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What relay would I need to use to switch some 12v LED's using an amplifier?

I have some LED's that run off 12v with resistors I have. I want them to flash in time to music so I am using a small 9v amp soldering that I made a while ago in a design class. What voltage and current do you think the amp will give from the speaker terminals which will be required to switch the relay? I don't really know how powerful the amp is, I have a multimeter but when I tried to measure the voltage it didn't really work. I think it may have been peaking at about 6v

Not enough data, how powerful was the amp ! Conceptually it won't really work well . What you might like to do is somesort of LED power meter ? Loud sounds light up more LEDS ?
pyro-jim (author)  steveastrouk4 years ago
edited. Also why shouldn't it work well? There is an instructable similar to this; if you search for music lights it is the first
Just because there's an instructable doesn't make it a good idea. A relay will only come on if the voltage on its terminals exceeds a minimum pull in voltage which is usually pretty close to its rated voltage. So a 6V relay will only pull in on 5.9V. That means your amp has to be banging on at close to its maximum output power to do ANYTHING. It will only respond to sounds below about 10Hz, because relays, being mechanical thingies have inertia which means you can't yank them on and off very quickly (and if you do something will wear out) That will limit just how you can shape the response of your lights. You'd get a much better visual experience with Lemonie's idea, which is on the right tracks with a transistor switch. It would only take one transistor and a pot and you could tweak the lights to suit the volume. A bit more refinement, and playing with something like the LM3915 bargraph chip and you'd have some really neat effects, for very little outlay.
pyro-jim (author)  steveastrouk4 years ago
How would I do it using a transistor?
How many LEDs have you got ? Basically you're adding another amp to your exisitng one. Try an old multimedia speaker amp ?
pyro-jim (author)  steveastrouk4 years ago
sorry I don't really understand what you're saying why would I need another amp?
Because you aren't going to get decent brightness out of 12V LEDS (assuming you can't change the resistors)) on an amp with only a 9V supply. Even if you run the amp flat out, you are only going to see something like 7V on the output max. The bigger amp will probably have a higher output voltage, which you LEDS will like. Steve
pyro-jim (author)  steveastrouk4 years ago
The amp could use a seperate voltage supply. Ive been doing a bit of research on transisitors and I'm starting to understand it now
EXACTLY ! Steve
pyro-jim (author)  steveastrouk4 years ago
Could you show me in a circuit diagram or something you mean please. It would really help me, thanks
pyro-jim (author)  pyro-jim4 years ago
I will look into how to do what you talked about; light up more LED's with more sound
http://www.national.com/ds/LM/LM3915.pdf

....is the classic chip for it. There are some simple circuit ideas in there too, nothing too hard to build.

Steve
pyro-jim (author)  steveastrouk4 years ago
It doesn't really give the effects I'm looking for so I think I'll leave it for another project. For now I'll stick with just the transistor. Would I need a pot aswell do you think?
Descrie what you are after and I'll have a think..See what the amplifier idea we suggested earlier actually looks like before you mess aout making soimehing
pyro-jim (author)  steveastrouk4 years ago
Well I've seen an instructable that does the same thing using a transistor but they didn't use an amp. I'm not sure whether it would be better with or without it
lemonie4 years ago
You could probably switch with a transistor. What exactly do you have, how are they wired and can you knock-up a circuit-diagram? L
pyro-jim (author)  lemonie4 years ago
I already know that the relays needs to be able to handle 12v it's the switching voltage I'm not sure of
OK, if you're intent on using the relay, the technical spec' should tell you. Because we don't know what relay you've got I can't. But the relay is going to be a heavier draw than the LEDs themselves or a transistor. You also have a slower response with a mechanical device. I don't think this is the best solution, as you're asking the amp to drive both the speakers and a relay coil.
Give more information on the relay, amp and speakers and I might be able to say more.

L
pyro-jim (author)  lemonie4 years ago
There are no speakers because I am using the amp to poer just the relay and I haven't got a relay yet; that's what this question is for
Without knowing the amp it's not so easy to give advice, but I think you'd be better with a transistor switch over a relay. L
pyro-jim (author)  lemonie4 years ago
I think I'm heading towards that idea
And the relay is going to make a clicking noise which will probably annoy you rather than adding to the soundtrack. L
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