Activate a relay using laptop's parallel port

[getfree]

Hi, guys. I'm new here. Forgiveme if this is too newbie.

I have a relay that I want to switch on using one of the output pins on the parallel port.

This is what I've got:
**broken link removed**

The relay is powered by a 5V DC adapter and the transistor should act as a switch.
But i dont know where to connect the parallel port's ground pin.

If I just connect the 5V pin to the transistor's base (like the diagram shows), it doesn't work.

 

[sunil_n]

hmmm connect the ground to the negative terminal of that 5V. i.e, connect the negative of the 5V and emitter of the transistor together to the ground. see the figure

 

[flashking]

the parallel port has several ground pins (from pin 18 to 25)

just connect the ground to any of them

 

[getfree]

hmmm connect the ground to the negative terminal of that 5V. i.e, connect the negative of the 5V and emitter of the transistor together to the ground. see the figure

I thought of doing that but I was afraid. Wouldn't that burn my port if for example I use a larger relay which needs to be powered by say a 12V supply?

 

[sunil_n]

anyways it gets the closed path through the ground. it just gives path to current and its a ground so should not be a problem,try doing this for the 5V relay, if it works then try for 12V

 

[getfree]

anyways it gets the closed path through the ground. it just gives path to current and its a ground so should not be a problem,try doing this for the 5V relay, if it works then try for 12V

It works with 5V. Thanks.
Now I dont know if I want to try with 12V.

 

[sunil_n]

it should be fine with the 12V using a 12V relay. I am 80 percent sure of that:|

 

[aryajur]

Another suggestion is to put a resistor from your parallel port output pin to the base of the transistor since right now a very large base current will be flowing when the transistor goes into saturation. You should limit that using a resistor.

 

[hussain_kiet]

adding a series resistor is necessary to limit the max current drawn from the port and it will work 12v relay..
but if you are still not satisfied you can add a optocoupler between the port and transistor...

 

[pranam77]

When you use 12 volts, use a MCT2E Optocoupler for safety. Parellel port Pin 2 > 470E resistor> Pin 1 of MCT2E, Pin 2 Gnd (18 to 25 all pins shorted). PIN5 > 2K2> +12Volts. Pin 4 > Transistor base. transistor base >10K to 12volts Negetive.
Cheers