Author Topic: Another ebay buy: 0-30V panel voltmeter  (Read 7387 times)

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Offline brainwash

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Another ebay buy: 0-30V panel voltmeter
« on: July 04, 2013, 04:25:02 AM »
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.
 

Offline mariush

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #1 on: July 04, 2013, 05:07:22 AM »
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..


« Last Edit: July 04, 2013, 05:11:31 AM by mariush »
 

duskglow

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #2 on: July 04, 2013, 05:43:42 AM »
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. :D
« Last Edit: July 04, 2013, 05:52:08 AM by duskglow »
 

Offline brainwash

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #3 on: July 04, 2013, 07:01:07 AM »
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.

I'd have to take the thing apart (unsolder digits) there's nothing on the back side. It probably has a voltmeter IC.
 

Offline bilko

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #4 on: July 04, 2013, 07:45:22 AM »
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..
Spot on !
I have one, the cpu is
STM8S003F3

Here's the seller's link and photos of the rear of the board
http://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
« Last Edit: July 04, 2013, 08:00:33 AM by bilko »
 

Offline brainwash

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #5 on: July 04, 2013, 12:37:25 PM »
You're right! As overkill as it seems it is an STM8 chip.
Pinout at the connector:
Code: [Select]
  SWIM / PD1 (HS)
  TX / AIN5(HS) / PD5
  RX / AIN6 (HS) / PD6
  NRST
  VDD
  VSS

Since 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.pdf
2.95-5.5V, 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.
« Last Edit: July 04, 2013, 12:45:42 PM by brainwash »
 

Offline ftransform

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #6 on: July 07, 2013, 11:23:00 AM »
How noisey is it? Is it a SMPSU or Linear?
 

Offline rexxar

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #7 on: July 07, 2013, 01:26:27 PM »
How noisey is it? Is it a SMPSU or Linear?

It's got a little LDO linear regulator on the board.
 

Offline brainwash

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #8 on: July 07, 2013, 01:48:03 PM »
Holtek 7130 3V/30mA LDO, you can see it in the picture.
 

Offline baoshi

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #9 on: August 11, 2013, 11:25:56 AM »
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 here http://www.ba0sh1.com/hacking-a-cheap-led-voltmeter/ (sameless self promoting)
 

Offline echen1024

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #10 on: August 11, 2013, 12:39:39 PM »
I just ordered a couple ICL7107 samples from Intersil, and am about to build a panel voltmeter from that.
I'm not saying we should kill all stupid people. I'm just saying that we should remove all product safety labels and let natural selection do its work.

http://www.youtube.com/user/echen1024
 

Offline Electro Fan

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #11 on: August 11, 2013, 12:47:12 PM »
www.adafruit.com (Arduino site) sells several similar meters for volts and amps; they can be handy for roughly approximate metering in circuit breadboarding

http://www.adafruit.com/index.php?main_page=adasearch&q=volt+meter
 

Offline quantumvolt

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #12 on: August 11, 2013, 10:05:13 PM »
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 here http://www.ba0sh1.com/hacking-a-cheap-led-voltmeter/ (sameless self promoting)

Very 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.
 

Offline amyk

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Re: Another ebay buy: 0-30V panel voltmeter
« Reply #13 on: August 11, 2013, 11:25:33 PM »
Holtek 7130 3V/30mA LDO, you can see it in the picture.
That's not a Holtek, it's a Mingda 7130H. 30V maximum input voltage.
 


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