DSO138: a fancy way to measure DCC voltage

I have previously found a way to measure the DCC track voltage using a home made meter. This method has worked for me for some time now - it's reliable and very easy to use. But how about something more extravagant? How about using a real oscilloscope for that?

Oscilloscopes are rather expensive devices and are not really portable. And if they are portable, they tend to be even more expensive. I can't afford spending this kind of money on something that is not crucial for my hobby. Luckily there's a cheaper option.

DSO138 is a DIY oscilloscope kit from JYE Tech. It is easily obtainable on Ebay with prices going below $20 including shipping. I've ordered one from China and it arrived quickly and in good shape. I wasn't expecting a nice box at this price point and I did not get one:

The kit comes in a plastic bag. It contains everything you need to build your own budget oscilloscope. There's a PCB, lots of elements and a probe. It even features a colour LCD screen. Considering the price, it looks really nice.

The set includes a build manual. Its quality is a huge surprise to me. It's several pages long, it's written in a proper English and it really covers the installation  very, very well. It's difficult to make a mistake here, and if you do - troubleshooting steps are provided, too!

Soldering the kit took me around 3 hours. Some SMD elements were rather tricky and I failed to install the USB port properly. But other than that - it was an interesting soldering exercise.

Alright, let's test this thing. The device offers a 1kHz 3.3V test signal that can be used during bring-up and troubleshooting. As soon as that worked, I connected the probe to my 16V transformer. Here's the image I got:

Looks like a proper AC signal to me. The peak voltage seems to exceed 20V which is expected for a 16V sinusoid. Unfortunately this test already shows the biggest limitation of the device. While it is supposed to handle up to 50V peak voltage, the screen and the controls don't allow to see such signal in full. The oscilloscope is limited to a maximum of 5V per division which results in just a 40V peak-to-peak display.

OK, let's finally see some DCC signal. Here's the output of the Fleischmann/Roco system I use for my N-scale trains. I placed the device next to my home made DCC voltage meter and compared the results.

It's definitely a digital signal I was expecting. The peak voltage seems to be just below 15V. My meter shows 13.5V but I have to remember to add around 1.3V due to the bridge rectifier in the meter. So it matches: 13.5V + 1.3V is almost 15V!

Let's try another system. Now I'm using the output of Uhlenbrock Power 8 booster intended for my G-scale trains.

My voltmeter indicates 24.8V (23.5V + 1.3V) and the DSO138 device... went out of range again. The voltage is more than 40V peak-to-peak and cannot be shown correctly. What a pity. I think I need a different probe...

Update

Very soon after I had posted this article, I've learnt of a hidden functionality in the DSO138 firmware. This undocumented feature is triggered by holding the OK button for 3 seconds and it allows the user to display a summary information for the signal under analysis. It looks as follows:

The interesting part here is that the measured voltage values seem to be correct even despite the signal being out of range for the display to show. So perhaps an extra probe is not needed at all...

Summary

So what is DSO138 and is it useful? It's first of all a soldering exercise that makes you feel good once it turns out to be working. Is it useful? Well, it does allow me to measure the DCC voltage. Of course the precision is probably not there but... it's not really needed for my use. The biggest caveat is the limited sensitivity control which makes the device less useful for large scale unless a different probe is used. Would I recommend buying it? Yes, but just for the fun of soldering!