DIY Tone Generator
I needed a simple battery powered tone generator to help isolate and diagnose a noise floor issue with the Beat Friend circuit. If I can inject a clean triangle wave to the ADC input without any chance of ground loops I’ll be able to measure the problem, and (hopefully?) fix it.
You can buy cheap function generators online, but I’d rather build a new circuit, learn how it works and know that I can repair it. That’s the fun, right?
I found a few schematics, but the 9V Battery Powered Mini Function Generator by Music From Outer Space looked reliable and (like all the MFOS circuits) has a great write-up. It’s all based around a TL074 quad op amp, with a LM13700 OTA to shape the triangle wave into a sine. I wanted to keep it simple to build it on perfboard and I don’t particularly need a sine wave, so I scribbled out the sine wave part and kept the triangle and square wave outputs.
MFOS Function Generator schematic with the sine wave function scribbled out
With perfboard I usually just start soldering and hope for the best, but I’ve learned that a little planning saves a whole lot of tedious bodges and testing later. So I downloaded DIY Layout Creator, every DIY guitar pedal maker’s favourite circuit sketching tool. It’s actually really great. By the end of the evening I had an ok looking layout and had triple checked all the connections.
MFOS Function Generator perfboard layout
Here’s a PDF of the perfboard layout if you’re interested. It took a couple of hours to put it all together, using bits and bobs I had lying around.
Function generator on perfboard
I only made one stupid mistake (the TL074 was the wrong way around so it got very hot). Once I figured that out, it worked a charm.
Function generator output on the scope
I put it in an empty box that used to hold COVID-19 tests. Not the most hard-wearing enclosure, but it’ll do.
Function generator in its new enclosure
Right. So much for the warm up. Time to test some drum machines…