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The idea behind the Apollo electrical system was that there were multiple sources of electricity -- for most of a normal mission, the fuel cells provided the bulk of the power, but batteries supplied some of the power during heavy loads, and all of the power during reentry. These power sources were connected to two electrical distribution systems -- Main Bus A and Main Bus B -- and all of the equipment drew power from one of the two buses. Main Bus A and Main Bus B were DC (direct current) distribution systems.About 40% of the power produced by the fuel cells was fed into a set of inverters, which converted the DC to AC (alternating current), and this in turn was distributed to the equipment that required AC by two distribution systems, AC Bus 1 and AC Bus 2. So equipment that required DC was connected to either Main Bus A or B, and equipment that required AC was connected to AC Bus 1 or AC Bus 2. A lot of the equipment required both AC and DC.There were voltage sensors at various places throughout the Apollo electrical system. The astronauts could display the voltages on any of the batteries, fuel cells, buses, and the battery charger. However, they could only display the voltage of one thing at a time. A rotary switch enabled them to select which voltage sensor was currently displayed.However, if a voltage became critically low in any of the buses, they weren't likely to notice it on the voltmeter -- that's where the Caution and Warning System (CWS) came in. It monitored critical signals from various systems, and among these were the voltages on the two DC buses (Main A and Main B) and the two AC buses (AC Bus 1 and AC Bus 2). If it detected a serious drop in voltage, it set off an alarm.The main panel is shown below. On the far left and far right are two Master Alarm lights (circled below in red). Whenever the Caution & Warning System detected a problem, these two lights lit up, and an audible alarm sounded. Pushing either of the Master Alarm lights silenced the audible alarm.At the top center of the control panel were the Caution and Warning Lights (circled above in blue). Whenever the Master Alarm went off, the Caution and Warning Lights would indicate the cause.Below is a close-up, and you can see the two lights that indicate undervoltages in Main Bus A and B.Losing the DC buses means you're going to lose the AC buses as well, since Apollo produced AC by converting DC to AC.On Apollo 13 the first symptom Lovell reports is a “Main B Undervolt”, which lit up the Master Alarm lights and the MN BUS B UNDERVOLT light. No doubt you’re thinking of the movie “Apollo 13”, and if you look at the movie around 55 minutes into the movie, when the accident occurs, you can see the Master Alarm light, and the lights in the Caution and Warning System.Hope this gives you the details you want. If not, ask more.(Note: Answer edited a couple of times to fix typos.)UPDATE: ROUND 2Jamie asked some very good follow-up questions in the comments, but I didn’t want to bury the answers in a reply comment, so I’m including it here. Sorry about the length …You've got it exactly right, Jamie, in your comment about the Master Alarm — the Master Alarm was part of the Caution & Warning System (C&WS), and was triggered only when something that was being monitored by the Caution & Warning System had a problem, and each one of those things is represented by a light on the Caution & Warning panel, shown above.The one exception is kind of interesting. You'll notice that one of the lights in the Caution & Warning panel is CREW ALERT. This is the only light that wasn't lit automatically when something happened with the spacecraft -- this light was lit by a signal from Mission Control. The controllers on the ground had access to all the data coming from the spacecraft, and they had whole teams of people watching that data. As a result, they often had a better idea of what was going on than the astronauts did. The CREW ALERT light gave them a way to trigger an alarm when they saw a problem that the Caution & Warning System didn't know about.Remember that the Caution & Warning System didn't have any real computing power -- it was just sensors and circuits -- and it really didn't have any intelligence. For example, to go back to your original question about the undervoltage, all the C&WS could do was trigger an alarm when the voltage dropped below a certain threshold (26.25 volts). It couldn't look at trends, or notice that the voltage had been slowly decreasing over the past few hours (as long as it hadn't dropped below the trigger point). But Mission Control could spot this sort of stuff, so they might alert the crew to a possible problem long before the C&WS triggered an alarm.The only computer on board the spacecraft was the AGC (Apollo Guidance Computer) and it was strictly concerned with controlling the maneuvering functions of the spacecraft. It wasn't involved at all with monitoring or controlling spacecraft systems such as the electrical system or the environmental system. (Incidentally, in the movie "Apollo 13", right after the MN BUS B UNDERVOLT light comes on, the light next to it -- CMC -- comes on. CMC is Command Module Computer -- another name for the AGC -- and indicates that there was a problem with the computer, most likely that it had momentarily lost power.)So the C&WS really was just for stuff that went suddenly wrong. It obviously got a serious workout on Apollo 13, and also on Apollo 12. You may not be aware, but Apollo 12 got hit by lightning just after launch. The C&WS panel must have been quite a sight. Here's Pete Conrad, Apollo 12 Commander, at 1 minute 2 seconds after launch:Okay, we just lost the platform, gang. I don't know what happened here; we had everything in the world drop out. I got three fuel cell lights, an AC bus light, a fuel cell disconnect, AC bus overload 1 and 2, Main Bus A and B out.Thanks to some quick thinking on the ground, they got everything reset and went on to have a very successful mission, but it was really tense for a few seconds.The reason I mention that the C&WS doesn't have any smarts is because I'm not sure it's a good model for you to use if you're designing something today. As much as we can admire the people that built Apollo, we have to keep in mind that Apollo was designed over half a century ago. You have more smarts in your Arduino than they ever could have dreamed of in the Apollo.As far as how you can learn about Apollo, you have amazing resources available on the Internet. I was almost exactly your age at the end of the Apollo program, and I never could have imagined having all the documentation available that you do.If you want to learn about the electrical system, here's some of the training materials, including schematics:http://www.ibiblio.org/apollo/Documents/ApolloTrainingElectricalPowerSystemStudyGuide.pdfHere's an overview of the entire Apollo spacecraft, with diagrams of all the major systems:http://www.ibiblio.org/apollo/Documents/ApolloTrainingOverview.pdfFinally, here's the Apollo Operations Handbook -- it's essentially the "owner's manual" for the Apollo spacecraft, and all together it's about 900 pages. It's not exactly an easy read, but it’s pretty understandable by us mere mortals, and if you want to know the spacecraft inside-out, it's all there, and of course if something doesn't make sense, you can ask questions on Quora. Note that there's an entire section on the Caution & Warning system -- that's where I learned about it. The section on "Controls and Displays" is very cool — it explains what every single knob, button, light, and switch does! What I wouldn't have given to have a copy of this when I was your age ...Apollo Operations HandbookYou asked about maneuvers. Maybe others here can add to this, but I've heard a lot of people -- including an astronaut! -- say that if you want to understand orbital mechanics, play Kerbal Space Program.Good luck, Jamie! It's fantastic that you've got an interest in this stuff at your age.