Research and Development..

So, the next evening we reconvene, and start to work on a plan.

I have a way of controlling the USB part already, I am going to use the Velleman experimental USB board.This is a handy bit of kit, even though it is obviously far too large to use for a finished product, it will be ideal to test the prototype. It gives me a number of inputs / outputs, both digital and analogue as well as some sample software to enable us to test the theory of whether we can get it to work.

We intend to use the 2 analogue out channels from the board, which will give us a variable 0-5V supply at a about 200mA, we can use this to power a relay / transistor or what ever we come up with to power the motors. Here you can see the 2 analogue output channels with 2 LED indicators. They both use a simple common ground and we can then use the inbuilt software to control the voltage.

We do a bit of research and find that a great way to do the control is to use an Opto-isolator.

Opto-isolators, or opto-couplers, are made up of a light emitting device, and a light sensitive device, all wrapped up in one package, but with no electrical connection between the two, just a beam of light. The light emitter is nearly always an LED. The light sensitive device may be a photodiode, phototransistor, or more esoteric devices such as thyristors, triacs etc.


This makes them ideal for this type of job.
We can make up a circuit similar to the one above, where the two side are isolated by the opto-isolator (means we can't blow up the USB end) and we can then use the optoisolator to power a transistor, to regulate the two motors.

We sit down and do some calculations on what we are going to need to buy and we come up with.
1: A 2 channel opto-isolator TLP-624-2 (a channel to control each of the motors)

2: A couple of 560 ohm resistors to regulate the voltage into the optoisolator (R1 in the diagram)

3: A couple of transistors (BD136) these are to allow the power to the 2 motors. They are quite big transistors, but are spec up to 1.5amps and will cope if the motor is ever "jammed" and starts to pull more current than normal. (best to be safe than sorry)

4: 2 Diodes, to protect the transistors if the motors ever get wound backwards, and start generating a reverse current.



A quick trip down to the local electronics store and we have all the bits we need. I also got some Vero strip board on which to mount the circuit, and already had some wires etc to make up the terminals. (you notice I have 4 transistors here, the others are BD140s which are slightly higher rated and we can substiture in, just in case the others get a bit too hot.)
The other bit you may notice is the 8 pin socket for the optoisolator chip, this will make life much easy when soldering (and there is no danger in messing up the chip through heat damage)

We sit down and draw out the diagram and mark up what we gonna put on the board and where, then we leave it as it is getting late. (and the pub has opened)


We will start constuction... tomorrow.