# Powering 12v Air Pump with Audio?

1. Aug 3, 2007

### nbontra

Hi, I'm currently an art student at the University of Houston, and I've run into a bit of a roadblock.

I'm working on an installation in which an audio signal from a headphone jack will be sent to an A/V Receiver Unit. From the A/V unit, the wiring for the left speaker will be hooked up to the pos/neg connections from an old air compressor (12v dc). In theory, as the audio signal fluctuates, the voltage sent through the speaker wires will power the air compressor. Louder sound = Higher voltage = More air through the compressor.

I've only done similar experiments when playing with old tube televisions, and turning them into o-scopes hooked up to audio signals. From what I figured, I was easily able to get around 10 volts from a small radio.

Ok, here are my questions.

What is the standard voltage output range going from a home receiver unit to an 8 ohm speaker?

If I'm able to achieve the proper voltage to power a 12vDC air pump, will there be proper amperage? If not, what can i do to compensate?

And lastly, if I am able to get the proper amount of power, will the fluctuations in voltage have a damaging effect on the pump itself? Or will it simply act as a relay, powering on and off as the voltage/amperage is reached?

Thanks so much for any help!

-Nick B

2. Aug 3, 2007

### chroot

Staff Emeritus
This is never going to work, at all.

First off, the air compressor is expecting DC current, not AC current. Your audio signal is AC, the direction of the current is constantly changing. Futhermore, your air compressor probably needs some tens of amps of current (tens to hundreds of watts) to run properly, and your audio signal is going to contain no more than about a watt of power, at most.

You are also correct that attempting to run the air compressor at wildly differing voltages may damage it. Also, since the air compressor is likely to have an extremely small resistance, you will no doubt damage your A/V receiver as well.

Your best bet is to use the air compressor as you normally would, connected to a normal, stable 12V source, and then use some kind of a variable valve downstream to regulate the flow of air from it. You can use an envelope detector to provide a signal to that valve.

- Warren

3. Aug 3, 2007

### Danger

Hi, Nick. I can't for the life of me figure out why you want to do this.
Anyhow, could you just drive a powerful speaker from your audio signal, then funnel the sound into a tube and use it as a fluidics control signal for the compressed air?

4. Aug 3, 2007

### nbontra

Warren, thanks for confirming my suspicions. Could you explain the bit about using a variable valve and the envelope detector?

Danger, I have a habit of making simple mechanics extremely elaborate just because i like the way it all looks when working together. Similar to the old rube goldberg designs i suppose. Could you explain the fluidics control signal method?

Thanks guys!

-Nick

5. Aug 3, 2007

### chroot

Staff Emeritus
I honestly do not know enough about valves to help you pick one out, but you should be able to find electronically controlled valves somewhere. Try McMaster-Carr. An envelope detector is the sort of circuit that you see on stereos, where it shows you the "average power" in the audio signal by a little stack of lights. When the music is loud, more lights light up. You can build the same sort of circuit, and then use its output to control the valve.

- Warren

6. Aug 3, 2007

### Danger

Not in great detail, because it's not one of my areas of knowledge. Basically, though, it acts like a transistor. A control stream of fluid (air, in your case) impinges upon and proportionally redirects a much larger 'working' stream.
Chroot's suggestion of a proportional electronic valve driven directly from the audio signal is much more practical, though, and was going to be my next offering.

7. Aug 3, 2007

### NoTime

Many amps sold as "A/V" recievers can put a couple hundred watts into a 2 ohm load. (Check the reciever spec).
Most 12v auto cigarette lighter air pumps draw about 10 amps at full load (about 120 watts) or a lot less if they are pumping air to ambiant air preasure.

You can use a bridge rectifier to convert the AC amp output to DC.

The output voltage of a amp will vary depending on the load.
You can use P=EI and E=IR to calculate it.
It can range from 20v to 100v for a big amp at full power.

Note: To protect the amp put a resistor in the circuit.
The Ohm value depends on your amp and pump.
The resistor needs to be big enough to handle the power of the amp. A 100/200 watt 1 to 4 ohm resistor is not that easy to find and is going to get HOT. You could use a collection of parallel resistors though.

8. Aug 4, 2007

### chroot

Staff Emeritus
I wouldn't use a resistor to limit current. I'd use a properly-rated fuse.

The bigger problem with your idea, NoTime, is that the compressor will likely not run at all at voltages below 10V or so, and voltages higher than 14V might well destroy it. Futhermore, rapid fluctuations or pulsations in the supply voltage might also destroy it.

- Warren

9. Aug 4, 2007

### NoTime

The point of the resistor is to keep the output load of the Amp within its design range.
Not to limit current.
Amps come with electronic or mechanical (or both) fuses.
No need to add another one.
The fuseing can not be relied upon to protect an amp from too small a load value.

