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Adding a manual/computer control toggle; optoisolator help

Hi,

I have a circuit that controls several motors and lights (all 12VDC, 2A max) through push buttons and switches. I'd like to expand the circuit to allow for computer control through a rs232c parallel port.

What should I be concerned about with the parallel port/opto isolator (input resistances, maximum currents, etc). Since I am controlling motors would it be least expensive to use low-current opto isolators to energize relays? If someone has knowledge of (or a site that discusses) the most significant details of wiring up a circuit such as this, I'd appreciate a few pointers; identification of the various opto isolator variables would be helpful too ('If' for example).

I am also looking for a way to implement the "manual/automatic" mode switching. I'll need a total of 10-15 relays if I add one in for each button/switch, so suggestions on a simpler method to implement this would be helpful.

Thanks

David.

Reply to
davidd31415
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Looking through the Jameco catalog results in finding part #174449CF which is a 5 volt/125 ohm coil relay with 24 volt/2 amp contacts. $1.79 each. The relay draws 40 milliamps at 5 volts. I don't know if the computer can supply this current but I think it can. ERS

Reply to
Eric R Snow

Not the parallel port. But you can still get ULN2803s to drive relays with, and that's a darlington input, so the parallel port would drive it easily.

Oh, and just FYI, rs232c is the protocol on the serial port. It and the parallel port are very different things.

Good Luck! Rich

Reply to
Rich Grise

This will be much easier with a parallel port. The serial port is designed to talk RS232, and uses strange voltage levels.

If you are really using the RS232 port, you can use a chip like a MAX232 to interface with the computer.

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It takes +5V, and generates the odd voltages required.

Now, your serial port will want to talk async, so you'll need a uart chip of some kind to handle all the RS232 protocolish stuff. Programming a uart isn't trivial, so you will want to use a microcontroller or something like that. Some PICs have built-in uarts, so using one of those would give you a lower chip count.

Yet another way to go would be to use a pre-built USB interface module, like these:

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The bottom one has up to 24 I/O points, which you can undoubtedly program arbitrarily.

--
Regards,
  Bob Monsen

If a little knowledge is dangerous, where is the man who has
so much as to be out of danger?
                                  Thomas Henry Huxley, 1877
Reply to
Bob Monsen

I'll probably end up going parallel with this instead of RS232; sorry for the confusion in the OP. Are there certain optoisolator specifications I should look out for and is it as simple as just putting the optoisolator in line with the parallel port output or do I need a resistor (or other component) in there as well?

Reply to
davidd31415

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Have Fun! Rich

Reply to
Rich Grise

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