Our homes are full of technological marvels, and, as a Hackaday reader, we are betting you know the basic ideas behind a microwave oven even if you haven’t torn one apart for transformers and magnetrons. So we aren’t going to explain how the magnetron rotates water molecules to produce uniform dielectric heating. However, when we see our microwave, we think about two things: 1) this thing is one of the most dangerous things in our house and 2) what makes that little turntable flip a different direction every time you run the thing?

First, a Little History

People think that Raytheon engineer Percy Spenser, the chief of their power tube division, noticed that while working with a magnetron he found his candy bar had melted. This is, apparently, true, but Spenser wasn’t the first to notice. He was, however, the first to investigate it and legend holds that he popped popcorn and blew up an egg on a colleague’s face (this sounds like an urban legend about “egg on your face” to us). The Raytheon patent goes back to 1945.

However, cooking with radio energy was not a new idea. In 1933, Westinghouse demonstrated cooking foods with a 10 kW 60 MHz transmitter (jump to page 394). According to reports, the device could toast bread in six seconds.  The same equipment could beam power and — reportedly — exposing yourself to the field caused “artificial fever” and an experience like having a cocktail, including a hangover on overindulgence. In fact, doctors would develop radiothermy to heat parts of the body locally, but we don’t suggest spending an hour in the device.

The first Raytheon “Radarange” was nearly 6 feet tall. The 750-pound beast cost about $5,000 which is nearly $70,000 today. You also needed three kilowatts of electricity to feed it. By 1954, the cost came down to about half, along with the energy usage. As you might expect, these were commercial or niche items.

Eventually, of course, the cost and power requirements came down. They solved the problem of running the oven empty, causing damage. There were two other major problems to solve: safety and uniform heating.

Safety Dance

Like any electric appliance, a microwave oven could catch fire or cause an electric short circuit. However, if you open your washing machine and it doesn’t stop, the worst that can happen is you get wet. With the microwave, you could get a big dose of (non-ionizing) radiation! Studies show this probably isn’t as bad as you might imagine, but it is bad enough to burn you.

Of course, if your fridge can turn the light off when you close the door, why not just put an interlock on the microwave door? Microwaves do have an interlock by legal requirement. But not just any interlock: they have to meet strict requirements. In the United States, 21 CFR Part 1030.10. This states, in part:

Safety interlocks.

(i) Microwave ovens shall have a minimum of two operative safety interlocks. At least one operative safety interlock on a fully assembled microwave oven shall not be operable by any part of the human body, or any object with a straight insertable length of 10 centimeters. Such interlock must also be concealed, unless its actuation is prevented when access to the interlock is possible. Any visible actuator or device to prevent actuation of this safety interlock must not be removable without disassembly of the oven or its door. A magnetically operated interlock is considered to be concealed, or its actuation is considered to be prevented, only if a test magnet held in place on the oven by gravity or its own attraction cannot operate the safety interlock. The test magnet shall be capable …

(ii) Failure of any single mechanical or electrical component of the microwave oven shall not cause all safety interlocks to be inoperative.

…

(v) One (the primary) required safety interlock shall prevent microwave radiation emission in excess of the requirement of paragraph (c)(1) of this section; the other (secondary) required safety interlock shall prevent microwave radiation emission in excess of 5 milliwatts per square centimeter at any point 5 centimeters or more from the external surface of the oven. The two required safety interlocks shall be designated as primary or secondary in the service instructions for the oven.

(vi) A means of monitoring one or both of the required safety interlocks shall be provided which shall cause the oven to become inoperable and remain so until repaired if the required safety interlock(s) should fail to perform required functions as specified in this section. Interlock failures shall not disrupt the monitoring function.

Naturally, there are many ways you could meet these requirements, but most of the microwaves we’ve seen do it with three microswitches. Usually, two are normally open, and one is normally closed.

