Hardware considerations


About the construction and the choise of components...


It is possible to use a PIC16F84-10 instead of using a PIC16F84A. Read more about this in the Software section.


The display can be of any manufacturer as long as the LCD-module comply with these requirements:


The diodes can be any general purpose small signal silicon diodes. Just make sure that if you're modifying the battery-circuitry the diodes that carry charging current can handle the current. I use BAX14 in my instruments.


The zener diodes may be BZX55/C6V8 and BZX55/C3V3.


Since this is a test instrument the crystal should be one with a high frequency stability. I suggest using a HC49 crystal with a frequency stability of 10ppm. (The price is three times the price of a 50ppm device, but it's worth it.)

ELFA order-code: 74-505-13


The capacitors (C2 and C3) at the 10 MHz crystal may need to be less than 27pF. I've experienced some problems with the oscillator not starting. Sometimes it started, and sometimes it didn't. The cure was to reduce the load-capacitance. If you're having this problem, you'll see that the oscillator will start if you help it by "piggy-backing" another 10 MHz crystal directly to pins 15 and 16 of the microcontroller. You could also de-solder C2 and C3. That would make the oscillator a little unstable but it works. I suggest to try 15pF or 22pF for C2 and C3. These capacitors are with 2,54mm pin-spacing.


ELFA have got some keys which can be used as S1 and S2. The order-code is 35-653-06


I soldered S3 directly to the solder-side of the PCB as an SMD-component. I did not use any of the holes for this component, and the copper pads/tracks under R2 through R8 (component-side) was scraped off with a knife.


The probe tip can be made with the metal from a standard test probe. The problem is that many test probes are made of a metal that is impossible to solder. What I did, was using a Hirschmann test probe, removing the insulation, and cutting it to 57mm. This material was very easy to solder and it works perfect!

ELFA order-code: 40-399-05 or 40-399-21

Another approach is to use an ordinary sewing needle, one of the thickest you can find. These are surprisingly easy to solder, almost un-beakable and still very cheap!

I cut off the part of the needle where you're supposed to have the thread (the eye of the needle). Initially the needle was 75mm, after cutting it the length was 56mm. The diameter is 1,8mm.

The idea of using a sewing needle came from mr. Voja Antonic.


The connectors between the LCD-module and the main PCB should not be too high. Have a look at the drawing in figure 15 in AN689 to see how much space there is for the connectors. Just 7mm!


The female charge connector is a 3,5mm "telephone jack". Both ELFA and Clas Ohlson has got the connector that fits the PCB layout.

ELFA order-code: 42-700-47
Clas Ohlson order-code:


LED4 - the "Discharge" LED is shown the wrong way (wrong polarity) in figure 16 in AN689 (the Microchip PDF file). The anode (plus) shall be connected to the track that has +5V directly from the battery.


This is a Swedish company that sells all kinds of electronic components.


Clas Ohlson

This is a Scandinavian company that sells almost anything, also some electronic components.


Erik Grindheim, 14.04.2000