jsdavis

This is also very dangerous without some kind of protection mechanisms. I was hanging out at my friend's car stereo place where they have big capacitors like 1 or 2 farads. While working on a 1 farad cap, one guy inadvertently bridged the terminals. A flash of light later, the screwdriver fused to the cap terminals and melted such that it formed an arc as if it were made of melted cheese hanging between the terminals.

dougmc

Personally, I'd just suggest an actual car horn -- go down to Autozone and pick one that sounds like you want it to sound. Perhaps something with a low pitch so you don't sound like a compact car.

For power, get a R/C LiPo pack, 3 or 4 cells. A fully charged 3s pack puts out 12.6 volts, and a fully charged 4s pack puts out 16.8 volts. A car with the engine running is usually right around 13.8 volts, and I imagine that the exact voltage probably doesn't matter much to the horn -- I'll bet they work reasonably well from 11 volts to 17 volts, but the higher voltages might be somewhat louder and higher pitched.

A 1000 mAh pack is probably more than big enough -- I think most car horns take around six amps, and a 1000 mAh battery can deliver that for a full ten minutes before dying. You just need to make sure you have a pretty good sized switch, big enough to support six amps. (You could go with a relay, but that complicates things.) Autozone probably sells an appropriate switch button as well.

Considering how small your battery would need to be, I wouldn't even think of messing with a generator or bank of capacitors. (Though if you already have the generator to power your lights, and you're the kind of guy who can make a charger to keep your LiPo battery fully charged from that (it'll probably need to boost the voltage and make sure it doesn't exceed 4.2 v/cell), that might be a neat project.)

Winfried

Thanks much for the infos. I don't have the skills to build this myself, but I might meet someone who can.

PatrickGSR94

Why the need to sound specifically like a car horn? Car horns have many different sounds among all the makes and models - from the lower-pitched American car horns, to the dinky "beep beep" sound of most Japanese and other Asian cars, to the high-pitched air horn sound from a Ferrari:


I say as long as the horn is loud enough, it really doesn't matter what it sounds like. It WILL get motorists' attention if it's in the 110+ dB range.

proileri

What you need is a small air tank you can pump up before riding, and an electronic valve for push-button action.

ItsJustMe

I just use a real car horn, a very small 12V lead-acid gel cell and a horn relay, both of which are smaller than fist sized together, and a button. Didn't take long to hook up.

Winfried

I'd be interested in any picture/infos about how to build this myself, provided it's doable by someone who has zero knowledge of electricity.

nerys

why not try 2 9v batteries? it just might work depending on how often you expect to use it you might only get a few or maybe a dozen little squirts out of the batteries but it will be small and light.

dougmc

9v batteries (the little ones used in smoke alarms) won't put out anywhere near the amperage needed to power a car horn. Not even once. Not even with two in parallel.

8-10 AA batteries might, however -- especially if they're NiCd or NiMH.

nerys

Hmm I am not so sure of that. I will have to try it when time allows. Horns don't cross me as "high amp" devices considering the dinky wires they use to wire them up. maybe we are thinking different kinds of horns? I think thinking the little shell shaped thing bolted in front of the radio of most every car on the road.

not the "air horns" on the roofs of trucks and rv's

I have run quite a few of the 12v car neon lights on pairs of 9v batteries with no issues (runs about 20 minutes) including 2 3ft green neon tubes. they are not "high power" but they are not led's either :-)

PatrickGSR94

The guy who leads a local "social" group ride every month hauls this behind him, powered by 8 AA batteries, that runs for upwards of 3 hours sometimes. Surely the same could power a car horn for a *very* long time.

dougmc

Well, remember that 8 AA batteries has much more energy than a 9v battery -- but even more important, it has a much lower internal resistance. 9v batteries are not made for high drain devices.

According to wikipedia vehicle horns draw about 5-6 amps. AA's can't really provide that much power very effectively (but maybe if you did 10 or 12 it might work), and 9v batteries certainly can not.

And really, if he's going to haul a speaker that large he might as well give it more power, or if he doesn't need it he should go for a smaller speaker. 8 AA's divided by three hours works out to an average of about six watts. The large speaker can make six watts sound decent, but he could go a lot smaller.

I've got a bike stereo that's 1/4th the size of that, powered by a 7 Ah gel cell (I didn't have a larger LiPo at the time I put it together) that does about 25 watts and it'll go for three hours as well. It's probably a good deal louder than this as well, mounts to my rack rather than needing a trailer. (Though if I put the amplifier and battery to this speaker, it would probably work significantly better than the small enclosure I've made.)

I'm not sure what you're talking about then. Speakers?

Oh, you meant radiator, not radio. Yes, that's what I'm talking about. They take around sixty watts, which must be at least ten times what your friend with the bike stereo is using.