The two normally open microswitches prevent the magnetron from receiving power when the door is open. One of them actually breaks the power to the tube. The other is used as a digital input to the control board. Closing the door actuates the switches and allows power to flow. You would think the switches would be in each leg of the magnetron, but it isn’t that simple. If the switch connected to the board fails, the light, fan, and turntable will operate when the door is open. If you ever open the door and your turntable starts spinning, it is probably one of the normally open switches shorted.

So why is there a normally closed switch? That shorts out the power to the magnetron. So even if, somehow, both normally open switches fail, the normally closed one will short the power and blow a fuse. If it fails, you assume one of the other two switches will still cause a failure if the door opens. A very smart appliance repairman explains it in detail in the video below. Watch it all the way through to get a good tip about checking the transformer power without a lot of trouble.

Our pro tip: buy your microswitches from the usual places, not the appliance part places. You’ll be able to replace all three switches for way less than one switch will cost from the parts house. Just make sure they are exactly the same switch. Obviously the normally open and closed part is important, but the mounting holes, actuator, and the voltage/current ratings need to match, too.

Turntable

The other issue is getting things to heat evenly. The radio energy will have standing waves, which can cause cool places. Some older microwaves have a mode stirrer to reflect microwaves in the oven. But Sharp started using turntables around 1964, and that’s what most modern microwave ovens use. Have you ever noticed that, usually, the turntable will spin one way until you turn it off. When you turn it on again, it will usually — but not always — turn in the other direction.

We were always fascinated about how that might work internally. It turns out the real answer is anticlimactic. Microwave ovens are price-sensitive. The cheaper you can make them, the more you can sell and the more profit you make on each one.

As a result, the motor is almost certainly a synchronous AC motor. The natural direction the motor spins will depend on where it was in the cycle when it stopped. In many cases, the drive gears in the motor will require a little torque to start, and this will cause the motor to change direction. Clock motors, for example, have a spring arrangement, so if it starts in the wrong direction, it gets pushed into the correct direction. The microwave doesn’t have that.

If you’ve taken apart a microwave, you might say that the turntable doesn’t have a drive train, just a motor. But the motor has a surprising number of gears in it, as you can see in the video below.

In Plain Sight

You probably use your microwave every day, but you probably never thought about it having a triple interlock switch and a gear train. These parts aren’t as sexy as the high-voltage transformer or the magnetron, but they are no less interesting. The deep theory of why it all works is pretty interesting, too.

Delightful banner image: Cover of Short Wave Craft magazine.

If you connect a generator to your home’s main electrical panel when the power goes out, you need to make sure the main breaker is shut off. Otherwise, when the power comes back on, you (or the linemen) are going to have a bad time. There are commercial interlock plates which physically prevent the generator and main breakers from being switched on at the same time, but since they tend to be expensive, [HowToLou] decided to make one himself.

The hardest part of this project is designing the template. It needs to be carefully shaped so its resting position prevents the generator’s breaker from being switched on under normal circumstances, but once the main is turned off and out of the way, you should be able to lift it up and have the clearance to flip the lower breaker. Spending some quality time at the breaker box with tape and a few pieces of cardboard is going to be the easiest way of finding the proper shape.

In the video after the break, [HowToLou] demonstrates the ideal shape for his particular application, which should help you get your mind wrapped around the idea. There are a lot of variables involved, not least of which the size and placement of the breakers, so taking the time to get the template right is critical.

Once you have the shape, you could really make the plate however you want. [HowToLou] cuts his by hand out of a piece of thin aluminum, but you could certainly 3D print it or even CNC it out of a thicker piece of metal. The important thing is that its stiff enough that somebody can’t just bend it out of the way if they’re fumbling around with it in the dark.

It probably goes without saying that a homemade interlock isn’t going to be up to code, but even if you don’t have any inspectors sniffing around your electrical panel, it’s a sensible precaution to have something like this installed. The middle of winter is a bad time to realize you don’t have any way to safely power your home when the grid goes down, so the key is getting something like this ready to go before you actually need it.

