powering relays

[coma]

ok im using the parallel port to run my circuit. on this circuit there are 3 relays now relays 1 and 2 are both 2 channel relays, relay 3 is a single channel relay. the voltage when i set one of the bits high is around 2.7 i think. any ways it enough to power the single channel relay to switch it on. but for the 2 channel relays it requires 3.1v - 5v so im going to add an external supply of 5v. my problem is how to i get a high signal from the port to send the 5v to the relay to power it up and hence turn it on?

 

[Gandledorf]

ok im using the parallel port to run my circuit. on this circuit there are 3 relays now relays 1 and 2 are both 2 channel relays, relay 3 is a single channel relay. the voltage when i set one of the bits high is around 2.7 i think. any ways it enough to power the single channel relay to switch it on. but for the 2 channel relays it requires 3.1v - 5v so im going to add an external supply of 5v. my problem is how to i get a high signal from the port to send the 5v to the relay to power it up and hence turn it on?

How much current do they require? You could just send your high through an amplifier and boost the signal to 5V if all you require is a voltage level without much current to back it up.

 

[coma]

well it very small what ever it is, i dont think current will be a problem. i was thinking of amping it up but i dont know how or how i could control it by sending a high signal from the parallel port?

 

[Gandledorf]

Either an op-amp wired as an amplifier, or using an amplifying transistor would work.

Shouldn't there be a 5V lead from the parallel port for powering external devices? I haven't worked with a parallel port in a long time, so maybe I am mistaken.

 

[coma]

even if there is a pin that supplies 5v i still need to control the relay with a 2.5v high signal

 

[nettron1000]

You could use a ULN2803 transistor array IC to boost the output from the parallel port to drive relays, heres my circuit:

http://www.geocities.com/nettron1000/ParaPort3.html

 

[Nigel Goodwin]

Shouldn't there be a 5V lead from the parallel port for powering external devices? I haven't worked with a parallel port in a long time, so maybe I am mistaken.

No, there's no 5V output, one way to 'fudge' a low current 'almost' 5V output, is to AND any unused outputs together with diodes and set those outputs all permanently high.

Certainly for running relays off a parallel port, I would be using driver transistors (or a ULN chip) and an external PSU.