I recently had to get rid of my rarely-used oscilloscope due to space restrictions and started looking at alternatives. The handheld devices started from about $70, but had known of some PC kits.
I thought, couldn’t I just use the input for my PC sound card? So came across the Poor Man’s Oscilloscope. I also knew of $300 iPad oscilloscope, which was crazy money.
Sat lonely in my box of old electronics, however, was a upgrade-made-redundant Android device. A brainwave hit and I set about making my $0 oscilloscope. Here’s a video explaining it:
How to Build
The probe is a very simple modified mic and headphone set with a potential divider and some crocodile clips. You will need:
- Android phone with OsciPrime Oscilloscope Legacy
- 4 pole 3.5mm jack (e.g. a handsfree set that comes with a phone)
- 2 resistors in a ratio of roughly 1:10
- Probes (I used crocodile clips) with some cable (multicore is best)
- Perfboard (optional – you could solder directly)
- Heat-shrink / insulation tape
Below is a diagram of how to wire. The potential divider simply reduces the voltage in to the device, lowering the risk of killing the mic-in circuit in the phone.
The circuit is very basic and you could make it all in under an hour, but it’s certainly not fool proof. A more complex alternative using op amps can be found on this Instructable.
You may also run in to the problem of the headphone wire being annoyingly coated. I simply burnt these with a butane lighter, but you can ready more on the technique here.
If you also build one, would be great to hear how you get on.
[...] to debug a circuit and you don’t have an oscilloscope. That’s not a problem thanks to [retronics] $0 Android oscilloscope, made with parts he just happened to have lying [...]
[...] you need to debug a circuit and you don’t have an oscilloscope. That’s not a problem thanks to [retronics] $0 Android oscilloscope, made with parts he just happened to have lying [...]
What sort of frequencies is this good for? I’m guessing since the device most likely samples at 44.1 kHz so based on Nyquist Criteria we will be talking between 20Hz T
to 20 kHz?
That’s a good guess. I don’t know what the ADC is in the device nor if that varies device to device.
Please use a tripod next time.
Am I stupid, or should the resistors be swapped? In config shown here, the voltage won’t be reduced by much…
This config reduces the current by a factor of x0.18, I believe (although I used different values). Will update post with more details.
Have uploaded a screengrab from eTools calculation.
In your diagram, the output (3.5mm jack) is across the 82k resistor, while in you eTools it’s across the 18k one, which would be correct
You’re right. I’ll update that!
[...] to debug a circuit and you don’t have an oscilloscope. That’s not a problem thanks to [retronics] $0 Android oscilloscope, made with parts he just happened to have lying [...]
Does anyone know the maximum voltage input to that mic jack? or is it a more of an impedance change sensing(since its a resistive mic (im guessing here)).
If its a voltage input would it be wise to have a zener across those inputs? and have a low pass filter too
I don’t know. There’s a link to a more complex circuit in the article. What the diode and filter do?
A zener diode would limit the maximum voltage we feed on the jack (keeping the android phone safe) while the low pas filter is for anti-aliasing. if we have a 40KHz sampling scope then the maximum signal it can capture without being influenced by aliasing is 20KHz (half of sampling freq) thus we need to keep the tones above the 20KHz limit out of the input.