The current situation has given closet germaphobes the world over a chance to get out there and clean the hell out of everything. Some of it may be overdone; we ourselves can cop to a certain excess as we wipe down cans and boxes when returning from a run to the grocery store. But sometimes disinfection is clearly indicated, and having an easy way to kill the bugs on things like face masks can make a big difference by extending the life of something that would normally be disposable. That’s where this quick and easy UV-C germicidal cabinet really shines.

The idea behind [Deeplocal]’s “YouVee” is to be something that can be quickly cobbled together from parts that can be picked up at any big-box home store, thereby limiting the number of trips out. You might even have everything needed already, which would make this a super simple build. The business end is a UV-C germicidal fluorescent lamp, of the kind used in clarifiers for backyard ponds. A fluorescent droplight is modified to accept the lamp by snipping off a bit of plastic, and the lamp is attached to the inside of the lid of a sturdy black plastic tote. The interior of the tote is lined with aluminum tape and a stand for items to be disinfected is made from a paint roller screen. The clever bit is the safety interlock; to prevent exposure to UV, the lamp needs to be unplugged before removing the lid. Check out the full build tutorial for details.

We can’t vouch for YouVee’s germicidal efficacy, but it seems like a solid design. If you have doubts, you could always measure the UV-C flux easily, or you could build a smaller version of this peroxide vapor PPE sterilizer, just to be sure.

The year is 1894. You are designing a train system for a large city. Your boss informs you that the mayor’s office wants assurances that trains can’t have wrecks. The system will start small, but it is going to get big and complex over time with tracks crossing and switching. Remember, it is 1894, so computing and wireless tech are barely science fiction at this point. The answer — at least for the New York City subway system — is a clever system of signals and interlocks that make great use of the technology of the day. Bernard S. Greenberg does a great job of describing the system in great detail.

The subway began operation in 1904, well over 30 years since the above-ground trains began running. A clever system of signals and the tracks themselves worked together with some mechanical devices to make the subway very safe. Even if you tried to run two trains together, the safety systems would prevent it.

On the face of it, the system is very simple. There are lights that show red, yellow, and green. If you drive, you know what these mean. But what’s really interesting is the scheme used at the time to make them light.

Smart Rails

These days we make everything “smart.” If it were done today, the tracks would be billed as smart tracks because they can tell if a train is on them. The system was built with 1,000 foot blocks of track that are electrically isolated from each other. The total system in New York has almost 15,000 blocks. If a train has wheels on a block it will short the two rails in that block together. Without the train, the two rails show as an open circuit. So each block (sometimes called a circuit or a section) is basically a switch where the train itself completes the circuit.

On a single piece of track, a particular signal will turn red when a block is occupied. Adjacent blocks may also turn red based on how long it takes to stop a fully-loaded train. For the New York subway, a red light also raises a train stop which is a T-shaped bar that is usually just at track level. When the light is red, the bar will engage a trip cock under each car. The trip cock removes power from the wheels and applies emergency braking. So — barring some sort of mechanical failure —  a train trying to pass a red light will stop even if the engineer doesn’t want to stop.

Up until 1970, trains would sometimes override the train stop mechanism. This was made against the rules after — you guessed it — there were several collisions caused by overriding the safety system.

History and Evolution

Once you know where there is a train, you get into the subject of interlocks. The idea is that you shouldn’t be able to operate the tracks in an unsafe manner. So if you light a signal green for a piece of track that crosses another piece of track, the second block must be both empty and red. Enforcing this kind of behavior is the job of interlocking.

Interlocking started in Britain. In June 1856, John Saxby received the first patent for interlocking switches and signals. In 1868, there was a patent for what is known today in North America as “preliminary latch locking”. By 1873, well over 10,000 mechanical locking levers were on the London and North Western Railway.

Up until the 1950s interlocking machines used levers like the shown here that locked each other out and, apparently, some of those are still in use, but they now mostly use at least relay logic. All this, of course, is not unique to the New York subway.

