populated Relay doc.

This is a rough transfer of my personal notes in to the public doc's
for the Relay project. I've not gone through it properly so expect
mistakes and it needing a good proof-read.

relay/attachments/main-proj-architecture.dot

@@ -0,0 +1,20 @@
+digraph overview {
+	node [fontname = Arial];
+	l1 -> server;
+	l2 -> server;
+	server -> r1;
+	server -> r2;
+	r1 -> server;
+	r2 -> server;
+	l1 [label = "Light Meter 1", color = lightblue, style = filled];
+	l2 [label = "Light Meter 2", color = orange, style = filled];
+	r1 [label = "Relay 1", color = lightblue, style = filled];
+	r2 [label = "Relay 2", color = orange, style = filled ];
+	server [label = "Midpoint (Server)"];
+		subgraph cluster0 {
+		style = filled;
+		color = lightgrey;
+		r1;
+		r2;
+	}
+}

relay/rtr-relay.md

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+# Return to Ritherdon: Relay
+
+Relay is one of three smaller projects which make-up the 'Personal
+Flash in Real-Time' -- which is one artwork with the main 'Return to
+Ritherdon' project.You should view the three smaller projects as one
+project. And, for the purpose of this documentation, I will refer to
+'Personal Flash in Real-Time' as a system instead of an artwork.
+
+For more information on the 'Return to Ritherdon' project, use the
+following link:
+
+- [Return to Ritherdon Overview](https://git.abbether.net/return-to-ritherdon/rtr-docs)
+
+### 'Personal Flash in Real-Time': Project Overview
+
+The overall system ('Personal Flash in Real-Time') consists of three
+separate/smaller projects. You should not view them as individual
+pieces within the overall project. From an artwork point-of-view,
+'Personal Flash in Real-Time' is one piece. The name of the software
+projects are 'Light Meter' (which is this one), 'Midpoint' and
+'Relay'. All three projects reside in their own git repositories. You
+can find the repositories at the following links:
+
+- [Light Meter](https://git.abbether.net/return-to-ritherdon/light-meter)
+- [Mid-Point](https://git.abbether.net/return-to-ritherdon/midpoint)
+- [Relay](https://git.abbether.net/return-to-ritherdon/relay)
+
+The (technical) aim of the project is to turn a set of lights on at
+the gallery when the welding machines are active in the welding booths
+at Ritherdon. The solution we arrived at was a three-stage
+process. The stages are as follows:
+
+1. Monitor the light levels in the welding booths at Ritherdon and
+   send that information to a sever (Light Meter).
+2. Receive the light readings and store them in a database and make
+   them available for others to access (Midpoint).
+3. Have the lights installed at the gallery connected to wi-fi enabled
+   relays which request the latest light readings from the server. If
+   the readings are above a certain threshold, have the light in the
+   gallery turn on (otherwise, turn off). The relays are responsible
+   for turning the lights on and off (Relay).
+
+Each step should require no human intervention.
+
+For more information on how each project accomplishes its task, please
+use the (repo.) links above. Otherwise, here is an diagram to help
+explain the three stages mentioned above.
+
+![Main Project Architecture](attachments/main-proj-architecture.png)
+
+## Hardware Specifications
+
+Here are a list of parts used in this project:
+
+- [Raspbian](https://www.raspberrypi.org/downloads/raspbian/) (You can
+  use the G.U.I. or "headless" version)
+- [Raspberry Pi
+  4](https://www.raspberrypi.org/products/raspberry-pi-4-model-b/) (I
+  am assuming you have the appropriate power cable, S.D. cards Etc.)
