88 lines
5.1 KiB
Markdown
88 lines
5.1 KiB
Markdown
Title: Slot Car Racing over IP (SCRoIP)
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Date: 2022-08-28 16:05
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Author: Error
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Slug: scroip
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Summary: I built a remote control for a slot car racing track to play with it over the LAN. It's unnecessary complicated and inefficient, but at least it supports IPv6. Ladies, Gentlemen and everyone else, start your engines.
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License: CC-BY-NC
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https://creativecommons.org/licenses/by-nc/4.0/
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## The Idea
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When I was a child, I used to play a lot with my slot car racing track.
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After stubling upon it again recently, I started wondering if I could build something to control it over the network.
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Not that I needed a remote control for it, but it seemed like a fun project.
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However, simply creating an API that offers precise control over the device seemed way to sriaght forward - the controling mechanism itself should add its challenges.
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So I came up with the idea to control it with the traffic itself.
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The more traffic is sent to the remote control, the faster the car is supposed to go.[°Use more bandwidth!!!]
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## Reversing
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In order to build the remote control, the first step was to figure out how the slot car racing system works. So I disassembled it and took a look at its wiring.
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The underlying mechanism is fairly simple. The power supply provides about 14.8 V DC voltage, which a used to power the car's motor.
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The speed of the car is controlled by a variable resistor within the controller, which allows to control the voltage the car receives.
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Pushing the controller further down lowers the resistance, which raises the voltage available to the car and makes is go faster.
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A third wire is used to brake the car when the controller is in its neutral position by utilizing the current generated by the motor when the car is rolling out.
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A look at wikipedia confirms this analysis. [^1]
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## Building a digital Controller
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Now that I understood the mechanism, the next step was to create a digital version of it, so that I could control the slot car racing track with a Raspberry Pi.
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My first idea was to simply use a digital potentiometer.
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However, searching for a suitable digipot, I quickly realized that there were no options with the desired dimensions of about 30 Ω available.
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Therefore I built it myself using a few resistors, solid state relais and multiplexers.[°I'm sure there are simpler solutions for this, but, well, it worked for me.]
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For the sake of simplicity,[°or maybe out of laziness, who knows? :)] I did not add the braking mechanism from the analog controller to my digital version.
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## Implementing the Server
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With the hardware fully assembled and soldered, it was time to have a look at the software.
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Python offers an easy way of implementing TCP servers.[°This project will use TCP. Even though the payload is immediately discarded, reliability is of the utmost importance and packet loss will not be accepted!!!]
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By default, it only supports IPv4, but IPv6 support can be added by setting `address_family` to `AF_INET6`.
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if not legacy:
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self.address_family = AF_INET6
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super(SCRoIPServer, self).__init__((address, port), SCRoIPRequestHandler)
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The server's logic itself is implemented in the method `handle()` of the class `SCRoIPRequestHandler`. While there is an active connection, it reads from the socket and sums up the ammount of data that was received.
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Once every 100 ms, it then calculates the average throughput for that time period and sets the raspi's GPIO pins accordingly.
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while self.data != b'':
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try:
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self.data = self.request.recv(4096)
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self.traffic_counter += len(self.data)
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except TimeoutError:
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pass
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time = clock_gettime_ns(CLOCK_REALTIME)
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diff = time - self.last
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if (diff > 100000000):
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self.last = time
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traffic: float = self.traffic_counter / diff * 1000000000
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print(f"{traffic} Bytes/s")
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self.server.set_throttle(traffic)
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self.server.apply_throttle()
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self.traffic_counter = 0
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The full server code can be found [here](https://git.undefinedbehavior.de/undef/SCRoIP "Slot Car Racing over IP - SCRoIP - undefined git server").
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## Lights out and away we go
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Once the server was implemented as well, all that was left to do was to test it.
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So I deployed the server script to a Raspberry Pi, connected its GPIO pins to the controller and the controller to the slot car racing track.
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Using netcat, I was now able to accelerate the car with the network traffic I generated:
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<video controls preload="metadata">
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<source src="{static}/posts/scroip/demo.webm" type="video/webm">
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<p>
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The video could not be loaded.
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You can try to download it directly <a href="/posts/scroip/demo.webm">here</a> instead.
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</p>
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</video>
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[^1]: [https://de.wikipedia.org/wiki/Autorennbahn#Fahrzeugsteuerung](https://de.wikipedia.org/wiki/Autorennbahn#Fahrzeugsteuerung "Autorennbahn – Wikipedia")
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