Variable capacitor

Yea, I am interested... Let me know what the solution is. Would it be controlled by a pot? I feel that I am indeed missing something clever.

I see. Let me rephrase what you said to make sure that I understand.

Current that would be normally charging a fixed capacitor, in fact is an input to a variable gain of the opamp. The output of the opamp is what charges the capacitor. The gain is controlled by a pot. So, the rate f charge is controlled and thus XR2206 sees this situation as changing capacitance.

I will check around. I may do different implementations of this later on and start with simple caps, and would switch to a controlling pot later.

A PIC. Generate the pulse train directly, rather than mess with all that icky stuff. Read two pots -- one for frequency, the other for duty cycle -- and put the PWM out on a digital pin. One chip, two pots, and a bit of wire; dead-bug manhattan style will do nicely. :)

What do you say? I'll stick your SO24's on for you too. All I want is first refusal on the blowed up 'zuki motor. (Actually, I think you were meant to have that. Someone wants you to weld up a kart frame and hurt yourself.)

What he said, but look also for thumbwheel switches with BCD output.

Seems like it would end up being a huge distraction for what I need. XR2206 and some bundles of radial caps will do the job. I already have the rotary switch and working XR2206 on a breadboard.

Programming PICs in C is not something that's worth doing for this simple app (simple frequency and duty cycle control). A single mistake could end up being very expensive.

I program in C++ all day long, so it's not like I am afraid of programming.

I'm not sure why I didn't think of this earlier, but probably the cheapest approach for frequencies below maybe 2kHz would be to generate them using a PIC microcontroller or similar. You could make programmable duty cycle and frequency, interface it to a cheap rotary encoder and maybe even a LCD.

I have done something similar in the past using the HiTech C compiler on a PIC16F84A, but I am aware that there are probably much more suitable microcontrollers around and someone else will be able to suggest one. I didn't want to use the timer module for some reason when I did it, so I ended up writing routines which were calibrated to execute in a specific number of cycles by padding them with asm("NOP"); lines or something like that. I calibrated the code using an oscilloscope on a port pin so that for example the delay would not change depending on whether an 'if' statement was true or not.

This will end up being much cheaper than a new rotary switch, though if you can find the rotary switch in your junk pile then it is a better proposition.

Chris

The only hard part is getting started. You need something to shove the hex file onto the chip. All mid-range PICs can be programmed in place through a simple 6-pin header; no complicated circuitry, no expensive mistakes. Just update the firmware to fix it, the same way you programmed it the first time. The PICkit2 costs $50; off-brand ones with in-circuit debug cost not much more. HiTech's PICClite C compiler is free for the download.

It's about 20 lines of C code to set up the CCP and A/D, plus maybe 30 more lines of boilerplate IRQ and device setup. You could almost be done before the iron warms up, and certainly before you're done paralleling the second or third batch of caps. (All of which seems a perfect waste.)

PIC 12F675, a 6-pin header, two panel pots, and 3 bypass caps. $5USD at single qty prices. How are you going to beat that?

The only hard part is getting started. That's plenty to scare someone off.

Can't see the point of doing all that now. I need a working TIG square wave inverter. A bunch of caps and a rotary switch can get me going in a couple of days and let me continue. PIC programming will take a long time to figure out.