EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: brainwash on July 03, 2013, 06:25:02 pm
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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.
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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..
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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
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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.
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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 (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)
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You're right! As overkill as it seems it is an STM8 chip.
Pinout at the connector:
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 (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.
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How noisey is it? Is it a SMPSU or Linear?
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How noisey is it? Is it a SMPSU or Linear?
It's got a little LDO linear regulator on the board.
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Holtek 7130 3V/30mA LDO, you can see it in the picture.
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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/ (http://www.ba0sh1.com/hacking-a-cheap-led-voltmeter/) (sameless self promoting)
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I just ordered a couple ICL7107 samples from Intersil, and am about to build a panel voltmeter from that.
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www.adafruit.com (http://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 (http://www.adafruit.com/index.php?main_page=adasearch&q=volt+meter)
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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/ (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.
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Holtek 7130 3V/30mA LDO, you can see it in the picture.
That's not a Holtek, it's a Mingda (http://md-ic.com.cn/en/product/Power_management_chip/Regulator_LDO/) 7130H (http://md-ic.com.cn/download/71xxh.pdf). 30V maximum input voltage.