Simulation

In addition to the fantastic guide linked at the top, Bernard Greenberg also wrote NXSYS which is a faithful simulation of operating the New York City subway system. You can see part of an NXSYS screen here. According to Bernard:

NXSYS can be viewed, utilized, or enjoyed at any of four levels:

• An entertaining rapid-transit video game for those who love the subways and wish to recreate the experience of navigating the tracks, and learn more about the subways and their signals.
• A comprehensive, interactive learning tool for the operation of rapid transit signalling and NX/UR control towers sufficiently detailed and accurate to be of value to those actually responsible for operating such equipment.
• A detailed guide to the sample implementation of such systems in electrical relay logic, observable in action down to the relay level.
• An interactive computer-aided design (CAD) tool for designing and debugging your own interlockings and signal circuitry.

Low High Tech

Pretty impressive. Even more impressive is that this whole thing could be done with levers and relays back in the old days. You can see an interlocking panel in the video below. Of course, the system gets all kinds of upgrades and you can read a lot of detail on all the technology in the subway on Wikipedia.

If you like this topic, you can check out what the British did — after all, it was their idea to start with. We’ve actually talked a little bit about block sensing trains before in a disruptive way.

Photo credits:

• Signal by Metro Transit Authority of the State of New York CC-BY-SA-2.0
• Interlocking levers by [JoeRoma] CC-BY-SA-2.5

If there’s one maker space that has an excess of mad scientist type hackers, it has to be LVL1 in Louisville, KY. They sure do a lot of crazy stuff, like this simple device to defeat the laser cutter smoke monster. Nobody got the memo about the “simple” part. Instead they created a functional, educational and aesthetically pleasing element for the hackerspace.

LVL1 has a large format laser cutter. Laser cutters emit nasty smoke. Said smoke needs to be vented outside. To do so, it needs to pass through a scrubber/filter so the neighbouring Pigs don’t complain. So they installed a larger, better filter. The Pigs are happy, until the filter gets clogged and the smoke monster decides to escape. Next they install a pressure switch which disables the laser when the filter gets clogged. Laser cutters have a myriad of safety interlocks, so quite often, it isn’t apparent which one caused it to trip. Hence, the Laser Cutter Enable Module – LCEM.

The simple part was to install an indicator that lights up when the pressure switch is enabled, and off when not. But when it’s off, it isn’t clear if the pressure switch is off, or the indicator has failed. Simple, just install a bi-color LED – Red for off, Green for On. But then what about color blind folks who cannot tell the two colors apart? So, finally, two LED’s with clearly labelled text marking them as Enabled and Disabled.

A simple (this time for real) circuit was finally agreed upon. The SPDT contacts of the pressure switch drive the LED in an optoisolator. Its output drives a DPDT relay via a transistor. One set of contacts light up the two indicator LED’s and the other set of contacts goes to the laser cutter enable contacts. Of course, the optoisolator is totally redundant and over kill too – it’s input LED shares the same power supply as the output transistor! Remember the missing memo?

It was time to assemble the circuit. This is where the mad scientist dudes got really creative. On one half of a piece of acrylic, the schematic diagram was etched using the laser. This ensures n00bs get some education. And the remaining half had the circuit laid out in old-skool wire wrap fashion. Holes were drilled and connections were drawn (using the laser, of course) for the various components. Parts were inserted, and wires were soldered to make the connections. The result is what they call the PCB/Mounting Plate/Educational Schematic/Acrylic thing. Of course, exposed connections and wires are no good. So they made a sandwich consisting of a flat acrylic base, and a cut out frame in the middle to accommodate the wire connections and joints. All of this to light up an indicator. Because.

Thanks [JAC_101] from LVL1 for sending in this tip.

If you want to read more about LVL1 shenanigans, check out this post about their Rocketry group, or this post when Hackaday visited LVL1.