DSO138 trigger understanding

Thread Starter


Joined Oct 17, 2023
I'm originally a software developer and on the side a (mediocre) self-taught electronic hobbyist.
I recently acquired a DSO138 to better see what is going on in my little electronic projects.
I've read the DSO138 manuals and tutorial upside down, and yet there are a few things I cannot understand regarding the trigger feature.

My playground is pretty simple, I have a 5v source, which is conditionally sent to the DSO138 probe through either a on-off button or a potentiometer.
I am setting the DSO138's trigger settings on NORM (UP) 1.0V, and I was hoping the screen would refresh everytime I press the button (providing 5V), or raise the potentiometer above 1V. This does not happen. Instead, the curve is frozen (the DSO138 is in "Running" mode) and never refreshed, regardless of my actions.

- Is Auto/Norm/Sing only working for repetitive signal e.g. square/sin/saw signal ? Or can I use it to capture a single wave form when it occurs?
- What is the rational that makes the trigger level change with the sensitivity/vertical scale, and not stay the same value?
- The left arrow seems to be a calibration for 0V. Why is the right arrow (trigger level) moving with it? imho it should not be linked to the left arrow, but snap to the grid according to the chosen value. Because my DSO138 has almost 1 division vertical offset, the right arrow's position seems pretty useless, as one cannot directly use its position to infer the value.
- Why dont I obtain the behavior I expect?

Thanks a bunch!


Joined Jan 27, 2019
Welcome to AAC.

Triggering happens when the voltage of the waveform passes through the value set, rising or falling depending on the selection.

If you want to be able to see a button press or other relatively long, single event you need to use Single mode for the trigger. It will capture the waveform it sees over the span represented by the full screen.

You have the timebase set to 0.2s/div. That means the entire span with be 2.4s, is that what you intend?

Single mode is like a snapshot, and it is very useful—but it will not retrigger by itself, you will have to hit the Run/Stop button to start it up again.

You've got the scope set to 2V/div so the peaks on that line of noise appear to be at about .5-.7V. It doesn't look like you've got a signal going into the scope.

First, take a DMM and make sure you are actually getting the 2V from your circuit.

If so, try setting the trigger mode to Auto, the vertical to .5v/div, and the timebase to .1s/div. Then apply your two volt signal to the probe.

The trace should jump up and down, not really make a meaningful waveform, but if it does, try Single mode with the Auto mode trigger.

I strongly suggest you get a better scope. That scope will be a frustration box, and has little use in practice. Ironically, it takes a very experienced user to interpret the data from something like that. If you are on a limited budget, you should consider something like the ZOTEK ZOYI ZT-702S, also sold by ANENG as the AOS-02. The links are to Ali Express where you can get either one for a bit under $60.00.

The scope has a 10MHz rating, in practice for other than clean sine waves it gets to about 5MHz—but for the money that's very impressive. It also has an excellent, intuitive UI which is another thing these cheaper meter/scope combos tend not to have.

It allows for screen capture and USB Mass Storage style retrieval from its USB Type-C connector which also provides a way to charge the internal 18650 cell that powers it. The cell is not a very good one—rated at 2000mAh—but it seems to provide about 6 hours of runtime. It is in a normal holder, so a quick upgrade would be to put in something around 3000 or 3500mAh.

It comes with a single, switchable 1x/10x probe—not the best but not bad. The 10x probe will be a big leap compared to what you have now. And, as a bonus, it is a very decent, 9999 count DMM with very high DC sensitivity. It's a win-win.

If you can, and want to spend more, there are a couple of ≤$200.00 options that AAC members have been very happy with.

Good luck.

Thread Starter


Joined Oct 17, 2023
Hello Yakov, thank you very much for your answer and your recommendations.
I am not sure I understood everything you mentioned. Here is a short video to better show my issue.
First part in AUTO mode, it works fine. As I press and release the button, the signal goes up (5V) and down (0V). Perfect.
Second part, I switch to NORM. When I press the button, you can see nothing happens, despite the trigger level being at 3.06V.
Same behavior in SING (not in video).

> Oh, and you should be able to align the 0V reference with the graph origin using a position control.
This seems to be what the left arrow ("Vertical Position Indicator") is about, right?

>I strongly suggest you get a better scope
Probably I will, if the DSO138 doesn't hold its promise. But so far I havent found out whether I'm not understanding something, or if this device is defective...