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803 Miscellaneous

Written by: David Williamson  from:   Link to his site

Editor: Phil Schulz

Length: 1000 words

Assumed Knowledge: None

Difficulty: Easy

Free Tutorial

This is an example of a writing robot

Handwriting is quite a difficult task for robots, but children find the result interesting.
This is a robot which carries a paper disc. The program is written round the edge of the disc with a black felt-tip, like a bar code.

The disc is carried by the robot rather like music on a music stand, it bears on one of the robot's axles,which rotates it so that the code passes in front of a simple light pen. This works a relay which reverses the motor driving the other axle. A felt-tip is carried in the middle which traces out a simple pattern according to the program. A disc about 3 inches diameter is big enough, the novelty of writing the programs soon wears off.


This next one is similar, but uses an electronic memory to store the program which it scans by rotating once on the paper disc, there is a downwards-pointing light pen at the end.

A read/write switch is then pressed and a felt-tip placed in the pen holder situated between the two axles, the robot then draws in accordance with the program it remembered from the paper disc. As with the previous robot only one of the motors reverses. The circuit shown is very laborious to build, this would all be done with a micro controller nowadays.


This one is similar to the previous one but it uses the scanned code to switch a single-channel radio transmitter taken from a cheap radio-controlled car. It doesn't have a pen holder and has only one wheel as it only goes in circles. The third balance-point is provided by the light pen.

The signal can be received by a simple morse-code writer

or by a robot similar to the first two but worked by a radio receiver. This can be then developed by switching the transmitter with a computer printer-port rather than with a paper disc.

The more slowly robots move, the more consistent they are however they become boring to watch.
All of the above were made using either walkman motors or Mabuchi solar motors; the Mabuchi motors are very good, almost as consistent as stepper motors and much simpler to use. If they are driven from a single AA battery the effect of voltage fluctuation is minimised. Best results are obtained using a single worm gear.


Finally a robot which uses two channel control to steer left/right or forward/reverse and raise or lower its pen.

It does this by using the pen to jack up one wheel, whereupon the robot circles round the pen tip which acts like the point of a pair of compasses.
The pen is mounted on a wishbone which can be raised to a neutral position so that the pen can write, both the robot's wheels now being on the ground. If it is raised further the pen lifts off the paper. Although this arrangement seems a bit complicated it overcomes various problems: the robot draws very straight lines and the pen stays central when the robot is changing direction. It also permits lifting the pen, this is useful if you want the robot to write upper-case.

Although these robots can do quite legible handwriting using dead-reckoning it is advisable to fit them with an electronic compass. This does away with all the design problems and raises the speed limit so the extra expense is well worth it.
Perhaps a better use for electronic light pens which can read felt-tip lines on paper is for line-following robots

given a good transducer design these work very well. If they can steer round a right-angle they can navigate using a grid. Another potential use of this type of circuit is for a paper wheel distance-measurer.
This is a good thing to start electronics with

Perhaps a good thing to end electronics with, too.
This is a way of making a reversing switch contributed by Richard Harris

Another way of doing this is to make two rings out of cooking foil: one for your left-hand thumb and one for your left-hand index finger (assuming you're right-handed); fix a wire to each ring using a paper clip; now if you hold an AA battery between your right hand thumb and forefinger you can use the rings to touch the battery terminals. To change the polarity just turn the battery round.

This is an extension of the same idea: a robot usually has a minimum of two motors both of which need to be reversible, this can be done with a slightly more complicated arrangement of rings and paperclips

Because of the finger tips' sensitivity, it is possible to touch either or both of the rings on either or both of the clips on the side of the cardboard tube. Squeezing the batteries in the tube insures good connections in the middle and at either end.
Another approach is to make a joystick out of micro switches

this is simple to make and very satisfying to use, but micro switches are quite expensive. It can be used with bigger batteries but then the circuits must be fused.

This is a way of making a writing robot just using scissors and sticky-fixers

the crank pushes on one side or the other of the hole at the top of the box and the box will tip one way or the other accordingly. As shown the robot is jacked up on the pen tip which acts as a pivot around which the robot travels, because one wheel has been pushed off the paper. If the box tips the other way it lifts the pen off the paper. If the crank is central the pen draws a straight line.



It can be controlled with two reversing switches (as above). If micro-switches are connected in series with the reversing switches it gives very good control.

my email is davidvwilliamson@hotmail.com
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