Oscilloscope 
Android Audio Oscilloscope. The app is available on F-Droid and here. See the Wiki for more screenshots and documentation.
- Timebase 0.1 ms - 0.5 sec
- Sync control
- Single shot
- Storage
- Index
- Spectrum
Using
The icons on the toolbar are, from left to right:
- Bright line - Defeats the sync and single shot
- Single shot - Touch the trigger icon to get a trace
- Trigger - Touch for a trace
- Timebase - Drops a submenu of timebase timings
- Storage - Does not clear the display between traces
- Clear - Clears the display
- Left - Moves the trace to the left
- Right - Moves the trace to the right
- Start - Moves the trace to the start
- End - Moves the trace to the end
- Spectrum - Spectrum display
- Settings - Settings display
The last two or six items may be on the menu, depending on the size of the device.
Oscilloscope
Touching the display will produce a vertical index line, which will show the value at that point. The value is arbitrary, as there is automatic gain control on the audio input so the trace does not get clipped. The display will respond to scroll and scale gestures. The index can be removed and scaling reset by touching the Start icon.
Touch the left edge Y scale to adjust the sync level. A pointer will appear which can be adjusted by sliding up and down. Touch the start icon to reset the level to zero.
In single shot mode the timebase may be changed to zoom in and pan left and right using the right and left icons.
Spectrum
The spectrum display shows a spectrum with a logarithmic frequency scale. An index line shows the dominant frequency in the input. The frequency and input level is also displayed in the toolbar. To lock the display, touch spectrum display or the lock icon in the toolbar. Touch it again to release the lock. To display the help screen, touch the help icon.
Settings
The audio input source may be selected. Some of the selections will not be available, depending on the device. The fill option selects spectrum trace fill. The hold option selects spectrum trace hold. The screen option will stop the screen turning off. The theme option selects the app theme. The about item shows the version and copyright.