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  1. #1
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    Low battery and high temperature circuit

    I thought I'd give something back to the community - here it is.

    I've just finished building my first set of lights based on a simple DX driver, so I wanted to add something to tell me when my batteries were running low (other than the driver simply going out of regulation) and something to tell me whether the set up is getting a bit hot.

    After a couple of redesigns, here it is (built on standard veroboard).

    Apologies for the non-standard electronic symbols...

    The 'blue' circuit is for the termperature and the 'red' circuit is for the voltage indicator.

    The X's are where you need to break the copper strips on the veroboard (really easy to do with a 3mm drill bit - just twist it between your fingers and the copper will drill out

    The two P's in triangles on the left of the board are preset potentiometers - I used 100K pots to keep the current draw down.

    L's are LEDs, the T is a 100K thermistor (neg temp coefficient) and the Z is a 4.7V zener diode.

    R1 is a 100K resistor to approximately match the thermistor in the temperature circuit. R3 is a 15K resistor to allow at least a little current to flow into the zener so it can provide its reference voltage. R2 and R4 are 330 ohm resistors to limit the current flowing through the LEDs. These values are based on a 6 cell NiMH pack, so if you're using more or less then you may need to increase or reduce them.

    The thick lines are jumper wires to transfer voltages around the board.

    The chip at the heart of it all is a LM393 dual comparator.

    The whole thing fits onto a 7 holes by 12 holes piece of veroboard - I allowed a margin all the way around it, and then mounted it to a 2mm thick piece of plastic (so that there were no shorts on the solder side of the veroboard, and it fit inside a square tube with internal measurements of 26mm but without much space to spare.

    With both pots at one end of the board, I'm able to set the temperature and voltage at which the LEDs come on before sealing my casing. If I decide to change battery packs (e.g., to move from a 6 cell NiMH pack to a 2 cell lithium pack) then I'll just pull the end cap of the case off (only held on by silicone adhesive) twiddle the potentiometer a bit and reseal...

    The parts are relatively cheap.

    My resistors were 8p each, thermistor 21p, veroboard was 1.19 but I only used a small piece of it, the zener was 2p, the pots were 10p each, the comparator was 10p and it sits in a 14p socket, and the LEDs were 20p and 28p respectively (one red, one green) giving a grand total of 2.66 (plus solder and a bit of wiring if you're counting).

    Anyway, hope this is helpful - there may be more elegant ways of doing this but my basic understanding of electronics was the limiting factor...
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  2. #2
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    Nice work. What are you using for a housing? Got some pics? I'd love to see it.

  3. #3
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    The housing is some 30mm square aluminium with 2mm wall thickness.

    Two pieces glued back to back. The front piece has a window cut into it and holds the LEDs and optics and the back piece has the electronics.

    Also in the front piece is a 5mm bar of aluminium to bring the optics closer to the front of the case as well as provide more of a heat path...

    The two sides are bits of plastic that I dremel'd out of one of those things you put on your desk to store magazines/letters etc.

    To the right of the cable gland is an on-off switch (completely kills power to everything) and to the right of that is a push to break to control the modes.

    To the left of the cable gland are the two LEDs for high temperature and low battery warnings, in their own little recessed panel mounts.

    The bar mounts came off a couple of DX bike lights - they were only USD7 each, and I fried the drivers when I was playing about with them... I didn't realise that having a driver on and then taking the LED away causes an open circuit which some drivers don't like... I'm still considering bolting them together and using them as a helmet light with new XREs and a new driver...
    Attached Thumbnails Attached Thumbnails Low battery and high temperature circuit-lightsfront.jpg  

    Low battery and high temperature circuit-lightsback.jpg  

    Low battery and high temperature circuit-lightspots.jpg  


  4. #4
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    Hi Jonboy, Thanks for the post. Could you also post the schematic? The pictorial was nice, but us electronic type need sche's. James

  5. #5
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    Here it is - hope it makes sense.

    I've not included the power supply lines for the two comparators as they just clutter the diagram up.

    When I built it, the two comparators are on the same chip (LM393).
    Attached Thumbnails Attached Thumbnails Low battery and high temperature circuit-circuit.jpg  


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