Driving 8 Ohm speaker from pic pwm pin

On a sunny day (Thu, 07 Feb 2008 18:46:50 GMT) it happened _ wrote in :

LM386?

Some ballpark numbers;

"mildly noisy" is really subjective but I'l take that as 70dB at the listener. A random persons view of mildly noisy could be 60-80dB

40feet is 12 meters. Assuming 6dB loss when you double the distance you loose about 20 dB from 1m to 12 meters

Call it 22dB. In practise it will vary with room shape, surface absobtion, audio frequency, number of people etc.

So assuming that having the beep about the same level as the noise is enough you need 92dB at 1 meter.

A typical cheap moving coil loudspeaker will give about 87dB at

2.83volts Thats, 1 watt into 8 ohms, about 0.35A. However this is not omnidirectional, if you can't point the loudspeaker at the listeners you may need more volume or a more sophisticated loudspeaker arrangement.

Again this will vary signficantly with loudspeaker model, loudspeaker impedance at the frequencys of the beep, load will be a bit reactive, current will depend on if you just try to switch one side of the supply or force the loudspeaker terminal voltage to be what you want etc.

So my conclusion is that you might get away with a 2n700 on a 5volt supply and a cheap little loudspeaker if the noise is 65-70dB if the beep is reasonably easily distinguished from the noise. Alternatively you might not. If you want to be sure I think you need more watts and more thought to the loudspeaker arrangement.

I don't think an LM386 would be any better than the FET with a five volt supply. They would both be limited by the supply rail and the loudspeaker impedance. If you use a higher supply voltage and a loudspeaker that can handle 10 watts or so then neither is suitable. Linearity is probably not important so a transistor with a higher current rating seems like the best idea to me.

Bob

Think for a moment. What the FET does, is yank the speaker against the supply, and then let go.

The amount the speaker cone moves is a lot when current flows (one way), but the amount it moves back is set only by the spring action of the thing it hangs from, its stiffness, and its resonance frequency. Unless you hit it at exactly its resonance, it will not move back a lot. And its resonance is likely not the frequency you want. It is a _bad_ way to drive a speaker. The LM386 will drive the cone both ways, so it will have a much bigger amplitude.

They would both be limited by the supply rail

I have about 12 volts to work with, and was thinking about a small horn speaker. The enviroment is a swimming pool, so a water-resistant speaker - plastic cone or horn - would probably be best, and directional would work just fine.

Anybody know of a source for small (~5watt, ~2inch opening) cheap horn speakers?

I'm getting 50w plus from 12v with a class H BTL into 4 R . Check out NXP for automotive amplifiers. The site is pretty crap, designed for accountants, but the products are useable, despite the datasheets

martin

With an acoustic resonator, and maybe some circuit tricks (like run the piezo at resonance itself), you can make it pretty blinkin' loud!

Good Luck! Rich

That's a reasonable point to raise. Depending on the choice of supply rail, loudspeaker and amp IC you might be right.

In the example linked by the orignal poster with a 5 volt supply and a little 2.5" loudspeaker I still think the 2n7000 would work better than an lm386

The FET should have a fairly low voltage drop across it when on so in the paticular example linked by the original poster the loudspeaker should get most of the 5V supply.

Acording to the National LM386 datasheet with a 5 volt supply you only get about 2.5volts peak to peak to peak output and the THD becomes grotty above about 1.4v

I think the extra volts from the FET switching would win.

A bit of googling suggests that the resonant frequency of moving coil loudspeakers is often quite low, somtimes 400Hz for a 2" loudspeaker so I think that with a lot of common loudspeakers you would be wrong about it not moving back enough under it's own elasticity.

5volts isn't going to physically damage an 8ohm loudspeaker, except maybe a tiny 1" one so I disagree that this is a bad way to drive a loudspeaker in the specific case we are talking about.

Bob

t

An LM386 at 40-feet is probably insufficient. Have you considered a Sonalert (Piezo) device?

What supply voltage do you have available?

-mpm

I have 12 volts, and I need to have three different frequencies of beep.

On a sunny day (Fri, 08 Feb 2008 11:36:28 GMT) it happened _ wrote in :

LM380

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ount

You should consider the Sonalert type idea. Different alerts can be taken care of by implementing various on-off switching sequences.

- mjkaras

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ount

Hmm, You mentioned using a two inch horn. A two inch horn will proably only work efficiently at rarther screeching trebble frequencys. With 12v you probably have enough volts to get enough sound out at a couple of kilohertz so it may not matter. I'm guessing a bit because I didn't turn up any data for really small horns with a quick google. Horns usually have a 15dB or more variation in efficiency across the frequency range listed in the spec. Going up from about 3000 to 10000hertz you need more sound level to get the same perceived volume so in that range the horn efficiency and human perception might cancel out nicely. The acoustics of leisure swimming pools are often very reverberant and resonant so that actuall effect would be anyones guess.

Piezo electric sounders are a nice idea but remember that the volume rating of little panel/pcb mount sounders is usually quoted at 10cm distance. The sound at 12 meters will be somewhere around 40-50dB lower.

Bob