Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Software => Topic started by: Katoda on November 04, 2017, 06:21:04 PM

Title: Simple budget oscilloscope for Android
Post by: Katoda on November 04, 2017, 06:21:04 PM
So I just wanted to share this with you, in case anybody needs to check some audio frequency waveforms and doesn't have the money (or is just cheap like me) to buy an actual oscilloscope.

First I disassembled and stripped the wires from an expired headphones set, the one with the microphone (TRRS), and soldered the wires to a piece of perfboard. Both of the headphone outputs were loaded by 1k resistors, one of them got connected to a 100nF capacitor for AC coupling. Than was the signal output / sine generator output. I loaded the mic input with a 20k resistor and put 3 header sockets onto the perfboard for an attenuator. The signal flow into the microphone input is as follows:
Probe wire -> Coupling capacitor -> one of the header sockets, where you can swap resistors to form a voltage divider -> microphone input.

I just put in a 100nF coupling cap, which together with a 20k resistor forms a high pass filter at 80Hz. Gotta upgrade that to extend the lower range of possible monitoring frequencies.

There are a lot of oscilloscope apps in Play store, I chose SmartScope.
I had a lot more problems finding an app that would continue to play sound when it was not in the foreground.  I ended up with an app called Note Fork from siliconfish, which is a nice little musical note generator, but you can still choose a note closest to the desired frequency. And, most importantly, it is the only app I found that continues to emit sound in the background, although if you have a better phone than I, chances are you will be luckier in finding a more suitable program.

Attached is a picture of a measurement I took to look into transformer distortion. The peak-to-peak value is not to be trusted, but the waveform is nicely visible. Depending on the phones sampling frequency, you can generally measure up to 20kHz.

So if anybody wants to analyze some clipping circuits on a budget, here you go. Hope somebody finds this useful.

Also, coaxial probe leads are quite beneficial in this application.
Title: Re: Simple budget oscilloscope for Android
Post by: trobbins on January 18, 2019, 12:47:05 AM
I'd be a little cautious about using a an expensive android phone for general use, as it may be easy to fry the microphone input. 

But if you have an out of contract old phone gathering dust, or other hand-me-down device, then go for it.  Often a 10:1 scope probe can be connected to a MIC input, and bandwidth may be tweakable by adjusting the probe compensation capacitor (even to the point of removing any fixed cap in the probe).

Ciao, Tim