Another ebay buy: 0-30V panel voltmeter - Page 1
Forum section: electronics / testgear

author: brainwash
author_rank: Frequent Contributor
timestamp: July 03, 2013, 06:25:02 pm
content: Not really test equipment but not sure where to put this.I just bought a little 3 digit panel voltmeter from ebay, cost was 2.12$ with shipping included. It's a great little module useful when building supplies or just to have one inside your car/motorcycle.Although it has 3 leads, you can short the blue (sense) and red (power) leads together and use it as a normal 2-lead panel voltmeter.Lowest supply voltage is 3.7V, it can take at least as high as 16V. I see no brightness change with supply voltage change and no flicker.It autoranges at the 10V mark so you only get one decimal resolution above 10V. Autorange time seems to be about the same as refresh rate, 5/s.Claimed accuracy is 0.1%, I haven't tested it but sounds a bit too good. Claimed input impedance is 100k, I haven't tested the input current.Interestingly enough, the PCB has two pads labeled TX and RX, I haven't looked into those but it would be nice to be able to read back voltage.

author: mariush
author_rank: Super Contributor
timestamp: July 03, 2013, 07:07:22 pm
content: Well, it would help if you'd actually post some real pictures of it, and the back side.My guess, microcontroller with 10bit adc running at something like 2.5v-3.3v, linear regulator to get 2.5-3.3v out of 3.7-30v, a handful of resistors.With a 10bit adc, 2.048v voltage reference you get 0.1v per bit for a 0-100v range. later edit.. actually no, should be less precision than 0.1 if I think about...you'd lose some accuracy when getting the test voltage down to 2v or so using a voltage divider or something like that, then the adc noise etc..

author: Unknown
author_rank: Unknown
timestamp: July 03, 2013, 07:43:42 pm
content: This might be perfect to put in parallel with that 7Ah battery I just got and am charging now. Hmm.EDIT: I just ordered one, or similar. I think this would be cool to hardwire to that battery w/ a switch, that way I'll know if it's charged.

author: brainwash
author_rank: Frequent Contributor
timestamp: July 03, 2013, 09:01:07 pm
content: Quote from: mariush on July 03, 2013, 07:07:22 pmI'd have to take the thing apart (unsolder digits) there's nothing on the back side. It probably has a voltmeter IC.

author: bilko
author_rank: Frequent Contributor
timestamp: July 03, 2013, 09:45:22 pm
content: Quote from: mariush on July 03, 2013, 07:07:22 pmSpot on !I have one, the cpu isSTM8S003F3Here's the seller's link and photos of the rear of the boardhttp://www.ebay.co.uk/itm/2x-4V-30V-2-Wire-Blue-LED-Display-Digital-DC-Voltmeter-Panel-Meter-F-Motorcycle-/290726855960?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item43b0abc518

author: brainwash
author_rank: Frequent Contributor
timestamp: July 04, 2013, 02:37:25 am
content: You're right! As overkill as it seems it is an STM8 chip.Pinout at the connector:Code:SWIM / PD1 (HS)TX / AIN5(HS) / PD5RX / AIN6 (HS) / PD6NRSTVDDVSSSince everything is already broken out this might be the cheapest dev board around there and it also serves a purpose.STM800S3F3 datasheet:http://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/DM00024550.pdf2.95-5.5V, internal RC up to 16Mhz, 3 timers, 4 capture/compare/pwm modules, 5 channels 10-bit ADC, 27 interrupts, UART, SPI, I2C, debugging.The inputs can take 6.5V, the HS outputs can sink/source 20mA.

author: ftransform
author_rank: Frequent Contributor
timestamp: July 07, 2013, 01:23:00 am
content: How noisey is it? Is it a SMPSU or Linear?

author: rexxar
author_rank: Frequent Contributor
timestamp: July 07, 2013, 03:26:27 am
content: Quote from: ftransform on July 07, 2013, 01:23:00 amIt's got a little LDO linear regulator on the board.

author: brainwash
author_rank: Frequent Contributor
timestamp: July 07, 2013, 03:48:03 am
content: Holtek 7130 3V/30mA LDO, you can see it in the picture.

author: baoshi
author_rank: Regular Contributor
timestamp: August 11, 2013, 01:25:56 am
content: I got several 4 digits one using the same STM8 controller. 10 bit ADC is definitely overrated for 4 digits. So i modified it to I2C slave LED like Adafruit's. Blog post herehttp://www.ba0sh1.com/hacking-a-cheap-led-voltmeter/(sameless self promoting)

author: echen1024
author_rank: Super Contributor
timestamp: August 11, 2013, 02:39:39 am
content: I just ordered a couple ICL7107 samples from Intersil, and am about to build a panel voltmeter from that.

author: Electro Fan
author_rank: Super Contributor
timestamp: August 11, 2013, 02:47:12 am
content: www.adafruit.com(Arduino site) sells several similar meters for volts and amps; they can be handy for roughly approximate metering in circuit breadboardinghttp://www.adafruit.com/index.php?main_page=adasearch&q=volt+meter

author: quantumvolt
author_rank: Frequent Contributor
timestamp: August 11, 2013, 12:05:13 pm
content: Quote from: baoshi on August 11, 2013, 01:25:56 amVery good blog piece. Something fresh instead of all the threads where everyone is presenting their last dusty broken junk 1970ish instrument they neither know how to use or need. Thanks a lot.

author: amyk
author_rank: Super Contributor
timestamp: August 11, 2013, 01:25:33 pm
content: Quote from: brainwash on July 07, 2013, 03:48:03 amThat's not a Holtek, it's aMingda7130H. 30V maximum input voltage.