Next stage of the build is to work out how to hold the pen. Some people call this the gantry or holder, I’ll be calling mine the gondola.
I decided to use lollipop sticks for this, they have a similar length to whiteboard maker pens, reasonably light weight and at only 50p for a pack of 50 sticks and it was an obvious choice!
Clamping the Sticks for drilling
Having cut a few sticks down to size I then clamped them together to drill a thread hole through them using my trusty dremel.
Lining them up for gluing.
First one clamped down.
With the first side clamped in place and the wood glue drying things are starting to take shape.
Sides nearly finished.
And the finished gondola! look forward to a video of the first test draw in a couple of days . . . things are a bit shaky!
With the research all done, I started thinking about how I wanted to build my PlotBot. Having looked at the other designs, I found they were either mounted on a wooden frame and then a piece of paper is taped onto the wooden panel, or they draw directly onto a surface like glass or a wall. Given that the aim is just to make something that catches peoples eye, rather than making posters or drawings for people, I think the best course of action would be to use a whiteboard. I can get one reasonably cheaply, and the mounting is pretty much already sorted.
The whiteboard and mountings.
Once I had bought a whiteboard (600mm x 450mm) I started lining up the parts I had as to how I would mount them. I had also bought 2 Pololu 1204 Stepper Motors and an Adafruit Motorshield v2 (AFMSv2). I did have a few concerns with these parts combined together, in that the motors only draw 600mA and the motorshield provides 1.2A per channel, therefore the motors might get a little hot if they start drawing more than they should – but we’ll see how it goes!
Roughly lining up the parts on a sheet of acrylic.
To mount the acrylic sheet to the whiteboard I used two of the mounts supplied with the whiteboard secured on the top of the sheet. These then hook onto the edge of the whiteboard, and the mounts on the side are adjustable to “lock in” the sheet to the sides of the board. Finally I decided to neatly mount the arduino and AFMSv2 in the center of the acrylic sheet.
I picked up two remote control car wheels at a local hobby store, along with 50m of fishing line, which would form the basis for my reels.
I found some nuts in the garage that fitted the inside of the wheel, and used Araldite (metal glue) to fill the gap around the stepper motor shaft hoping that this wouldn’t go wrong.
Araldite’s in, I was a little bit messy dripping it everywhere!
With the luck of the gods, after leaving it 24 hours to cure I was able to punch the stepper motor shaft out of the nut, leaving a nice shaped hole. The advantage of this method being that I can very easily remove the reels and use the steppers in other projects.
Now that I have the reels mounted on the steppers, I was able to complete the main build; mounting the steppers onto the acrylic sheet, and winding the fishing line onto the wheels – happy days!
So as time draws on we are getting closer to the start of the new academic year, and of course that means Fresher’s Fair! At Kent we have several creative societies including SpaceSoc and their “Build a Rocket” sessions, Engineering Soc with their focus on robotics, and TinkerSoc who want to help people build without limits. With TinkerSoc it has become somewhat of a tradition to build and showoff a project at the fresher’s fair. In previous years we have had a laser engraver making custom name tags and furbies singing bohemian rhapsody, basically something to grab peoples attention and imagination. Having seen a number of vertical plotters online I have decided now is the time to build one.
The standard vertical plotter is made up of 2 stepper motors, a servo, a motor controller and a microcontroller. By providing a stream of polar coordinates to the robot, the two motors can be wound in and out to move a pen across the whiteboard. This produces drawings where the pen never leaves the surface however that does not limit the styles of drawings that can be produced. Drawings can be developed further by adding a server or linear actuator to the pen carriage in order to push the pen off the drawing surface, thus allowing mush more freedom to implement different drawing styles.
Obviously we cannot draw above, or on either side of the motors, however the effectiveness of the plotter changes depending on the position of the pen carriage. As such the most effective drawing area is a rectangle in the centre of the drawing surface with the tension on a cord being too low on either side, and the resolution is too low at the top due to the large angles. (http://2e5.com/plotter/V/design/
There have been a great many vertical plotters in the past, a great list can be found at plotterbot.com. Overall there seem to be two different styles of drawing with vertical plotters.
Single line, where the pen never leaves the surface, is technically less challenging and can provide great results however you can be left with the odd scrawl across the surface that you didn’t want.
Multi line, where the pen can be lifted/pushed away from the surface, allows much more flexibility with regards to what can be drawn as the robot won’t scrawl connecting lines across the surface however does add the extra complexity of having a servo or linear actuator to push the pen carriage away from the surface.
Bearing in mind the saying, the more complex it is, the more likely it is to break.
