DSO150 Oscilloscope Module
As mentioned above, as I was rootling around on the manufacturer’s website  I noticed the DSO150 oscilloscope kit (see Figure 2). This device is a development of the DSO138, using the same display and microcontroller. However, this model is only available in kit form with an enclosure. The whole user interface apart from the switch to select the input coupling is driven using four buttons and a rotary encoder. The interface is straightforward and it is hard to see how it could be improved. This kit also comes with the SMD components already mounted. There are just a few leaded components that need to be soldered to the board and then the unit can be put together and tested. The instructions are of a high standard and none of this will present any difficulty to a hobbyist with a little experience. Because the device is supplied in kit form it is not subject to the various standards and regulations that apply when ready-assembled units are sold. The lack of a CE mark is not really a problem, as a manufacturer can always add it later to a finished unit.
I spotted the biggest shortcoming in the unit immediately after assembling it, when I first turned it on: the zero level shifts with changes in the power supply voltage. Since once again a circuit diagram is supplied it was easy to find the cause: unfortunately in the DSO150 the linear regulator to stabilize the supply for the input amplifier has been omitted! The offset for the ADC input is set using just a Zener diode, which does not provide a very well-regulated voltage. As before, you get what you pay for. Fortunately, however, there are leaded series resistors in the positive and negative supply lines, and it is a relatively easy job to replace these with 78L05 and 79L05 voltage regulators. The supply for the Zener diode is now moved to the regulated 5 V rail, and the zero level is stable.
As in the DSO138 the vertical offset is done in software in the microcontroller. The full-scale drive amplitude is 640 mVPP, and so only about 20 % of the ADC’s available input range is used. Compared to the DSO138 interference is much less visible on the DSO150, and occurs less frequently. It is also possible to add an external trigger input to the DSO150.
The unit is an excellent gadget for experimenting with and could make a good secondary oscilloscope for a hobbyist. After a bit of modification to the power supplies it works perfectly acceptably (see Figure 3), especially when you bear in mind that it only costs about £25 (US$30) including shipping from China. There is a chance that you will end up paying VAT and other import charges on top of this, as its cost may exceed the duty-free threshold, at least in Europe.