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Among other things, I teach a topic on networks at Flinders University. As much of my other work is focussed around providing communications in difficult places, I decided to make the topic as hands-on as possible, so that anyone, anywhere might be able to make use of it, and even teach it to others without even needing a computer.
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.
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.
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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.
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.
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(Did you know that the Reverend Lord George Murray invented a shutter-based telegraph that could send messages from London to Portsmouth [about 75miles] via a network of towers in just one minute? It was used during the Napoleonic Wars - IMO it was the inspiration for the system Pratchett described in Going Postal.)
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(Did you know the electrical telegraph was capable of sending pictures as well as text? There were many systems including purely analog ciphering methods but my heart is always with this proto-fax)
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Paul.
One nice thing I discovered with the PVC pipe is that it is very bendy, and so allows a taller tower to bend in the wind, just like the Dearheart's plans for the MK2 towers in Going Postal.
In fact, I will probably experiment with mounting WiFi antenna on a length of unguyed or lightly guyed PVC pipe as a cheaper easier option to guyed steel rod. Also, PVC doesn't attract lightning quite so much (thorry Igor).
Paul.
<sub>Oh, I've found more details. The shutter-telegraph was eventually replaced with an arm-telegraph very similar to your design, but larger.
First, thanks for your enthusiasm for my little project :)
Yes, the shutter-telegraphs were replaced with arm-telegraphs after Claude Chappe did experiments with visibility in France, and found that angled rods were resolvable at greater distance than were coloured panels.
From my perspective, I wanted the best bit-rate-per-dollar I could manage at bottom dollar, so a semaphore arm that can (under optimal conditions) encode an analog signal equivalent to many bits was the answer.
In fact, the difficulty of actually encoding more than 2 or 3 bits with the semaphore arm just adds to the educational value from my perspective, as (hopefully) the students will realise that 1 bit is easy, 2 bits is doable, and 3 bits probably slows the thing down so much and/or introduces such a high error rate that it is faster to use fewer bits-per-baud, and thus a valuable lesson will have been learned, that is directly relevant to the variable bit-rates of WiFi, and DSL latency versus throughput optimisations.
Paul.
:-D