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Efficient Speakers

Discussion in 'Electronic Basics' started by [email protected], Mar 15, 2007.

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  1. Guest

    From my other thread, I'm trying to arrange a very efficient
    solar/battery radio as a burglar deterrent while I'm away. Seems the
    speaker is the thing that eats Milly Watts (lucky Milly :)

    Homey tells me that a big speaker will give me more sound for a given
    signal power. I have in my hand a Panasonic "Oval Whizzer" car speaker
    that is nominally 20W and impedance of 4 ohms. It's about 5" diameter.
    I also have some large HiFi speakers about 10 or 12" and from memory,
    8 ohms impedance.

    What speaker do folk advocate for this very non-HiFi application?
    I think I might use a crystal set kit and a small half Watt amplifier
    kit to drive this speaker. All suggestions gratefully received, jack
  2. Bob Eld

    Bob Eld Guest

    Horns are the most efficient speaker systems. For your application a small
    PA horn being driven with a few milliwatts will be quite loud and should do
    the job. These are usually eight ohms. Cone speakers of any type have very
    low efficiencies. Since the advent of high power amplifiers, speaker
    efficiency has not been important so modern cone speakers in small boxes are
    always poor in efficiency. Efficiency has given way to frequency response
    and other qualities deemed more important.
  3. Since solid-state amps made audio power cheap, hi-fi speakers haven't been
    designed for efficiency. A PA trumpet speaker will give you more sound
    per watt. To make the place sound occupied, the radio will have to be
    loud enough to be heard through closed windows, so you will need more than
    a few milliwatts. Is there some reason you can't just use line power? If
    the radio is the only thing on, it won't cost much even if it's on for
    twelve hours at a time. If it's on continuously, a smart burglar might
    notice that.

    How about an alarm? I used to install traditional perimeter-loop systems
    that drew 3 mA in standby. Loop power was from two old-fashioned Number
    6 dry cells, which we replaced annually. The alarm sensor was just a
    little relay, with its coil, normally-open contact, the loop and the
    battery in series. Breaking the loop closed the normally-closed contact,
    which drove a bigger relay that applied 12 volts from a pair of lantern
    batteries (also replaced annually, because they were hardly ever loaded)
    to the bell. With transistor switches and an electronic siren, you could
    run the thing from four rechargeable D cells charged by your solar panel.
  4. Guest

    Thanks Stephen, I have a very old house with old wiring and I will
    feel happier with disconnecting the mains when I'm away for any length
    of time. Some friends went to Europe for a few weeks several years ago
    and returned to a burned out house. A wiring fault somewhere.

    In my area (fairly busy street) alarms are consistently ignored. I
    used to rush out to see what was occurring, but have given up after a
    dozen false alarms. The usual action is to ring the police to complain
    about the noise when the alarm has been going for several hours. Next
    door even have a "call to base" alarm, but the "cat burglars" got in
    through a slight window opening left for the bloody cat and cleared
    the place of anything of value before anyone arrived to investigate.
    This was after the bloody cat had apparently triggered several false
    alarms. The usual first attempt at burglary is probably a knock at the
    front door or a phone call to see if anyone is home. Perhaps a large
    dog bark triggered by an approaching person to the front door might
    work, but that's way beyond me, and so I will have a non-mains powered
    night light and 24/7 radio. I do that now from the mains for the odd
    night away, and so far it has worked, but then so has the nekkid
    dancing in the moonlight prevented elephants from invading my
    The radio I will be using based on the MK484 chip draws about 0.3 mA
    (300 microamps) and so the main drain will be the tiny amp and small
    horn speaker(s) I will use to get sufficvient sound. The radio will
    last for many months on one alkaline D cell. The amp will run at 12 V
    and whatever mA I require from a 12Ah SLA that I have (for another
    purpose not yet come to fruition.) The drain on this will be fine with
    intermittent charging from my solar PV panel.

    Thanks for all the help you and everyone have offered. A very helpful
    and knowledgeable group of folk. Regards to all, jack
  5. Rich Grise

    Rich Grise Guest

    Actually, it's probably not. Just get an old, but working, tape recorder,
    make a tape of a big, angry dog barking, cue it up, press play, and
    unplug the recorder.

    Then, get a PIR switch like or so, and use the
    output to power the tape recorder. You don't even need to solder anything!

    Good Luck!
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