I bough 3$ digital DC voltmeter from ebay. I needed one for my power supply and I have became too lazy to build such things myself. It looks awesome, is not very precise. I found SMD potentiometer from the back that calibrates the voltage for about 1V, so, yea. Not precise at all. So, teardown time!
PCB is reasonable quality, two sided, plate through holes. SO-16 package in the middle that does all the work, part number is sandpapered off. I searched a bit and now I know that it is not a AVR nor a PIC. Probably not ASIC either because cheap chinese things have these in die bounded to PCB, but you never know. Pin 1 is GND and pin 2 is Vcc. Power circuit has protective diode in series and then 3.0V SOT89 regulator with 10µF tantalum capacitor for power. The display is 3x 7 segment display with dots. It has 8+3 pins so everything is multiplexed, bad for me, I wanted to change the comma position.
The input goes through 330k resistor and 4.7k potentiometer, so it should affect the input by about 1.5% of the full 100V scale. There is 10k resistor and 10µF tantalum between signal and input. So, some heavy low pass filtering and about 1/34 voltage divider. The input signal goes to pin 14.
I tore it apart to see if it can be easily modified to a ammeter. The cheapest ammeters cost about 3-5x more than voltmeter. To modify it to ammeter I have to modify voltage divider ten times smaller to measure shunt voltages from 0-1V. Also more of an visual thing - I have to move the dot one place left. But the placement of the dot is determined by the chip, so I can't move it. I can use it for my own purposes but I can't sell an unit with wrong comma placement, even to a friend. Damn.