Alternative to relay

[james2k2]

Hi All,
I've recently got a bluetooth Hands-Free speaker for vehicles and I would like to link it to my car stereo's mute line. The bluetooth unit has some LED's that light up when a call is being made or receieved so I can use this to 'switch-on' the muting circuit. On my car stereo, muting is achieved by connecting the mute wire to ground so i figured that a relay would be suitable, but they use too much current.

Is there an alternative to this?

kind regards,
James

 

[LTX71CM]

You can use a suitable transistor (NPN type) to pull down the mute line when the LEDs are lit. The LEDs would feed the base connection. Proper precautions should be taken.

 

[james2k2]

Thanks for your reply. I've attached a circuit - could you tell me if this is what I should be looking at?

Many thanks,
James

 

[LTX71CM]

Looks about right, I would add a resistor between the /MUTE line and the transistor though. You might be safe without out one but I'd be concerned about the transistor sinking too much current should something go wrong.

 

[(*steve*)]

And I would also want to be sure that both components share a 0V line.

 

[james2k2]

This was one of my concerns also. I haven't got hours to analyze the circuitry on my bluetooth receiver, but i'm pretty certain all the LED's and switches share a common line, hopefully this is the 0V, because otherwise the transistor option isn't going to work i think. I did also have a thought, and that was if the +ve is the common rail and the negative is switched by the IC's, could I maybe simply connect the mute wire directly /through resistor to the negative on an LED/Switch somewhere?

Many thanks,
James

 

[LTX71CM]

If you're concerned perhaps a a photo-transistor would work for you. You the know the bluetooth dongle can drive an LED. The only question is whether the LEDs are steady on or not.

 

[james2k2]

Hmm, that's looking very promising. I suppose I could measure current flow with my multimeter first between the mute and ground, just to be sure it's not sinking a too much current. I believe the LED's are constant on as they are backlights for the two buttons (Make/End Call), also; they don't seem to flicker when the unit is 'waved' around. If they are PWM controlled, a capacitor would smooth this?

I've done some searching and found this part: http://maplin.co.uk/Module.aspx?moduleno=2228 would it be suitable?

 

[Resqueline]

Yes, that'll fit the bill, provided the mute current is not higher than 1.2 times the LED current. Yes, a capacitor on the transistor output would take care of any flicker.
One question however is where/ how to connect it to the bluetooth. Do you want to retain the original LED lighting for example?

 

[james2k2]

The Bluetooth PCB is multilayer and has some copper holes that go through the board and these are perfect

 

[Resqueline]

Yes, well, that's ok but not my point. I was thinking about the electronic design aspect. Do you intend to lose the original LED & replace it with the optocoupler?
Or do you intend to run it simultaneously, & if so in what way? Remember the forward voltage drop of IR-LED's is 1.1V as opposed to 2.1V for red & 3.1V for blue LED's

 

[james2k2]

Ok, i've now finally got my multimeter out and measured the current to be tiny at 200uA or 0.2mA. I'm pretty certain the opto-transistor idea should work.

Resqueline: Sorry, didn't actually answer your question, lol. Well, I was going to use the holes to solder some wires to and then try and fit the circuit inside the bluetooth casing - there isn't a huge amount of space though. Then i was going to fit a headphone style connector to connect the mute wire directly to as i'm going for the oem all in solution with regards to bluetooth. I may even get some switches to fit in the spare button spaces for my dash and hook them up to the make/end call buttons as these are needed to pair any new phones. Since there are two backlight LED's for each button and it's going to be hidden anyway, I suppose i could replace the LED with the opto-transistor, would ensure the fitting of the circuit since it's so simple and small.

I'll pop to maplin at some point soon and build the circuit and let you all know how it goes. Should be a simple enough task.

Many thanks to everybody who has helped, it's a great forum here