Mini Panel Meters - Page 1
Forum section: electronics / projects

author: steve30
author_rank: Frequent Contributor
timestamp: January 10, 2014, 10:38:49 pm
content: For a lab PSU project I'm working on, I decided to purchase some panel meters (I was originally going to make some). I was looking at some tiny LED 3 digit ones which keep cropping up on ebay. I bought one the other day (http://www.ebay.co.uk/itm/200981193732?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649), and it works fine.Does anyone have any technical details on these things? There seem to be a few different ones around. This one I got is based on a Holtek microcontroller. Also, they mostly seem to be available from China. There seem to be hardly any for sale in the UK. The one mentioned above was posted from England, but most ebay listings are chinese, and they don't seem to be very available from many retailers.

author: IO390
author_rank: Regular Contributor
timestamp: January 10, 2014, 11:11:59 pm
content: I was in a similar situation. They are really cheap so I bought a couple (this was a year ago, can't remember the exact ones) and they were fine. They met the specs as far as I remember (can't remember what they were...).In the end, I decided to use some decent Lascar meters. They are a bit more pricey, but worth every penny IMO.http://uk.rs-online.com/web/c/?searchTerm=lascar+voltmeter&sra=oss&r=tI've got a few of these, and they are particularly good:http://uk.rs-online.com/web/p/digital-panel-voltmeters/4199334/

author: rdl
author_rank: Super Contributor
timestamp: January 10, 2014, 11:35:32 pm
content: I bought three of the 2-wire version a couple of months ago for $5 US including shipping. They work pretty well, off by about 0.02 volts at worst according to my Fluke 87-III. I wish I had gotten the 3 wire (separate sense line) but I just put them on breadboards and stuff to monitor the supply voltage, so no big deal. They work down to about 2.5 v, but the display (red LED) begins to dim below about 3.3 V. They auto-range, so you get 0.01 resolution below 10 volts and 0.1 from 10 on up.

author: steve30
author_rank: Frequent Contributor
timestamp: January 11, 2014, 12:05:34 pm
content: I just sent off for a couple of 999mA meters from DealExtreme.It would be cool if anyone could find a schematic etc for these.

author: Bored@Work
author_rank: Super Contributor
timestamp: January 11, 2014, 12:42:56 pm
content: Old ones had ICL7107 clones. Typically just following the schematic in the datasheet. They are easy to spot, they are those requiring a separate, isolated(!) power supply.The more expensive ones had a small isolated DC-DC converter on board.Then the manufacturers simply started to use the cheapest microcontrollers, capable of sampling some voltage and driving a LED display. PICs, STM8s, Holteks. Whatever they can get the cheapest at the moment or have as leftovers from some other production run.There is barely anything in the circuit in addition to the MCU and display. The minimum external circuitry to get the MCU clocked and out of reset. Maybe an input voltage divider. Maybe an LDO or other voltage regulator. If the MCU has no voltage reference and if they have no voltage regulator then an external voltage reference - of course the cheapest they can get, like TL431. Maybe, but only maybe, a power supply bypass cap. Very unlikely some protection diodes.All they do in code is to set up the internal ADC, the voltage reference and LED output pins and then they read the ADC and update the display accordingly in an endless loop. The complexity of the code is just one notch above blinking a LED.

author: Monkeh
author_rank: Super Contributor
timestamp: January 11, 2014, 12:52:36 pm
content: Franky (iloveelectronics) supplied me with this ICL7107 based meter some time ago, which is currently operating beautifully in my DC load:It is, however, obviously far from mini, and as mentioned does require an isolated 5V supply.He also sent me a little 4 digit 2-wire meter which works remarkably well, although I don't have a picture to hand, very similar to the one you've tried. I believe he has 3-wire versions now. I'm sure if you give him a shout he'd be happy to help out with finding one to suit you, he's always been able to source things for me.

author: rdl
author_rank: Super Contributor
timestamp: January 11, 2014, 01:38:23 pm
content: Yeah, these newer mini meters are not much more than a micro controller and display. Works good enough for non-critical applications though, and only $1.65 delivered. This is the one I bought:http://www.ebay.com/itm/Mini-LED-DC2-5-30V-Red-Volt-Voltage-Meter-Display-Digital-Voltmeter-Self-Powered-/360732171649