Before anyone downvotes the question for my recklessness:

Disclaimer: I acknowledge that it is not advisable to make the connections mentioned in the question. This is just a hobby experiment where I can afford to dispose or damage parts in the name of learning.

I have the following:

  • Parallel Port in PC
  • LED
  • 9V battery
  • 6VDC 100-ohm relay

I can control Parallel Port signals through my software code.

  1. I tried connecting the parallel port to LED and it worked

  2. I tried connecting the 9V battery to relay and it worked.

  3. When I tried connecting parallel port to relay, it didnt work - leads me to believe voltage (or current) was not enough.

(yes yes.. I know.. some of you are itching to punch me in the face)

With just the parts I mentioned above, without using Darlington Transistor or other such devices, is there any way in which I can connect them all, so that I can control the relay from the PC - at least for a short period of time?

  • \$\begingroup\$ Without using anything else? I think you've already found the answer: no, there isn't enough power delivered from the parallel port. \$\endgroup\$ – pjc50 Jan 8 '13 at 17:15
  • \$\begingroup\$ @pjc50 I was wondering if it is possible to somehow use to 9V battery for power and arrange the rest so that the port signal will open or close the circuit? \$\endgroup\$ – user17643 Jan 8 '13 at 17:16
  • \$\begingroup\$ Well, you said "without using .. other such devices"; if you're willing to add extra components, add the right ones and get a transistor. \$\endgroup\$ – pjc50 Jan 9 '13 at 9:39

It's likely that your parallel port does not provide a high enough voltage. You can check this by connecting the relay to a variable power supply, start with ~4V and increase the voltage until your relay clicks. If the voltage on your power supply needed is higher than that from your parallel port, it does not give you a high enough voltage.

If your parallel port however provides enough voltage, the problem most likely is current. The parallel port will not provide enough current to switch the relay, and you're most likely damaging your computer.

So the answer to your question is no.

Anyway, for anyone looking for a working solution: it will work with a transistor like this circuit:

Electronic switch with a transistor

This uses a 50 Ohm resistor because the relay is 6V / 100 Ohm.

| improve this answer | |
  • \$\begingroup\$ The topic starter wants nothing else but the listed parts, so also no regular transistor. \$\endgroup\$ – Passerby Jan 8 '13 at 18:03
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    \$\begingroup\$ That's why I say the answer to his question is no. But for people who come here looking for a working solution, I give a solution with two extra components. \$\endgroup\$ – user17592 Jan 8 '13 at 18:04

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