computer controled relay

[theChris]

Ok, i am a second year EE student. that being said, dumb down any conversations on digital circuts or semiconductors or whatnot. also don't suggest things that are expensive!

problem: i have 2 cold cathodes in my computer case. they have a switch. i have to open the case to turn the switch off.

what i want: i want to control the switch via computer. my plan is to run some kinda signal to a transistor that will turn on a relay, or turn off a relay. the signal will come from the serial or parellel port.

what i had thought: some kinda Xor gate where the serial port is one input, and the ouptut is the other input. i don't have any idea why this would really work, and figure it is probably the hard way to do this all.

[SkinnyBoy]

Why don't you just put a switch on the outside of the case?

Do you have a USB printer? or no printer at all?

Serial wouldn't be possible....

Parallel... EASY!!!

Just a sec..

[SkinnyBoy]

http://www.aaroncake.net/circuits/interfac.htm

[theChris]

hmm, i had wanted something a little more dumbed down then that. I did think of something though. it involves 3 transistors involved as switches, and 1 op amp as a comparator, and a few resistors and a relay. possibly a coulple diodes. i'll attach the likely very wrong version. like i said, i'm new to semi-conductors.

[SkinnyBoy]

lol, I have a confession.. I don't even get the circuit I posted.. lol..... I have a circuit in magazine somewhere for a single parallel port controlled relay.... Don't know where it is though... as for the schematic you posted.. . umm.... I dunno..

[theChris]

see, my idea is that when the on signal is passed, it will make the transistor into a switch, and this will turn on the drive transistor. this will begin charging the relay. this will also send a signal to the op amp, which should be a non-inverting comparator. since the signal voltage is above 0, the opamp will send a signal to the drive transistor. once the "on" signal is let off, the drive transistor will still be on because it is controlled by the opamp, and the opamp will stay on because the drive transistor is on, so it will stay on. once the off signal is sent, the "off" transistor should act like a switch, and allow the same voltage to reach both terminals of the op amp, which will cuase the op amp to stop sending the signal to the drive transistor, which will cause the transistor to turn off, and the opamp should stay off.

i am wondering if it will work and if i drew it correctly, and if the relay's inductor will supply a signal to turn the op amp back on, which would mean that the thing would be hard to turn off.

[SkinnyBoy]

Ummm... okay.. lol... so, what exactly are you trying to do with this? have a program that you can use to toggle the relay on and off?? or what?? :P lol

[jackinnj]

Microsoft was never happy about controlling things on the outside from inside windows, thus you are going to have to noodle around a little in Visual Basic and perhaps get the DLL's which allow you to work around the operating system. The circuit below will control several relays or switches.

Be careful with the parallel port. You can fry your entire machine if you draw too much current on any pin.

[SkinnyBoy]

Well, as for software.. someone already wrote a rather nice Windows program....


http://www.aaroncake.net/files/index.html


To go with this circuit....


Also...

How To Program The Parallel Port In QBasic

How To Program The Parallel Port In Visual Basic

[theChris]

hmm. I'll look into these. I am new at semiconductors, so i don't know how to get these things to work for me. I wanted something where you would supply a pulse, and the relay would turn on. another pulse on another terminal would turn the relay off.

could someone explain how circut with the octal something with inverted output works?