Computer control is the
ultimate; it makes you feel more like a real scientist experimenting with different chemicals, sampling them on super-high-tech
pieces of equipment, overall its pretty cool. If you have a desktop PC, it is guaranteed to have a parallel port, which is
a great way to interface lab equipment to your computer. There is very little electronic skill needed to connect things to
your parallel port, because the port pretty much does all the controlling by itself. Below is a diagram of a circuit that
can switch a relay on an off using simple software commands in a programming language called Qbasic. You can control up to
eight relays from your parallel port with very little hardware or software necessary. Through a relay, you can switch mains
voltages (120v AC), meaning you can control electric airline valves for breathing masks, compressors for inflating suits,
lab projects like the exploding resistors, you name it, you can control it with a relay.
you can see from the drawing it is not that complicated when you really think about it, pins 2 to 8 can be controlled in Qbasic,
so those are the pins we connect our relay circuit to. Pins 18 to 25 should be soldered together as they are the ground of
the circuit. Both the circuits are exactly the same, the negative side of a 9-volt battery is connected to the ground of the
circuit (Pins 18 to 25) and the wire coming out of the circuit that is connected to the parallel port is the control line.
If you want to control eight things from the port you will have to build 8 separate circuits, I know this is a pain and there
are probably easier ways to do it, but this works fine and I would highly recommend it to people who are just starting off
or only need to control one or two things. When you are choosing a relay make sure that you get one that is rated for the
load you are planning on switching.
is a table with the component requirements to build the circuit above, which can control two separate devices.
|3K 1/4 Watt Resistor
|7.5K 1/4 Watt Resistor
|2N3904 NPN Transistor
|"Sugar Cube" Relay
|Standard AC Outlet
|Standard AC Plug with cord
|DB25 Male Connector
You could assemble
the circuit about using point-to-point soldering, a breadboard, however you want to do it. If you do not want to tackle this
project and would be interested in purchasing a ready-made circuit from me please send an email to email@example.com.