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Simple budget oscilloscope for Android

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Katoda:
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.

trobbins:
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

blackcorvo:
Buy one of these little digital Oscilloscope kits:

DSO138 - https://www.ebay.com/itm/263551058348
DSO150 - https://www.ebay.com/itm/223308701225

They're plenty good for most applications, and cheap to replace if you ever fry them.
And while at it, get some "proper" probes as well:

https://www.ebay.com/itm/153317744094

phatt:
I'd be interested to know about scope packages (USB and like) I know that sound card systems are rather limited.
And as *trobbins* has noted You can blow up the input if you are not savvy.

I have seen computer scope intefaces but they are quite expensive.
Is there a cheaper alternative?
I've never owned a scope but would look at the idea of computer based system if it was not too expen$ive.  I don't use mobile phones,,  :duh xP :loco :trouble cept for a basic unit so my other half can contact me if I'm out.
Phil.

trobbins:
I lucked upon an EMU0404 USB interface for just a few $.  It's key advantages are a 1Megohm unbalanced input with wide attenuator/gain stage, and a wide range headphone/line output, and a very low noide floor, and a 192kHz sampling rate.  Add a standard scope 10:1 or 100:1 compensated probe, and a battery power supply, and a battery powered computer or USB isolator, and REW software, and you get a top class spectrum analyser for audio for under $50.  Other software can be used for time-domain scope waveforms.

Using an interface without all those attributes forces compromises, and may be not so easy to work through to a practical outcome.  I've tweaked $1 USB soundcard interfaces - not too difficult, but tweaking requires the MIC low impedance ADC input to be raised to 1 Megohm if a standard scope probes is used.  A 100:1 probe goes a long way for protection.
 
An Android device would likely have a similar MIC input issue - I just updated an old Nexus with the aim of setting up a BNC probe input adaptor if I get the chance.  Could certainly make a convenient portable tool.  USB OTG allows a 5V supply for powering any interface opamp.

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