What is the relevance of all this you ask? Well, if I wasn't going to use a computer to teach networks, then I needed some kind of cheap network device that I could use as a teaching tool. Something that would have analogues (pun duly observed) to the real-world vagueries of wireless communications, with line-of-sight constraints, appreciation of dissipating power over distance, directional receivers for excluding interference and improving gain, and the plain old unreliable nature of physical layers generally.
I've been reading too much Pratchett lately, so immediately decided that the best solution would be a simple clacks or semaphore line system.
Pratchett's clacks system uses shutter semaphore, but that is really too tricky to build for $20 a tower (even old Pony needed more in AM$), let alone drive in duplex with a single operator sitting in a $7 chair.
So I have opted for a cable operated single arm semaphore with a range of about 170 degrees, since the single actuator can be easily operated by foot, allowing the operator's hands to be free to write down what they see on the next semaphore tower.
You can see the arm of the semaphore tower in the image, with actuator cable attached.
So let's get started.
Step 1: Get Your Stuff Together
You will need:
0. A power drill with 2.5mm, 3.5mm and 8mm wood, metal or plastic bits, a hacksaw, and the right sized philips-head screw driver for the screws.
1. approximately 6m of 20mm PVC pipe. In Australia, you can get this in Bunnings for about $11 as a single 6m length, or for $6 per 4.5m length for "Telstra 20mm conduit", which is the same stuff, but with slightly thinner wall as it is not rated for high-pressure water.
If you only have a regular car, buy the longer length, but take a hack-saw and measuring tape with you, and cut the stuff up in the car park -- it only takes a few minutes.
2. 3x 20mm PVC Tee Connectors.
3. 5 x 6G 25mm chip-board screws, or some other fixing.
4. Some thin cable rope. If you are making only one tower, you could go to your local bike shop and get a brake cable, otherwise Bunnings has plenty of options for cable rope. I suggest something in the 1mm - 2mm size range, with thinner being better, as it is bendier.
5. A cable-rope grip for forming the hand/foot piece.
6. Optionally, a $7 fold-up camping chair for the operator to sit on.
7. Optionally, something to make a nice carry bag, or even just some velcro to bundle the pipes together when transporting.
Step 2: Cut Up the PVC
Cut 6x600mm lengths and one length of somewhere around 2-3m. If you are using a 6m length, then the left over bit after you cut the 600mm bits will be the right length.
You should have six bits that all look much the same, and like the ones in the picture.
Step 3: Make the Base Strut
1. Attach two of the 600mm lengths to one of the Tee connectors to form a ~1.2m length.
2. Now REALLY push the pipes into the Tee connector so that they go all the way in.
3. Put Tee connectors on the end, at right angles so that they point horizontally when the middle Tee points Due Up. The easy way to get this right, is to put the other four 600mm pieces into the ends of the two Tee connectors and then fiddle the middle Tee so that it points straight up. Put the 2.something metre piece in the middle Tee if you like to give you a guide to make sure that the middle Tee does in fact point straight up.
3. Drill 2.5mm pilot holes to take the screws to accomodate the four screws as shown in the figure, and then insert and tighten the screws. They should easily self tap in the soft PVC pipe. You could use pipe cement instead of screws, but screws are a whole lot less fiddly, and besides, you will need a screw for the semaphore arm attachment later on.
You should now have a lovely thing like in the figure.
Step 4: Assemble the Base
If you haven't already added the other four 600mm lengths in the previous step, do it now.
DO NOT GLUE OR SCREW THEM IN if you want your tower to be a portable one.
The pipes tend to hold in well enough for use, and by being able to remove them, you can collapse the base down to a ~1.2m x 100mm bundle of pipes.
This makes a nice ~1.2m x 1.2m base to keep your tower upright and generally stable.
Sure, it's crude and has zillions of drawbacks, but it does work, and costs all of about $12.
Step 5: Make the Semaphore Arm & Actuator Cable
This is the most involved part of the project, and it really isn't that involved.
Start by cutting about a 20cm length of PVC pipe lengthways (the same way that politicians cut red-tape) to make two where there was only one. But now they are half-pipes, which is nice and convenient.