I'm not the OP so I'm not sure exactly what he's referring to, but I think those are closer to what he was after -- either that, or horns like this. (That would be the sort that takes 5-6 amps, probably each.) And really, the air horns on the roofs of trucks and rvs might be just perfect for what he wants. Certainly, they're small enough to mount on a bike.

If you want a "horn" that's not much louder than a bike bell, here's an option -- and it runs on a 9v battery. If you want something that sounds like a car horn, it's hard to beat an actual car horn -- but that will require something beefier. 8 AA's might be able to do it (especially if rechargeable or the high discharge rate alkalines, as they can handle higher currents than the standard alkalines) but it would be marginal.

PatrickGSR94

He meant "shell shaped thing bolted in front of the radiator". Radio was a typo, probably mobile device auto correct. Damnyouautocorrect.com hahaha

nerys

yeah I have NO clue where "radio" came from :-) hehehe

yep one of these buggers your typical on 99% of the cars out their horn



I could be flat out wrong but I think a pair of 9v in series would run that just fine in short little blasts.

dougmc

Well, let's do the math.

According to this page the internal resistance he measured from an alkaline 9v battery is 1.5 ohms (and that is pretty close to my guess, so I have no reason to doubt it.)

The maximum power that can be extracted from a battery when the external resistance is equal to the internal resistance (which gives you 50% efficiency, by the way) -- so that's 1.5 ohms externally, 3.0 ohms total. 9 volts / 3 ohms = 3 amps, and since half the resistance is internal to the battery, our load only sees 4.5 volts * 3 amps, or 13.5 watts. Considering that these horns require about 60 watts, and that 13.5 watts/battery is an absolute maximum under ideal conditions ... no. Your horn might make a noise, but it would sound sick at best.

Maybe if you had four 9v batteries -- two in series, two in parallel -- you could get close to 60 watts, assuming that your batteries are fully charged. (If the horn uses 6 A at 12 V, then it has about 2 ohms, so it's pretty close to right for this, by the way.) Six 9v batteries, 3 in parallel, two in series, would probably do it.

AA batteries aren't really designed for this level of discharge either, but they'll work far better, especially NiMH or NiCd cells (which have lower internal resistances than the alkaline cells.) Ten of them would probably work acceptably well for short blasts.

Really, I'd suggest a R/C LiPo battery. They *are* designed for this level of discharge (and a lot more) -- a 20C 2000 mAh 3s pack is pretty typical and yet cheap, and the 20C means it's rated for a continuous discharge rate of 40 amps -- so six amps (or 12 amps if you use two of them like many cars do) would be no sweat.

nerys

Like I said I could be flat out wrong here but I just can't see that horn taking 60watts. that is the power a halogen headlight takes. More.

when I turn on a headlight There is a audible drop in engine RPM as the alternator loads down to power the headlight (yes one was running a relay test)

when I use the horn under the same conditions there is zero audible change in engine rpm.

again maybe it does take 60watts but I find that hard to believe. that is a crap ton of power for such a little horn.

I guess I am just going to have to try it and see what happens :-)

PatrickGSR94

Here you go, Hella supertone or universal disc horns - they all need 5.5 amps at 12 volts:

https://cat.hella.com/hella-nafta/upl...structions.pdf

nerys

man thats a ton of power. guess my metro horn is a really wussy horn cause it does NOT take no 5-6 amps of power :-)

I will have to see what my oooga horn takes.

Winfried

Thanks for the infos. So it looks like to an electric alternative to the AirZound could use about 3 amps and be powered with a 2000mAh 3s R/C LiPo battery (www.rchelicopterfun.com/rc-lipo-batteries.html)?

dougmc

If you use an actual car horn, you're looking at six amps rather than three, but that battery would handle it just fine, and would probably give you 20 minutes or so of blasting (so about 20x what the AirZound would give you.)

If you're using something else, then you'll have to see how much current it draws, but as long as it draws less than 40 amps a 2000 mAh LiPo battery will probably be fine (especially in small bursts.)

TScott27

Although I haven't ever tried this before because my wife says it is a stupid idea, I would love to see my Thunderpower megaphone become my horn. It would definitely cause a riot on the road. Hmmm... Can this be truly possible? Just an idea worth exploring.

10 Wheels

Found this one:

Winfried

This project looks good...

https://www.orpland.com/

... although I'd rather something louder and sounds more like a car horn instead, even if it means having to hook up a bigger battery.

nerys

wow that orp looks pretty nice. I like the friendly tone option too for when you want to alert but not annoy :-)

fietsbob

a 12V motorcycle Gel cell, battery and the actual horn out of a car in the bone yard?

Here there is an older guy mounted a big air-compressor and tank in the back of his pickup truck
and mounted 2 horns off the exhaust stack of a couple ocean going Ships,
to sound "for emergencies"..