[3 July 2010 – this project has been retired, but the posts left for reference]
Our bbboost journey continues today with the prototype voltmeter module. Once complete, the bbboost will have a digital voltmeter to accurately measure the output voltage. (And maybe… an ammeter… stay tuned). Over the last week I have been pondering and experimenting on what sort of voltmeter to have. Of course it had to be digital, but what to drive it? First there was a PICAXE microcontroller using a voltage divider and the PICAXE’s analogue to digital converter. Great… except that chip can only use integers. Next…
There seem to be many cheap digital LED voltmeter modules on eBay at the moment, but where’s the fun in that?
Around six months ago I stumbled upon an interesting IC when wasting time browsing through the Farnell website, so I ordered two of them planning to do something with them later on. And promptly forgot about them. However research for the voltmeter showed this chip as ideal – the Intersil ICL7107. An analogue to digital converter and 3.5-digit LED driver all in one. Woohoo!
Data sheet is here… ICL7107 datasheet
Furthermore, this seemed like an inexpensive solution, volumes of this IC can be had for ~$2. Good so far, but after studying the data sheet – there was a catch.
It needs positive and negative five volts DC… ah fudge. The ICL7107 data sheet suggests using a CD4009 hex buffer/converter, with some diodes and capacitors. Nope, too hard and messy. However, thanks to the internet a solution was found – the ICL7660 CMOS voltage inverter!
Check it out – ICL7660 Voltage converter
So now the theoretical solution had been found, it was time to move from thinking to doing. Originally I used the circuit diagram from the Intersil web site, however calibration was a problem due to only having a single-turn potentiometer (see parts list below). So for the real unit, a multi-turn potentiometer will be used for R7.
Just a side note… breadboarding circuits can be a joy or a pain. If you use those cheap-ass breadboards from China via eBay, you will suffer. Take my word for it.
Anyhoo, before getting the circuit together, I like to line up all measure all the components – note my trusty capacitance meter …
Then put them in order according to the parts list. This will save confusion and time later on, as you have checked all the values and ensured they are correct before installation!
Here is my lovingly-crafted schematic for the voltmeter module. Note that this could be made as a standalone voltmeter, it will measure up to 20v DC. In our finished product, the +5V will be sourced from an LM78L05 voltage regulator.
And the parts list:
- IC1 – Intersil ICL7107CPLZ
- IC2 – Intersil ICL7660
- D1~D3 – 1N4148 diodes
- LED displays – Agilent HDSP521G 2 x 7-segment green displays (common anode). You can use anything really, as long as it is common anode, and each segment is ~8mA
- R1 – 220 ohm – all resistors 0.25W
- R2 – 10k ohm
- R3 – 1M ohm
- R4 – 47k ohm
- R5 – 15k ohm
- R6 – 100k ohm
- R7 – 1k ohm multiturn potentiometer/trimpot (for calibration)
- C1 – 10nF – all capacitors must be rated for at least 25V
- C2 – 20nF
- C3 – 470 nF
- C4 – 100 nF
- C5 – 100 pF
- C6,7 – 10 uF electrolytic
Please note that this is a work in progress and errors may have been made, or values altered at any time after publication.
And now for the finished mess:
Wow – what a mess. If you are going to use a breadboard – take care with the very low value capacitors. Try to keep the legs as short as possible to improve the meter stability. When you turn it on, the display will flick a few numbers around until settling on zero. You will need to calibrate it, so just measure a solid, reliable power source (such as the output from an LM7812 and LM7805 (12V and 5V DC) in turn. So when you are measuring the (for example) 5V output from the 7805, adjust the trimpot until the display says 5.00. Try this if you can with a few different reliable voltage sources, to check your new meter’s accuracy.
Finally, this wouldn’t be complete without one of my soundless videos. In this clip, I measure a 9v battery, then an alkaline AA cell, then the same again but with reverse polarity.
So thanks once more for reading. Please leave feedback and constructive criticism or comments at your leisure… and to keep track, subscribe using the services at the top right of this page!