VCP200 breakout board

So, I had 3 of these VCP200 chips and I wanted to use them in something so I built a breakout board. In the process I found a bunch of interesting tweaks to the design that I may try in the future, but let’s start with the first one I found and replicated. It’s found in Radio Electronics April 1991.

The whole article has the theory of operation, but the highlight is that the sound is amplified in 3 stages, then clipped, hard, in the last stage to digitize it. This digital signal is fed into the processor to analyze and it picks which set of sounds it most thinks matches what you said. Right away there’s some things I don’t like about this, first is using 4011 gates to drive these LEDs without any series resistors. That’s probably fine, but it’s not my style. Second is that the outputs from this board have series resistors but DON’T got through the 4011 as a buffer. That would limit the current drawn from the chip but not protect it in any way if something pretty bad happened. They also use a 7805 which is pretty standard, but these days we have 5v power everywhere so why would I ever power it from anything but 5v.

After prototyping the circuit on some protoboard (I lost that picture) I made a PCB that I’d be willing to use. This is almost identical for the left half of the board to the design from that magazine. The right half is all me though.

I decided to add LED indicators for all the control lines. I also decided to replace the 4011s with 4070s and using XOR gates as selectable inverters I can configure the signal polarity for any output. If you use this to drive a microcontroller you may not care. If you use this to drive more discrete logic, you may like this built in feature. I had a lot of issues powering this from a usb charger or my bench supply though. What fixed it?

A big cap across the opamp. I think the switching noise of modern supplies just don’t work well for this design. It’s very sensitive. I found that putting it on the back is overkill and I can just add it to the input section, but it’s still worth noting that this circuit works fine on an old linear bench supply, but my switcher was not quite good enough for it.

There was also a layout issue, I forgot the ground pin on the 4070s… oops. Easily fixed though. Now there’s about 5 or 10 variants to the analog part of this circuit that make it more or less sensitive. I think this one works alright. I may do a writeup of the history of this design and the dumping of the rom from this chip later (but we did dump it). I don’t know if anyone still has one of these chips in a drawer somewhere, but I have 3. these boards came in groups of 5. Anyone who wants one can have one (my boards don’t have design fixes, the design on github does).

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