Activate tiny RELAY by headphone jack?? (PLEASE HELP)

the question that needs to be answered, does the signal on the jack need to do something other then turn on a relay?

 

This looks impossible without an amplifier and power supply.
What is very small?

Gas central heating boilers use a thermocouple to drive a relay but this is DC.
If you rectify the signal to get DC you will lose voltage in the diodes.
If you need the relay to hold for a time, you will need a capacitor and sufficient power to charge it up.

 

You should be able to get something working with a LM3915, once adjusted/calibrated for your input levels, just use a transistor on the appropriate output pin to drive the relay... Hardly the most eloquent solution but for someone without much experience they are likely to get it working without much effort... You can even purchase DIY UV meter kits built around this chip that would only require the small modification/addition of the transistor and relay...

 

You can use a transistor or intgrated circuit to amplify the signal to switch a high power. In order to do this, you will need a power supply which I thought you did not want.

 

I have one idea..... a 3.5mm headphone jack normally has three pins - so that the 'speakers' can be cut off while using headphones.

So there's your switch. now you just need sufficient DC to a) drive the relay. and use an RC to determine the off time as suggested by (*steve*).

trouble is we are assuming there is sufficient 'sound' at the point of insertion to drive all this? what if it's a quiet bit? can you energy harvest the signal before plugging in the jack - with a rectifier and a cap, then the switch-on would happen when you want it to & not 'some time' later?

 

That changes a few things. What Steve said stil make the most sense if you don't want a power supply.
I think it would look like this, although the polarity of the diode might depend on the type of MOSFET and the image shows a polarized cap and a regular cap should be fine.


Just curious, since your messin around with the wires connecting to the back of the CD player and there is 12V +/- back there, why not use it. It opens up a lot more ways to do this.
--tim

 

!!! So not "I am trying to make a 3.5mm headphone jack activate a small relay" then.

Okay. For clarification here, are these two wires at line-level or the amplifier output? I know you said speakers but you did also say headphone jack before.
do you require this event to be a one off or pulse continuously with the sound levels?

/edit
I think IRF640N is a bit of overkill to drive a relay coil!
/

 

I have not looked up the data for the IRF640 but it probably has a threshold voltage of 3 or 4V so will not do any thing with a signal below this. There are mosfets suitable for digital circuits with lower threshold voltages.

You could consider an audio transformer to raise the voltage to a level that will work.

The diagrams shown previously shows the FET to be off and turned harder off with signal input.

You should try to get a measure of the output voltage that you are dealing with so that an appropriate solution can be found.

 

Thanks. No need to shout though.
So we are back to a headphone output stage, with not many volts and a few ohms output impedance (apparently an ipod is 5 ohms - not verified ) driving a switch...........hmmm

 

True, but i just don't think there is going to be enough juice available to hard switch anything!
I'm expecting anywhere between bouncing relay to nothing at all happening