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Button cells
and LEDs

I’ve got a “Light Sabre” key-ring that you can see here: http://www.zazz.com.au/pastproducts.php?past=1466

It is very simple – press the button and it lights up. Unfortunately, the four LR41 batteries which came with the key-ring are running out (and really didn’t last that long either to be honest). I’ve been looking around and a local electronics ship has the battery for sale in single units for $4.

Given that I need four batteries, I was hoping to find a cheaper equivalent, especially since the lifespan is monthly! A hearing-aid shop has a pack of six for about $6 which is more reasonable but they are PR41.

From the Wikipedia page http://en.wikipedia.org/wiki/List_of_battery_sizes you can see that LR41 is around 25-32/38-45mAh whilst the PR41 is 160mAh. Can I safely substitute the PR41 for the LR41? (B. G., via email).

• All button cells have a pretty short life when they are used to drive LEDs – as yours is. In practice, you can substitute any button cell of the same voltage, provided it will physically fit. A 160mAh cell will last about four times longer than a 38mAh cell so it would be a better choice. Jaycar also have an equivalent at $2 each and if you do a Google search for LR41 you will find plenty of suppliers who can do 10-packs for $1/cell or less. They might not last quite as long but they are much cheaper. Or just go to your local $2 shop and you should be able to pick up a similar deal.

Mind you, if you buy the original product, it will probably be about the same cost as replacing the batteries!

Battery Capacity Meter In A 4WD

I run a 100Ah auxiliary battery in my 4WD. To do this I have a second alternator to charge it, a solar panel and an automatic battery charger (which is connected whenever the vehicle is parked). A portable generator is used when there is not enough sun.

Having purchased your Battery
Capacity Meter, how should I connect these? I presume the solar panel and alternator go through the shunt while the automatic charger goes direct to battery. Am I right?

Secondly, on the Capacity Meter there is a provision for a 14-pin socket. Is this for an external display? I wish to put one on the dash or in a more convenient position than under the bonnet.

Will two displays run OK? And perhaps I could add a second keypad or would I need to use optocouplers? (E. L., via email).

• We made provision for two different LCD modules in this project. The 14-pin connector is for the Jaycar LCD module, while the other is for the Altronics LCD module. You cannot easily connect two LCD displays but if you wish to have the display mounted somewhere else you could run ribbon cable to the PC board.

The way to connect your equipment is as follows: connect them as you would normally, with the chargers connected to the battery and the load connected to the battery. Then disconnect the battery positive terminal and connect it instead to the left terminal of the shunt (looking at the meter in the normal orientation) and connect the chargers to the other shunt terminal, as shown on page 23 of the June 2009 issue.

You could connect the automatic battery charger directly to the battery or via the shunt. If it is connected directly to the battery it means that you won’t be able to measure its charging current.

Speed controller for a bowling machine

I purchased two 12VDC 10A Speed Controller kits (SILICON CHIP, June 1997) for a cricket bowling machine I am designing. The controllers work very well, save for one small problem that I have and am not sure if is correct or not.

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