230V LFO

birt · October 21, 2007, 02:59:58 PM

I want to build an LFO with an output of 230V. i want to be able to control an electric horn with this. the horn normally works with line voltage. it has a metal hammer that hits another piece of metal at 50Hz. you can see where this is going. if i can control the frequency of the power i can control the frequency of the horn. but i need an expensive frequency controller to do that. but i was wondering if the same might be possible with some kind of LFO.

MartyMart · October 21, 2007, 04:25:10 PM

How about a "variac" ?? .... as used by tube amp guys

birt · October 21, 2007, 07:13:09 PM

a variac doesn't change the frequency. it'll just allow me to find out what minimum voltage that thing needs to make a sound. frequency modulators are also far more expensive. if it's not DIY ofcourse

MartyMart · October 21, 2007, 08:35:03 PM

ooops, sorry Birt, should have read more carefully... my bad !!

Paul Perry (Frostwave) · October 21, 2007, 09:07:03 PM

Well, 230v mains is just an audio signal - a LOT of audio signal, but that's what it is.

So if you just take a powerful amplifier - with response good down to 50 Hz or whatever your lower note is going to be - and crank it up, and run the output through one of those public address system line distribution transformers, maybe you will get there.

BUT.... what can go wrong?
likeliest thing, is that the original horn is optimised for 50Hz (or 60hz, depending on country) operation. It takes much less power to drive a resonant structure, and maybe that is happening here. My barber got me to convert a USA handpiece (110v 60Hz) to run on local 230v, but the cutting mechanism was designed to vibrate at 60hz so it didn't like the 50Hz we have here in Australia.
The other thing is, the alarms i see here, have a hammer that whacks a bell. Which it does 100 times a second. So what you hear is not the mains frequency, but the resonant frequency of the bell, which is much closer to peak hearing range. Whacking the bell at a different frequency isn't going to change the pitch of the sound much at all.

R.G. · October 21, 2007, 10:25:20 PM

Moving things takes a lot of power. Have you thought about how you'll provide that much power?

birt · October 22, 2007, 06:14:23 AM

i was thinking about a big rectifier on mains and then modulate that the way i want it. but i don't know how. i figured if the rectifier is big enough the circuit should have enough power to drive that horn.

this horn doesn't have a bell. the hammer just hits a flat thick piece of metal. it's a pretty loud low sound and kinda honky with the horn in front. it was used in a factory as an alarm when machines started operating.

i found a recording of that sound (not that good but you'll recognize it immediatly):
26. Warehouse type buzzer
on this page: http://www.audiosparx.com/sa/display/sounds.cfm/sound_group_iid.779

R.G. · October 22, 2007, 09:53:32 AM

Quotei was thinking about a big rectifier on mains and then modulate that the way i want it. but i don't know how. i figured if the rectifier is big enough the circuit should have enough power to drive that horn.

Yep, that would provide enough power.

But do you have any idea
(a) how hard it is to do the modulation
and
(b) how incredibly dangerous this is?

There is a real chance of dying from electrocution with this setup, not only in building it but forever in using it.

gez · October 22, 2007, 10:11:19 AM

Birt, out of curiosity what exactly are you going to do with this horn? There might be an easier (safer) alternative?

birt · October 22, 2007, 05:25:34 PM

i realise how dangerous it is, i am used to work in tube amps.
what i want to try is to make some kind of crazy organ with keys controlling the frequency of some of those horns. but the first thing is just checking how these things react to different frequencies and what the (if it is possible) usable frequency range is. to test that i need to build this LFO.

i am a theatre technician and this "organ" would be used in a play. so i want to make it look special too, that's the easy part for me

Andre · October 23, 2007, 03:34:21 AM

There are schematics around of 12V DC to 230V AC inverters.
The ones I saw all have a 50Hz square wave oscillator fed into the secondary winding of a 12V transformer using some high power transistors like 2N3055.
The primary winding will then give you 230V AC.

You could try to adapt this principle and change the frequency of the oscillator.
I'm not sure what the transformer will do with different frequencies, but there's only one way to find out.

I could dig up a schematic of such a converter if you want, but that would have to be tonight, because I'm at work right now.

André

the_random_hero · October 23, 2007, 05:33:50 AM

What about wave generator > power amplifier > step up transformer? There's probably a reason why this won't work.

soulsonic · October 23, 2007, 08:31:45 AM

What you need is a variable frequency power regenerator. As has already been stated, the required parts are an oscillator, amplifier, and then a step-up transformer to get the required voltage.

First and foremost, you need to determine the current requirements to run the thing in the first place. This is the deciding factor for what 2/3 of the project will require.
If it's needs are very light, then it may be feasible, but if it requires a large amount of current, I'd seriously consider different options.....

Might I suggest the wondrous 21st century technology of digital sampling?

birt · October 23, 2007, 02:21:22 PM

hèhè, digital sampling doesn't have the looks :p

i'm gonna measure the current draw and post it here.

soulsonic · October 24, 2007, 03:02:13 PM

If it's being used for Theatre, then it doesn't even matter if it's making sound or not - it just has to look interesting. I think it would be easy to build up the "organ" as a prop and then use the sampled/pitched sound of the horn played through the sound system (or through a speaker hidden in the prop if you're wanting to be really slick). It seems like that would be way easier.

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230V LFO