+- [Single Channel 5v Relay](https://www.amazon.co.uk/Dealikee-Module-Indicator-Channel-Arduino/dp/B08ZJ9T355/ref=sr_1_12?dchild=1&keywords=5v+relay+module&linkCode=gs3&qid=1635877670&qsid=262-0370227-8667213&sr=8-12&sres=B08YK9V3V6%2CB07RKH9KLM%2CB06XK6HCQC%2CB01H2D2RI0%2CB08MTD5BPJ%2CB07PZC9ZMM%2CB07CNR7K9B%2CB08ZJ9T355%2CB07V1YQQGL%2CB07MY2R2ML%2CB07G71S7VP%2CB09C2ZHXXP%2CB08GPF9FNX%2CB09CN81J32%2CB07SDLJSWB%2CB08YJ6TS29%2CB08V15SNZS%2CB09DS3DD1J%2CB08KWPMP2V%2CB07TTVYGC8&srpt=RELAY)
+
+### Checking Your Raspberry Pi's Hardware Layout
+
+Because there are multiple versions of the Raspberry Pi, the layout of
+the General Purpose Input/Output (G.P.I.O.) Pins differ. A quick way
+to find out how the G.P.I.O. pins are laid out, for the Pi you are
+currently working on, is to open a terminal and entering the following
+command `pinout`.
+
+Doing this will lead to you seeing something similar to the image
+below,
+
+![pinout-terminal](attachments/pinout-terminal.png)
+
+More information about this can be found at the following URL:
+
+- [Checking Raspberry Pi Board Version](https://www.raspberrypi-spy.co.uk/2012/09/checking-your-raspberry-pi-board-version/][Raspberry Pi Board Information)
+
+## Wiring-Up the Relay
+
+- [Wiring Single Channel Relay with Raspberry Pi](https://www.14core.com/wiring-single-channel-relay-with-raspberry-pi-on-wiringpi-python/]
+ 
+I used this article/blog post (linked above) as a starting-point for
+the relay part of the project. It includes software written in C and
+Python but I have stuck to just Python for this project. You can see
+how the relay is wired-up to the Pi in the image below.
+
+#+Caption: Wiring Diagram for the Relay Project
+#+Name: wiring-diagram-relay
+![Wiring Diagram Relay](attachments/wiring-diagram-relay.png)
+
+### General Overview of Controlling Relays
+
+- [Four Ways to Control Electronic Relays](https://makezine.com/2018/03/19/control-electronic-relays/)
+	
+This is a primer for understanding how to control relays and the
+various types of relays at your disposal. The list is not exhaustive
+but is enough to get you going.
+
+- [YouTube Guide](https://www.youtube.com/watch?v=58XWVDnB7Ss)
+
+I find the guy in this video difficult to understand but he gets his
+message across well enough. He explains how a relay works and how to
+connect it to an *Arduino Board* and control a light bulb. If you are
+not familiar with how relays work, this video should give you a good
+grounding in it. /Note: This video does not use a Raspberry Pi so the
+information is limited to just the relay and the light part of the
+project.
+
+- [How to Set-up a 5v Relay](https://www.circuitbasics.com/setting-up-a-5v-relay-on-the-arduino/)
+
+The link above focuses on using an *Arduino Board* but you can
+determine what pin does what. The example in the link, also,
+includes a thermistor in its set-up which does not apply to this
+project. The image below  is taken from the site and
+highlights what each pin is.
+
+![Relay Pins Layout](attachments/5v-relay-pins-layout.png)
+
+- **NO (Normally Open)**: In the normally open configuration, when the
+  relay receives a HIGH signal the 120-240V switch closes and allows
+  current to flow from the C terminal to the NO terminal. A LOW signal
+  deactivates the relay and stops the current. So if you want the HIGH
+  signal to turn ON the relay, use the normally open terminal. See
+  /figure [[no-configuration]]/ for further information.
+- **NC (Normally Closed)**: In the normally closed configuration, a HIGH
+  signal opens the switch and interrupts the 120-240V current. A LOW
+  signal closes the switch and allows current to flow from the C
+  terminal to the NC terminal. Therefore, if you want the HIGH signal
+  to turn OFF the 120-240V current, use the normally closed
+  terminal. See /figure [[nc-configuration]]/ for further information.
+
+![Relay Normally Open Layout](attachments/5v-relay-normally-open-layout.png)
+
+![Relay Normally Closed Layout](attachments/5v-relay-normally-closed-layout.png)