$25 Color Oscilloscope! The Incredible DSO138 (4K)

I wish I had this when I was learning electronics in high school. After the resistor, capacitor, and inductor basics, oscilloscopes are almost indispensable in next stage electronics. Most electronic equipment contains oscillators, from clocks to computers, and an o-scope is a critical diagnostic tool.

Video of it in use with joule thief and frequency generator:    • Using The $25 DSO138 Oscilloscope - J...  
$23 Fnirsi-138 Pro Vs. $25 DSO138 Oscilloscopes    • $23 Fnirsi-138 Pro Vs. $25 DSO138 Osc...  

This one cost about $25 with the case (but I'd skip the case next time). It has many of the functions of my $600 scope.
Personal Observations: Its upper limit is 500KHz, but is better starting at 400KHz down. It is not automatic, you have to be close with the timing before it syncs and gives good detail data.
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INFO I HAVE SCRAPED TOGETHER. Supplied "as is". You take all the risks for how you use it!

OPERATIONS:
[SEL] button: Selects parameter to change. Selected parameter will be highlighted blue. Press on [+] or [-] button to change parameters options. Press & hold SEL to save settings.

[OK] button. Quick press: Run / Stop. Long press: detail measure on/off.

[CPL] switch: Sets coupling to GND, AC, DC. If GND, scope input is isolated from input and connected to ground (0V input).

[SEN1] & [SEN2] switch: Adjusts sensitivity. The [SEN1] X [SEN2] = actual sensitivity. Displayed at panel lower-left corner.

[RESET] button: Resets system & reboots the oscilloscope.


TRIGGERS / MODES
[AUTO] MODE: Is the same as Normal mode, except it forces the oscilloscope to trigger if the trigger conditions are not met.

[NORMAL] MODE: Displays a waveform when the trigger conditions are met, otherwise the oscilloscope does not trigger and the display remains unchanged. Normal and single modes are useful for capturing sparse or single waveform.

[SINGLE] MODE: Is the same as normal mode except that oscilloscope will enter HOLD state after a trigger has been detected and waveform display has been updated.

TURN ON/OFF MEASUREMENT DETAIL: Press [OK] button for about 2 seconds.

WAVEFORM SAVE/RECALL:
a) Press [SEL] & [+] simultaneously: Save displayed waveform to non-volatile memory.
b) Press [SEL] & [-] simultaneously: Recall saved waveform.

0V LINE ALIGNMENT
If the 0V line (the trace corresponding to 0V input voltage) does not match the VPOS indicator at the screen left border, it can be fixed by performing the “0V line alignment” function. 1. Set the [CPL] switch to GND position. Then press [SEL] button until VPOS indicator is highlighted and hold down [OK] button for about 2 seconds. The trace with be aligned to VPOS indicator when you release the [OK] button. It is normal for some residue mismatch to remain at the highest sensitivity settings.

PROBE CALIBRATION
Problems may need calibrating for more accurate results at high frequencies.
1. Connect the clip to the test signal terminal and leave the black clip (ground) disconnected.
2. Set [SEN1] switch to 0.1V and [SEN2] switch to X5. Set [CPL] switch to AC or DC.
3. Adjust time base to 0.2ms. You should see a square waveform. If traces are not stable adjust
trigger level (the pink triangle on right screen border) until it is stable.
4. Turn C4 (capacitor trimmer) with a small screw driver so that the waveform displays sharp right angle.
5. Set [SEN1] switch to 1V and [SEN2] switch to X1. Keep all other settings unchanged. Adjust C6 until a sharp right angle waveform is displayed.

TEST MODE: Is used to find possible opens (for all port pins) and shorts (for pins PB0 - 15, PC13-15). It first checks PB and PC pins with special patterns to find possible shorts. LED will blink fast if found. Otherwise, it will generate 3.3V and 0V alternatively at each port pins (PA, PB, PC, PD) at about 4 seconds. These signals can be used to check for opens.
How to use: Hold down SW4 and press RESET to enter Test Mode.
If LED is blinking fast that means there are shorts on PB or PC pins.
Find out the shorts before continuing.
If LED is blinking slowly, use a volt-meter to check each pin related connections that are suspected open. If there is no voltage change at a point which should be connected to a port ppin there may be open between the point and port pin.