DC 5-30V Voltmeter on eBay - Page 1
Forum section: electronics / projects

author: 4CX35000
author_rank: Regular Contributor
timestamp: June 20, 2017, 04:24:02 pm
content: No idea if this item has been commented on before.Searching eBay I came across this 5 to 30 volt meter. Anyway it cost next to nothing (£1) so I bought two. See attached pictures and eBay item number:http://www.ebay.co.uk/itm/DC-5-30V-Wire-LED-3-Digital-Display-Voltage-Voltmeter-Volt-Meter-Panel-Car-Motor/292078199175?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649Any idea what the main IC is in the middle ?

author: strawberry
author_rank: Super Contributor
timestamp: June 20, 2017, 04:41:02 pm
content: Some cheap 5V microcontroller,try to track down power traces and match to some controllers, PIC..

author: ebclr
author_rank: Super Contributor
timestamp: June 20, 2017, 04:45:11 pm
content: I believe the is on the middle because is the shortest way to gel the Led display pins

author: DBecker
author_rank: Frequent Contributor
timestamp: June 20, 2017, 05:29:52 pm
content: I have handfuls of those around.Most are based on a STM8S processor, some are based on an STC processor, and a few are based on an unmarked chip I haven't identified.The STM8S is appealing because it's cheap, operates on 5V, is cheap, can directly drive a multiplexed LED, is cheap, has enough pins, and is cheap.Operating from 5V is important because most varieties are operated directly from a 7805 type regulator. The cheap way to get 3.3V is to use a bipolar regulator to 5V and then a LDO to 3.3V, which would double the component count and cost.Most designs use resistors only on the digit common pins, or skip them entirely. I'm guessing that they adjust the drive time based on the number of segments illuminated.The STM8 has only 10 bit resolution, but there is plenty of noise to do averaging. Even with this extra work, the measurement update rate is often quite slow.

author: 4CX35000
author_rank: Regular Contributor
timestamp: June 20, 2017, 07:02:09 pm
content: One of those smack around the back of the head moments for me as the clue was the voltage limits being 5v to 35v, seems obvious that is was some type of microcontroller. Had me thinking a manufacturer has released a new cheap measurement chip.I have to admit for £1 the eight segment and black chassis case probably cost more than that to buy.Anyway thankyou all those who replied.

author: floobydust
author_rank: Super Contributor
timestamp: June 20, 2017, 07:09:06 pm
content: I've bought them and was not happy.They have low resolution A/D and readings are quantized, I think it is a low end chinese MCU. Notice no dropping resistors to the LED display, so very noisy power and find the capacitor...I have had really good luck with"Electrical Parameter Tester"which has V, I, Ahr, time, temperature etc. using STM32 and OLED.

author: DBecker
author_rank: Frequent Contributor
timestamp: June 20, 2017, 09:03:38 pm
content: Quote from: floobydust on June 20, 2017, 07:09:06 pmDon't judge the whole category by a single sample. There is a huge range of performance and behavior in the $1-$2 range.Something that costs 10x isn't in the same category. Of course it should be vastly better.

author: TechieTX
author_rank: Contributor
timestamp: January 25, 2018, 04:51:02 am
content: There's a cheaper variant out now using a 0.28" high display (see the pix). I haven't figure out what chip is doing the driving, but for just under a buck each who cares?Specs are 4V to 30V supply, 0-100V sense in the 3-wire version, and 4-30V sense in the 2-wire version. For 2-wire they install a 0 ohm resistor at the empty pads on back, and it only displays the voltage across the two wires. Looking at the schematic someone else drew, I'm at a loss for the 16 SOIC part.Look for 0.28 voltmeter on your favorite site (dx, aliexpress, ebay, banggood, etc.).