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Very compact voltage dropdown

Discussion in 'General Electronics Discussion' started by AnonyMouse, Apr 25, 2016.

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  1. AnonyMouse

    AnonyMouse

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    Apr 25, 2016
    Hi. Probably too simple for you guys, but I need it.

    I have a pair of small alarms that each have a different wireless trigger - tone indicates location of trigger. Each needs 3v, normally 2 x penlites internally to each. I want to run them both side-by-side from the same power source. I figured a 9v battery would last longer.

    So the requirement is for a circuit with a 9v battery as input and 2 x 3v outputs. But...

    I have found several circuits online, but what bought me here is that there is very limited space where I need to house the circuit, so I need a circuit with the absolute minimum number of components.

    Am I in the right place?

    TIA

    - Billy
     
  2. Alec_t

    Alec_t

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    Jul 7, 2015
    Welcome to EP!
    Actually, penlite (AA?) cells have a greater life than a 9V battery (PP3?). You would need to connect the two alarms in parallel, so the load current for the 9V battery would be 2c, i.e. twice the load current 'c' that one alarm draws. Dropping from 9V to 3V at 2c load current would waste (9-3)x2c=12c Watts, whereas each alarm uses only 3c Watts.
     
  3. dorke

    dorke

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    Jun 20, 2015
    You can try the 6v camera grade lithium battery 28L 2CR11108 L544 .
    It is small and will last longer the the 9V one.
     
  4. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    How long do those alarms usually operate on two AA cells and what's the minimum run time that will suffice on the new battery?
     
  5. AnonyMouse

    AnonyMouse

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    Apr 25, 2016
    OK, I can see a better explanation would have helped.

    My elderly mother lives in a large old house. If a visitor knocks she sometimes can't tell whether the knock is coming from the front door or the back. Heading off in the wrong direction is a strain on her because of her compromised mobility.

    Last week I bought two cheap, identical wireless doorbells, chosen because a different tone can be set for each and the tone is quite loud. Then before I got around to installing them I got to thinking about making maintenance as easy as possible for her.

    I figured I would mount the receivers together in the middle of the house, both running off the one easily accessible battery, instead of the 2 x AA (1.5v) batteries in each which are under a cover not easily removed by arthritic fingers.

    I also wanted to be able to switch them both off with a single switch. There was recently a spate of doorbells going off at 3 AM - morons pressing and running. So I want her to be able to switch them off at night.

    Battery, switch and circuitry to be in a separate box, with the receivers mounted adjacent. I've got a very neat arrangement to mount it all so long as the circuitry doesn't take up too much room.

    I realise this is a trivial exercise for you guys, but although I can solder and identify most components, I am by no stretch of the imagination an electro-tech, so I need plain-speak.
     
  6. Alec_t

    Alec_t

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    Jul 7, 2015
    Minimum maintenance would be if the bells were powered by a mains-supplied converter. Nowadays they're not much bigger than the mains plug itself.
    Using a USB (5V) charger, just 3 diodes to drop the voltage would give you roughly 3V, like this:
    Bells-supply.PNG
     
  7. AnonyMouse

    AnonyMouse

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    Apr 25, 2016
    Thanks Alec, that's a good idea that I hadn't considered, but unfortunately it won't work in this case, as there is no mains outlet anywhere near where I want to place the receivers. Mum's house is about 100 years old and those old places are not well endowed with power points.

    Changing a single battery once in a while will be no trouble for her as I will make it readily accessible. I've designed a small console to 3D-print that will neatly mount the two receivers and the on/off switch, and put a small circuit board and battery behind an easily removed cover.

    The goal is to replace 4 batteries behind 2 difficult to remove covers with 1 easily accessible battery, and add an on/off switch.

    Hate to be a nuisance, but can anybody provide me with a circuit that will provide the correct voltage to the two receivers and which includes a switch. Using a suitable battery of course, such as the "6v camera grade lithium battery 28L 2CR11108 L544" suggested by dorke, or whatever you might want to suggest.

    Thanks to all for your assistance.
     
  8. BobK

    BobK

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    Jan 5, 2010
    A CR123A 3V lithium battery comes to mind. No voltage conversion necessary.

    Bob
     
  9. Alec_t

    Alec_t

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    Jul 7, 2015
    You could run both bells in parallel from either one CR123A battery as Bob suggests, or two AA alkaline cells. The CR123A has a rated capacity of 1500mAh whereas AA alkalines have a ~2500mAh rated capacity and (depending where sourced) are cheaper.
     
  10. AnonyMouse

    AnonyMouse

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    Apr 25, 2016
    Well thanks for that Bob and Alec.

    I started out with the idea that the 9V battery would last longer, hence the consideration for step down to 3V.

    So if I understand you correctly 2 x AA batteries will outlast the CR123A ?

    In either case no need for a step down, and it's a lot easier to get a battery holder for the AAs. I could always print one for the CR123A, but I've got plenty to do without making jobs.

    Sound about right?
     
  11. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    Correct, the 9V would last the least long of those 3 options.

    I only suggested the CR123A because you specifically said 1 battery (cell actually.) If I were building it, I would use 2xAA or even C for longer life.

    Bob
     
  12. Alec_t

    Alec_t

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    Jul 7, 2015
    If you have the space, then C cells look to be the most cost-efficient. Depending whose figures you believe, an alkaline C-cell has about 3.6 times the mAh capacity of an AA cell but costs about 1.7 times as much as an AA.
     
  13. AnonyMouse

    AnonyMouse

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    0
    Apr 25, 2016
    Thanks again Bob & Alec.

    The information about battery life is an eye-opener to me and will be useful in the future.

    A couple of C cells will take up too much room, but I can certainly go with a pair of AA cells. Down at that level the cost differences are irrelevant so long as we get reasonable life from the batteries, and I can't imagine a couple of door buzzers consuming that much, even if they are wireless.

    Your assistance much appreciated.

    - Billy
     
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