Efficient Speakers

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.

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.

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 curtilage.

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

On Fri, 16 Mar 2007 12:40:57 +0900, spamfree wrote: ...

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! Rich