Re: Introduction

David Johnson wrote:

> From: "David Johnson" <dcjohnsn@...>
>
> Hi, my name is Dave Johnson. A friend of mine asked to take a look at the
> list, so here I am. That friend is Tim Goldstein. I am in the process of
> building a cnc flame/plasma cutting machine. I am using Camtronics 5 amp
> controller and Desknc. Everything is moving accurately, but I am unable to
> trip a solid state relay for the cutting O2. I am getting a signal from the
> parallel port, but it must not be enough current to pull in the relay. I
> applied 5 volts from the logic portion of the controller board and measured
> the current. It only takes 7ma to pull in the relay. Any suggestions on
> how to pull in the relay from the signal on the parallel port? I am not an
> electronic genius, so the simpler the better.

OK, I assume the 'output on' signal is a logic 1 (5 V)? What you need is
an NPN transistor. A 2N2222 would be fine, but any NPN transistor
with a current rating of, say, 50 mA, and a voltage rating of 50 V will do.
Now, connect a 10K Ohm resistor from the port bit to the Base of the
transistor. Connect the Emitter of the transistor to ground. Connect the
Collector of the transistor to the relay. Connect the other side of the
relay to +5 Volts. Finally, you need to protect the transistor from the
inductance of the relay coil. Connect a diode, such as 1N4148 or 1N4002
across the relay coil. Connect the cathode (banded end) to the +5 volt
end. Connect the anode (other end) to the transistor-collector end of
the relay coil. This diode will only conduct when the transistor turns
off, shunting the inductive current around the coil until the current dies out.

You may also need to connect a 1 K ohm resistor from the parallel port
bit to +5 Volts, if the port's pull-up is weak.

Jon