Recently I bought a bunch of components and modules from a new website called InkOcean.in, of which one was a Two Wire 0-30 Digital Voltmeter module. The model number printed on the PCB was DSN-DVM-368. I don't know who makes it, how they make it or the exact specifications. But let's reverse engineer this module.
Here's the schematic of the module.
It has three terminals; one voltage input for measuring, one supply voltage and a ground. There are total 4 PCB jumpers on the board. J1 near the terminals is used to short the voltage input and supply voltage (Vcc) to be the same. Therefore if you short J1, you can use just two wires for measuring a voltage. Because it has a 7805 +5V regulator inside. Due to this, when using two wires for measuring, the input voltage must be greater than 5V. Otherwise the display will dim or stop working due to insufficient voltage. To overcome this, you can disconnect J1 and solder a separate wire for voltage input terminal. In this way, you can measure voltages starting from zero to 30. I was able to measure up to 33V. I used a separate 7809 regulator outside so that the heat generated by the on-board 5V regulator will be less.
Other jumpers J2, J3 and J4 are used to adjust sensitivity or measuring range. The default jumper used is J3 with 510K resistor.
To see the IC number and other component values, you have to desolder the seven segment display module. I didn't do this because I had only one of this and I didn't want damage it. So I had to use an image from Kinoshita Masashige, with his permission. Looking at the picture, it seems like there's no identifier printed on the IC package. But that's expected because most of these Chinese modules come with the part numbers scrapped off to prevent reverse engineering (what an irony!). The controller on the module could be any cheap 8-bit one with a 10-bit internal ADC.
The accuracy of the meter is satisfactory considering the price and its build. A maximum variation of +/-0.05 for voltages below 20V and +/-0.3 for voltages above 20V, from multimeter readings could be expected.
I used this module for my LM317 Benchtop Power Supply. You could buy this module from eBay, Amazon and other electronic parts suppliers in India.