Modifications to the "Robotics Kit" Hardware ...

14/9/97

The Power Supply - The original documentation suggests that plugpack units up to 30 volts may be used. Problems with overheating of the 12 volt regulator (see below) mean that this is not correct. The plugpack voltage should be such that the rectified voltage is as close to 12.6 volts DC as possible. The statement should read "...between 12 and 15 volts with a current rating of at least 0.5 amps".

14/9/79

The heatsink used on the 12 volt regulator - If the plugpack supply results in more than 12.6 volts appearing at the 12 volt regulator, the energy associated with any 'extra' voltage must be dissipated as heat. A heatsink must be attached to the 12 volt regulator to prevent it from getting too hot. The heatsink should be checked occasionally to ensure that it is firmly attached. The regulator can be operated without a heatsink, but it will get sufficiently hot to cause burns. If it gets too hot, the regulator will automatically shut down.

14/9/97

Reduce the value of the voltage regulator capacitor - The voltage regulator circuit incorporates a 1000uF capacitor. One solution to the heat problem outlined above is to reduce its value to 47uF which has the effect of preventing the rectified voltage from reaching its maximum (as determined by the plugpack power supply voltage). The capacitor should be rated at TWICE the input voltage of the plugpack. A 47uF, 50volt capacitor should do.

23/10/97

The MEL 12 "opto transistor" can be connected instead of a switch on the input lines. It is VERY sensitive. It would be best to inclose it in a long (light proof) tube to improve its response. The transistor has three 'legs': base, collector and emitter. The "base" is the middle leg and is never connected. The other two legs are connected via the white "connector" to the input lines. It will ONLY WORK if it is connected the correct way around. If yours doesn't work, swap the legs over (ie the one that was connected to ground is now connected to the input line and vice versa). But remember ... the BASE (the middle leg) is NEVER connected to anything!!

4/11/97

Stepper Motors ...

We have started looking at using 'stepper' motors out of old 5 1/4" disk drives for building a simple robot arm. There are LOTS of resources on the Web which describe the use of these motors and writing programmes to control them. (A good start would be: http://www.cs.uiowa.edu/~jones/step/index.html ). Just a few advance notes here about using stepper motors with the Robotics Kit interfaces:

Either interface can be used. Connect the "common" wire to the +12 volt line and the other four wires to each of the four output lines from the ULN2803 (before the LEDs). Cycle pulses on each of the four lines to make the stepper motor move.

For more details on Stepper motors see: steppers.html

11/11/97

Stepper Motors continued ...

The ex-5 1/4" disk drive stepper motor approach has exciting possibilities. You can connect TWO stepper motors to the demonstration interface and drive them with code addressing the 'upper' and 'lower' output lines at the same time. eg OUT 17, OUT 34, OUT 68, OUT 136. ie (1 + 16, 2 + 32, 4 + 64, 8 + 128).

This means you can construct a 'buggy' driven by two motors and use code to make it go forward, back, left and right. You will need some sort of swivelling 'jockey' wheel at the front and maybe some wheels (wooden, or plastic) made with a hole saw. All up, just a few dollars to duplicate what would otherwise cost up to $100!!

PS - Our Design and Technology staff have made a screw-on adapter for the stepper motor shaft. (To remove the old adapter grip it in a vice and use a centre punch to drive it off the shaft - actually you drive the shaft through the adapter.)

I've had a quick look at the stepper motors out of hard disks and they appear to be a "bipolar" motor which requires the voltage to be reversed. The Demonstration Interface can't reverse voltage, just turn it on and off. You could use the full (relay) interface,but it would require all four relays to control one motor. I don't think it would be worthwhile.

13/6/98

Using a Zener Diode when driving Stepper Motors

When using the ULN 2803 with stepper motors a 'zener' diode should be connected between pin 10 and the postive rail. This prevents "back emf" generated by the motor from damaging the IC. A value of between 15 and 30 volts should do. All current circuits make provision for the zener, but earlier boards did not. You may need to do a little creative wiring to modify your board.

30/9/98

The Relay Sub-board

This is the first major modification since the development of the Simplified I/O Board. Rather than build the 4 way Relay interface you can now just as a 'sub-board' to the Simplified I/O board. See: relaysub.html