

 

If you can read the colours on the resistors and the values on the capacitor and we can work out what that grey disc is, it shouldn't be difficult to trace out the circuit from your pictures. You already have a good PCB (even if you need a chisel to get some of the components off). I would suggest either repair or rebuild with new components on that PCB.

 

That gray disc is PTC 975

I wrote P-Q Controls. Sounds like they might be able to come with a schematic. Should here back today. So that'll save having to figure out what some of those cap values are. I think I can tell all the resistors. I think there's one zener diode, too. I hoping for a schematic. ,

 

I'm not sure about that but I'm pretty sure it's just voltage driven

That could still be done by PWM. The inductance of the valve coils would smooth the PWM into a steady DC current. Far less lossy than using a wholly analogue approach.

 

Ok I found this in the manual. The part number, and they call it square wave generator.

 

That's encouraging. Here's an article which explains the PWM method of driving proportional valves. The article mentions current feedback. Your circuit may involve that, as it seems more involved than a mere PWM voltage source.

 

Thanks. I think you right. I need to be look for swuarewave generator schematics instead of variable voltage...

That's encouraging. Here's an article which explains the PWM method of driving proportional valves. The article mentions current feedback. Your circuit may involve that, as it seems more involved than a mere PWM voltage source.

 

If you're going to build something from scratch, rather than just repair the circuit you have, then a good start woul be to reverse-engineer your circuit completely (if you can't get the schematic from P-Q) to make sure you can replicate its functionality.

 

If you're going to build something from scratch, rather than just repair the circuit you have, then a good start woul be to reverse-engineer your circuit completely (if you can't get the schematic from P-Q) to make sure you can replicate its functionality.

That's what I've been working on today. I came up with this schematic. I know nothing about op amps, and square wave generators, but I've triple checked my schematic from the board. Can anyone tell by looking at the schematic that it will work as a square wave generator. Or does any errors jump out at you? The only part I couldn't identify was Q3. Q1 & Q4 are the little guys (3906 & 3904), that control the two bigger ones, and Q2 was definitely a 2N4918, so I'm guessing it is the Q3 is it's NPN counterpart, which I came up with a 2N4923.

I'm going to order this PCB on monday if no one finds any errors. Thanks for looking.

 

PS. Since I don't know exactly how this functions, I probably don't have the schematic drawn up right to make sense of what it does...
I'll include the eagle files here if anyone can tune it up a bit so I understand it better, that would be great!
I notice that the leg of the POT is actually connected directly to the square wave output. I guess that's right, I've double checked it.

 

Oh, I left off a junction marker which made the schematic ambiguous. Here's an updated one.

 

The schematic is missing a junction dot which changes the appearance of its function. The right hand side of the PTC is connected to the squarewave output. {update: but you spotted that}
On the PCB the labels 'PUMP' and '12VDC' are swapped.
It certainly looks like it will generate a PWM signal under control of the pot.
Do the two black wires from the original board go to a switch connected to joystick lever that switches the PUMP power?

 

Nice work. What does 'DGND' signify?

 

Nice work. What does 'DGND' signify?

Ground. That symbol I used for ground had DGND as a label.

 

I'm pretty sure I got it right. I'll recheck everything tomorrow and order the pcb. the components are cheap. And the board is only about $25 for 3 from OSHpark. I heard back from the company that makes the board. They said they couldn't find any documentation on the board. JLG lift sells it for $285, though! That's a little steep.

 

Steep? It's a rip-of!
I tried a simulation of the post #13 circuit, but it fails to oscillate. There seems to be a problem with the biasing/driving of pin 3 of the 741. Can you re-check the circuit around that area?

 

There seems to be a problem with the biasing/driving of pin 3 of the 741.

Yes, there is a problem there somewhere. When the square wave output is at 0V, there is a path to +12V with D1 and D2 with no current limiting resistor.

 

Oh, thank you so much for checking that circuit for me. I will redraw the whole thing this afternoon and check for errors and get back with you. I sure appreciate someone being able to test it out for me. I really have no way to test it. I'll post back this evening.