DonsDeals Blog: Hacking cheap LED Voltmeters and use them as Displays for other Purposes

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Sunday, August 25, 2013

Here are a couple interesting Projects. That Hack Cheap LED Voltmeters and use them as Displays for other Purposes. Check em out...

Don

I’m planning to build a EEVblog-ish constant dummy load for battery and power supply testing. Dave in his build used a LCD voltmeter for the display. In the senseless pursuit of difference, I tried other display solutions including character LCD, graphics LCD, OLED, TFT, AMOLED, IPS, which resulted in several previous blogs but nothing ends up to be satisfactory. Along with the complexity grows exponentially from one solution to another, I slowly start to feel the importance of KISS concept, keep it simple stupid. All I need is just a number display, nothing fancy, nothing pricey, nothing takes my attention away from the analog circuitry. I decided to go back to the basic 7-segment LED display. “Simplicity is the ultimate sophistication”, how elegant it is!

I ordered several LED voltmeters from a taobao seller. It costs RMB16 each, about US$2.6 excluding shipping. The original design was for battery measurement on scooters. The advertised accuracy is 0.2%, which turns on my bullshit detector immediately. So the first thing I did after receiving it, was “don’t turn it on, tear it apart”. The manufacture did not scratch off the markings so the schematic can be easily traced out.

Read More...
http://www.ba0sh1.com/hacking-a-cheap-led-voltmeter/

I2C LED Display From Hacked Voltmeter (Magic Smoke Blog)

LED voltmeters are a cheap source of 7 segment displays and can easily be repurposed. Using the I2C protocol, a master microcontroller such as an Arduino Uno can govern multiple displays with just 2 I/O pins.

Hello, Pleased to Meter You

The Adafruit 4-digit 7-segment LED backpack is a handy little board, and Ladyada's tutorial and Arduino library make it a snap to use. It's as easy to connect 8 displays as a single one: just set them to have different slave addresses and hook them all up to your Arduino's I2C pins. The trouble is that 8 displays will set you back around $80 (or a little more if you like the colour blue).

Looking around for an alternative, I discovered an internet retailer selling cheap LED voltmeters in both red and blue. The listing titles mentioned "Stm8s003", the name of a series of small microcontrollers. For around $30 I bought myself 8 of the voltmeters and the STM8S discovery development board to reprogram them with. Similar voltmeters are available elsewhere, including eBay.

Then I had the most amazing stroke of luck. While I was waiting for the voltmeters to ship from the Far East, I came across an excellent blog post documenting a very similar project. The author had done an amazing job of writing firmware that would emulate the Adafruit board so that the hacked voltmeter would be compatible with the existing Arduino libraries. The hard work had all been done for me. All that remained to adapt the firmware to the slightly different voltmeter that I would be using.

Hardware

The voltmeters accepted between 3.5V to 30V on the red (V+) and black (V-) wires. On the reverse of I identified a voltage regulator that provided 3.3V to the STM8S003F3P6 MCU which drives the LEDs.

One or two of the voltmeters looked slightly different with a trimmer resistor for R3 and and extra capacitor C1:

With soldering iron and tweezers in hand, I sacrificed one of the voltmeters to recreate the schematic as best I could. The hardware I2C pins on the MCU were both in use, but it was apparent that pins 19 and 20 on the MCU could be reassigned to bit bang the I2C protocol.