Help with Relays!! School project!

[DoktorD]

Hi guys,

I'm working on a project for high school, and to be quite honest, I've never worked with too much electronics in the past. What I'm trying to do is get my mp3 player to activate a relay to turn on a DC model motor.

I know that the power coming from the wires in the headphones is AC (or is it?). My question is, would the AC power from the mp3 player be able to activate a relay switch even though its AC?

If it can't, is there any way to convert it to DC?

And what setting on my multimeter would be used to give me a reading of milliamperes? I have "400m, 40m/20A, 4m, and then 400:mu:A". Which one would give me the reading in plain mA provided my prongs are in the correct holes (which they are). I have a Techmaster DM-8400 if thats of any help there.

 

[Leftyretro]

Hi guys,

I'm working on a project for high school, and to be quite honest, I've never worked with too much electronics in the past. What I'm trying to do is get my mp3 player to activate a relay to turn on a DC model motor.

I know that the power coming from the wires in the headphones is AC (or is it?). My question is, would the AC power from the mp3 player be able to activate a relay switch even though its AC?

If it can't, is there any way to convert it to DC?

And what setting on my multimeter would be used to give me a reading of milliamperes? I have "400m, 40m/20A, 4m, and then 400:mu:A". Which one would give me the reading in plain mA provided my prongs are in the correct holes (which they are). I have a Techmaster DM-8400 if thats of any help there.

The audio AC signal (not power) coming from the audio output of your mp3 player would not be able to directly activate any normal commonly avalible relay coil.

To solve your task we first need to know the specific relay you are going to use, it's voltage (AC or DC coil voltage rating) and it's current requirement. You then need to figure out what power source you are going to need to activate the relay. Finally then a circuit to detect the mp3 audio into a digital on/off circuit to drive the relay will be needed.

On your multimeter, one normally starts on the highest current range and switches to lower range as needed.

Lefty


Lefty

 

[DoktorD]

So what exactly is the difference between the AC signal and AC power?

We knew the signal (power, audio, or whatever) coming from the mp3 player would not be constant, so we thought maybe if we played a constant tone on the mp3 player, it would be an almost constant signal, and the (almost) constant signal played through would be able to trip like a 1 mA relay?

We were just guessing sing we don't know too much about electronics.

And we have no idea what kind of relay we would need.

**broken link removed** as long as the mp3 player could put out 1.2mA where I imagined it could.

But, these were all ideas considering I don't know much about electronics.

 

[wmmullaney]

I think 2 diodes would help with an AC current, an mp3 player should drive it. (just turn up the volume) It will with 2 diodes control a transistor, is a transistor suitable for what you need to do?

 

[Wingmax]

Hi guys,

I'm working on a project for high school, and to be quite honest, I've never worked with too much electronics in the past. What I'm trying to do is get my mp3 player to activate a relay to turn on a DC model motor.

I know that the power coming from the wires in the headphones is AC (or is it?). My question is, would the AC power from the mp3 player be able to activate a relay switch even though its AC?

If it can't, is there any way to convert it to DC?

Assume you want to drive the relay from your headphone. As was suggested before you need to know what sort of relay is driving. What voltage required (usually DC), resistance of coil, current drawn etc. Once you know all that, you might need some sort of amplifier stage to amplify the audio output, and then rectify to DC voltage to drive the relay coil.
Or may be use sound activated method.

 

[Voltboy]

Why don't you rectify the AC signal and then power the motor via a power transistor or MOSFET??

 

[DoktorD]

I think 2 diodes would help with an AC current, an mp3 player should drive it. (just turn up the volume) It will with 2 diodes control a transistor, is a transistor suitable for what you need to do?

Well on the multimeter, I'm getting 79.5 mA on the 400m setting with volume all the way up. Could it trip a relay?

The diodes keep the current going in only one direction right? Like a water check valve. If I used the diodes, what type of diode do you think I should use? I wouldnt even know where to start trying to figure out what to use there.

I was going to use **broken link removed** relay switch.

Do you think that would work? I'm trying to stay away from the sound activation method.

 

[Leftyretro]

So what exactly is the difference between the AC signal and AC power?

We knew the signal (power, audio, or whatever) coming from the mp3 player would not be constant, so we thought maybe if we played a constant tone on the mp3 player, it would be an almost constant signal, and the (almost) constant signal played through would be able to trip like a 1 mA relay?

We were just guessing sing we don't know too much about electronics.

And we have no idea what kind of relay we would need.

**broken link removed** as long as the mp3 player could put out 1.2mA where I imagined it could.

But, these were all ideas considering I don't know much about electronics.

Well find a DC relay that activates directly on 1ma at 1-2 volts dc or so and we can see what we can do for you. I'm sure such a sensitive relay has been made at sometime in the past at some high price, I just don't think you will actually find one. Otherwise my first post still applies, external power source, detection circuit, relay driver circuit.

PS: your link shows a 1ma current "trigger" but requires a power source to power the relay coils (12-24vdc at 30ma for each relay), just as I tried to explain before. See the specifications section.

Lefty

 

[Wingmax]

Why don't you rectify the AC signal and then power the motor via a power transistor or MOSFET??

The audio signal comming out from the earphone will not be large enough to drive a relay. Also, it won't be able to supply enough current.

 

[erosennin]

You could send the signal through an op-amp, gain the living daylights out of it, as I assume sound quality is not of the essence. You then could rectify the signal from the op-amp and use that to drive a transistor to switch the relay.

 

[audioguru]

The relay he has selected already has a transistor driving it. It needs to have an input of 5VDC to 24VDC at a low current.
I don't know the output AC voltage of the MP3 player so the opamp should have a trimpot to adjust the amount of gain.

 

[erosennin]

The relay he has selected already has a transistor driving it.

Sorry, didn't read into it in great detail...

 

[Krumlink]

wait, how is this a robotic thread? Oh wait its NOT!

Make sure you post your questions in the right category...