The pump is just a DC motor.
Fairly bullet proof and cheap, if you do manage to wreck it.
It will operate just fine over a large range of voltages.
The one concern here is overheating, but some consideration of volume settings and source signal should eliminate this as a concern.
I might note that variable speed DC and AC/DC motor controlers work in pulse mode.
The rapid fluctuations and pulsing are not a concern.

10. Aug 4, 2007

### chroot

Staff Emeritus
NoTime,

I'm sorry, but you're grossly oversimplifying. We don't even know the specific compressor the OP is using, so it's not possible to know whether or not it will survive this torture. The truth is that many of these cheap compressors are meant to be plugged into cigarette lighters to re-inflate low tires. They're meant to be turned on and off a couple times a year. If you start driving it with a rapidly varying signal, you might well over-stress and destroy the coupling between the motor and the impeller. In fact, I'd almost say it's a guarantee that that will eventually happen.

Besides, using a huge (100W!) power resistor is a pretty terrible way to limit current.

- Warren

11. Aug 4, 2007

### nbontra

Wow, thanks for all the input! If it helps in the discussion between NoTime and Chroot, the compressor will be pumping air at the bottom of a 10 gallon fish tank, under about 2.5-3 feet of water.

12. Aug 4, 2007

### chroot

Staff Emeritus
Another problem with the approach of modulating the compressor's power supply: there is going to be quite a bit of "storage" for air in the lines in this system. Pulsing the compressor off and on in time to music will just result in a slight rhythmic increase and decrease in the pressure in the lines, rather than a dramatic "on and off" effect. If you have enough air space in the system, you might not even be able to tell anything's happening to the compressor at all.

I reiterate my suggestion of powering the air compressor in a conventional way, and using some kind of electromechanical valve to direct the air where and when you want it.

- Warren

13. Aug 5, 2007

### nbontra

Thank you for all the help, I will do my best to learn about how to construct and use an electromechanical valve. Maybe i'll pay the kids in the engineering department on campus a visit.

14. Aug 5, 2007

### NoTime

I simply disagree with your statement "This is never going to work, at all.".
It is technically quite possible to do this.

Perhaps, but I have a friend that used one of these \$10 pumps to blow up his tire every day for 2 years. The pump still works. The weak point of these pumps seems to be the bearings. The pump will likely last a lot longer than the novelty value.
That is a value judgement, but as I said before the resistance depends on the amp minimum load and the pump impedance. The pump will present an impedance similar to a speaker, which is nothing more than a linear DC motor. The resistor may not be necessary, but this is something you would want to measure if you care about destroying the amp.

I tend to agree with this, but it's kinda hard to say for sure without some experimenting.

Another solution could be to screw a plastic sheet with some holes and mylar flap valves to the front of a speaker.
This would form a simple pump.

Or scavenge the pump body from an old AC aquarium pump and connect it to a speaker cone. These work at 60hz so it should work with an audio source. There would be a serious power limit with one of these. They are small and can only handle a couple watts.

15. Aug 7, 2007

### nbontra

Would it make a difference at all if I could run the whole unit [amp/speaker/compressor] off of a 12V car battery with an AC charging unit on the battery to keep the level topped off?

If so, then I could use a small car amplifier [200wX2] and the compressor without having to arrange for current limiting resistors. I think the car battery would have about 20-25amps also.

With this setup, I could then go with the fluidics approach or the envelope detector. Or if i can get around 10 volts of current from the amplifier, i could also build a small electromagnet that would open and close a small valve in the line from the compressor. If that actually worked, it would also be a variable valve in the sense that as the voltage fluctuates on the electromagnet, it will draw the valve more or less open rather than a simple open-close circuit.

Any thoughts?

16. Aug 7, 2007

### chroot

Staff Emeritus
You don't seem to understand the behavior of electricity at all. Batteries don't "have" amperes. A car battery can easily deliver several hundred amperes of current if the resistance you connect to it is low enough.

It doesn't really matter how you power your amplifier -- any stable 12V source capable of supplying the needed current will result in the same behavior.

If your amplifier is designed to use a 4 or 8 ohm load, and instead you connect it a compressor with a very small resistance (probably 0.1 ohms or less), you will destory the amplifier (or put it into protect mode, if it has one). It doesn't matter how you supply power to the amplifier.

"10 volts of current" makes no sense.

Your valve concept is probably workable, but you might be better off simply purchasing a well-designed valve from a reputable distributor. If you're only dealing with 10-30 psi, though, the dangers are pretty small.

- Warren

17. Aug 7, 2007

### nbontra

I can't thank you enough warren. My lack of knowledge in electrical systems is a constant setback when i think of these setups. You've been more help than the entire engineering department at my college.

Thanks!

18. Aug 7, 2007

### Loren Booda

A great way to convert an air mattress into a musical bed!

19. Aug 8, 2007

### ice109

why don't you ditch the compressor and get a tank of already compressed air and pulse that with an electric valve?

20. Aug 8, 2007

### nbontra

wouldn't the tank of compressed air run out eventually? its for an installation so it would need to be totally self sufficient. what kind of electric valve though?