Pratik's Build log

Aug '20: I graduated from SDMines in Dec 2020 and work as a controls engineer. I want to make my way up to becoming a design engineer at some point but job prospects for the same are slim for me right now (International student in the USA).

Since I graduated I’ve built a couple of things: I’ve reverse engineered my roku remote and rebuilt it from scratch using a stm32 on a pcb and everything. I’ve also played around with a analog voltmeter design using comparators (It fits in your wallet! but has a LED bar graph for voltage indication).

Something i’d really like to work on is a proper theremin. But i keep having issues that i don’t know how to fix. Especially with my oscillators i hope people here at CE can help me finally finish this project. It’s been bugging me for quite some time now.


Hey Pratik! The theremin sounds like a great project to improve skills. Is it digital or analog? Are the design files online anywhere?

Hey Chris! I wanted it to be analog. Feel like digital would be cheating. I was designing it from scratch. So no design files. I only have some notes in my design notebook.

I got my oscillators swinging around at 800kHz. But body capacitance wasn’t doing much at that frequency and I couldn’t find any down converters that go that low.

When designed for 7MHz and got it to work i couldn’t get a low enough output impedance to drive the SA602 mixer i was using. I just gave up on it after that.

Do you have any design resources for oscillators? I’m yet to find anything that comprehensive enough.


I found a oven on facebook marketplace and decided i needed a reflow oven. 2 weeks later I had all the parts I needed and it was built. But now i needed a test board. I made an Electronic load with a STM32F103. Wrote all the drivers for the ADC and the DAC and interfaced the buttons everything worked great. Till yesterday…

When I went down to ~6mV, I’m not sure why but HAL gives me HAL_Busy error. I might need to pull out my Bus pirate and investigate why. Then I set the the load to max current and i blew up my opamp. Not sure how this could have happened. The Opamp was biasing a FET so it couldn’t have sourced to much current. Long story short, now when I plug my board in, the Opamp gets really hot. No clue how this could have happened.

My guess: you drove the MOSFET out of saturation to a point where it conducted enough current but had relatively high resistance. Its max power dissipation was exceeded as a result and that caused it to short out. The short bonds drain, source, and gate with the result that VCC went into the op-amp output. Boom!

This is just a guess, perhaps others will chime in and tell a different story…