Tinkerlog’s “Der Krizler” is definitely one of the more popular V-plotters out there, drawing on glass to amuse passers-by.
Dan Royer’s Makelangelo is a very impressive V-plotter. Commercialised as a kit, it’s reliability has been tested extensively!
And probably the oldest V-plotter around from 1988 developed at MIT using lego!
So one day I was bored and had a few things lying around:
A pane of glass from a scanner
555 timer & passives
There was really only one logical thing to do to assist in my procrastination, put together an astable 555 circuit on the glass!
Through my time learning about electronics, I have come to realise that the 555 timer circuit, astable or monostable, is one of the first circuits anyone should make.
However for those who don’t know about it here is a short explanation of the astable circuit.
The 555 timer IC is a a circuit of over 40 components, including 25 transistors and 15 resistors, all printed on a silicone chip.
The circuit works by flipping the voltage states of different pins on the IC. Initially pin 7 is high and so the current flows though R1 & R2 to charge the capacitor. Pin 6 detects the high voltage build up on the capacitor and toggles pin 7 to be pulled low, this causes the capacitor to discharge through R2. While the capacitor is discharging, pin 3 is pulled low, turning off the output, however when pin 2 detects the low voltage on the capacitor, pin 7 is pulled high again, allowing the current to flow through R1 & R2 again.
And ofcourse there is some maths to work out the length of each high and low pulse for given component values, and thus the frequency as well.
f = 1 / ( ln(2) * C * ( R1 + 2R2 ) )
High = ln(2) * C * ( R1 + R2 )
Low = ln(2) * C * R2
And so with values of 1000Ω for R1 and 10KΩ for R2, and 100μF for C1, we get a high pulse of 0.76 seconds, and a low pulse of 0.69 seconds and a frequency of 0.69Hz (687 mHz).
So as part of my involvement with TinkerSoc, we now have our new hoodies. These hoodies are designed so that a Lilypad Arduino can be sewn onto it and then components added to it. We will be wearing hoodies that we can literally tinker with.
For those that don’t know, the Arduino Lilypad is an Arduino development platform intended for clothing and e-textiles. Using conductive thread you can sew tracks and components onto any fabric and then programmed.
The Lilypad doesn’t have a USB plug like an ordinary Arduino Uno. This is because the Lilypad does not have a FTDI Chip (Future Technology Devices International Ltd) unlike the Uno and many other Arduino’s. The FTDI chip converts the USB to serial communication.
So because the Lilypad doesn’t have a USB connection we can use an Arduino Uno instead. To do this we need to remove the ATMega 328p from the Arduino Uno, then break out the header on the Lilypad. As per the diagram above from left to right, the pins connect to Gnd, Gnd, 5v, Rx, Tx and Reset. With these connections made, we can proceed to connect the Uno to a computer, and then start up the Arduino IDE. While in the IDE, make sure you change the board to the correct version of the Lilypad you are using. (If in doubt try to read the number on the Microcontroller on the centre of the board)
And with that done you are ready to start programming, I suggest loading up the example blink program first. Enjoy.
Following the mod of the cheap lamp, I had a 12v supply lying around. I figured a good use of it would be to make a supply board for my Raspberry Pi and other devices I may want to attach to it.
The supply actually outputs around 13.4 v or so which can be attributed to the tolerances of components used. Regardless of output being greater than 12v, I can still use it with the two different regulators I ordered from Rapid, the L7805cv 5v 1A TO-220 package regulator, and the LM723 adjustable voltage regulator in a 14-DIP package.
Both regulators have a maximum input voltage of 40v, so the 12v supply will be just fine. However the supply outputs an Alternating Current (AC) signal, this can be converted to Direct Current (DC), which is needed for most general electronics, by passing the supply through a device known as a Bridge Rectifier.
The Bridge Rectifier
Moving On . . .
On my breadboard I first built the circuit to output 5v in order to power my Raspberry Pi.
This is a standard circuit found in the datasheet however C2 has a value of 100µF and C1 is equal to 10µF.
Oopps . . . Please do remember to put the capacitors the right way round, first time I’ve ever done it, but it turns out these capacitors don’t like 12v going in them the wrong way . . .
So after connecting the capacitors in the right polarity, and attaching a 7W 75Ω power resistor across the regulator’s output to load it, I attached the voltmeter to measure the output.
Using the 100µF and 10µF combination proved successful and outputted a solid 5.028v, however the datasheet recommends values of 0.33µF and 0.1µF. If anyone understands the reasons for the different values please do comment below because I am very curious as to why they both work.
Additionally I would be interested to know why the AC signal of the 12v supply distorts as seen below when the supply is under load.