Most hacksaws have about a 10cm clearance, so cut in from one direction first, and then once you have got at least half-way, flip it over and start in from the other direction. A vice is a handle for holding the pipe still while you try to cut it in this way.
Alternatively, if you have something fancier like a bandsaw, then you can do the job all in one go, give or take a few fingers.
Drill a 3mm hole approximately 4cm from one end of the half-pipe and on the long-axis of the half pipe.
Shove a suitable length of 20mm PVC pipe over the end of the half-pipe once it is attached.
Drill a 2.5mm hole approximately 1cm from one end, and also on the long-axis of the half-pipe. This is where the actuator cable will enter the arm.
Drill another 2.5mm hole 1cm from the end, but this time perhaps 1cm off-axis, and thus 1cm from the previous hole. This is where the cable will leave the arm, forming a tight angle that will create a good friction hold for the actuator cable.
In the 2-ish metre upright, drill a 2.5mm hole a few cm down from the top.
Use a 6G x 25mm chipboard screw to fix the half-pipe via the 3mm hole. Tighten the screw such that the half-pipe should swing freely, but not slop about the place.
Drill an 8mm or so hole perhaps 45 degrees around from the screw that holds the semaphore arm on the upright, and a little lower than the hole in the arm that feeds the actuator cable. The idea is that when the arm is upright, the cable should still be able to get from the hole in the arm to the hole in the upright, without limiting the range of movement of the semaphore arm.
Drill another 8mm or so hole about 600mm up from the bottom of the upright for the cable to emerge from. If using a bike brake cable, make sure it is long enough!
Thread the cable, beginning at the actuator arm, then into the pipe, and then out the other hole. If you have trouble getting the cable out the other hole, push it in further than the hole, and then rotate the cable in your fingers at the arm-end, and pull it gently backwards. With a bit of fiddling, the end of the cable should pop out of the hole.
If you get it wrong, just unscrew the semaphore arm, and pick a new place to attach the semaphore arm so that it DOES work. You can see evidence of me doing this in the photos.
Shove a length of pipe over the end of the half-pipe, squeezing the half-pipe so that it can be inserted into the length of pipe. This makes the arm longer (and hence more visible). It also makes the arm heavier so that when the actuator cable isn't tensioned, the arm should drop down to be near-vertical.
Note that if you use 2mm wire rope instead of bike break cable you might only get 120 degrees of movement in the arm. Such is life.
If you want to prevent the occasional jamming of the semaphore arm at the bottom, insert a screw on the upright such that it prevents the arm from passing through vertical at the bottom. You will understand what this prevents if it happens to you. Otherwise, ignore.
Step 6: Make a Nice Handle for the Actuator.
I just thread a piece of 13mm irrigation pipe or hose onto the cable, and then use a cable-rope grip to join the ends. The loop should be big enough to fit your foot in a shoe, since the idea is that to operate the semaphore duplex, you use your foot to move the arm, so YOUR arms are free to write down what is being sent.
Note that the photo shows the optional collapsable upright.
Step 7: Assemble the Whole Thing and Test Actuator
Stick upright in base. There really isn't much more too it.
Note that in this picture, I haven't put the handle on the actuator cable yet.
Basic operation: pull cable/handle, semaphore arm will raise; release cable/handle to lower arm.
Advanced operation: Devise physical layer protocols to transmit digital data with suitable forward error correction and clock recovery. Devise link-layer protocols to frame data into packets. Make meat-to-computer interface to transfer packets between internet and semaphore tower. Better yet, make a computer controller to drive the actuator cable, and an image recognition system to read the remote semaphore, and thus implement a complete unlicensed 500 tera-hertz internet connection. Tell RFC1149 and RFC4824 their grandmothers smell like elderberries.
Step 8: Packup for Transportation.
I did say that this was transportable, didn't I?
If you don't have a car with at 1.2m x 1.2m x 2.4m storage compartment, then try the following:
Remove upright from base.
Remove 4x 600mm pipes from base strut.
Rearrange to form a bundle of pipes ~2.4m long and <100mm in circumference.
If you want to get REALLY clever-pants, then cut the upright in half, and add a pipe-collar or use a piece of pipe with integrated joiner so that you can fold the thing down to only ~1.2m long, just like in the photo.