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Electromagnetics: Transformers, Generators, Motors, and Other AC Machinery Answered September 2017

I’m trying to make sense of everything coiled, but the only thing getting wound up is me! I thought I knew a bit about electromagnetics, but recently I’ve been trying to make sense of all these fields and flows.

What is the difference between the magnetic field and the flux? How does flux work in a transformer or a generator? Does anyone really understand Maxwell’s Equations?

So many textbooks dealing with electromagnetism speak in equations instead of English. I want to know HOW it all works, not just how to compute these things. Am I just reading the wrong books? Can you help me figure out what the flux is going on?

Taylor Street
Felton, CA


I have spent a lot of my life wondering about magnetics. Welcome to the club! Hopefully the information that I have is correct

Here are a few basics:

  1. When an electron moves, it generates a magnetic field in addition to the electric field that's always there.  — Why?: Because
  2.The field exists if the electron/electrons move in a wire, a stream (e.g. in a tube), or anything else.
  3. Magnets 'work' because (if I have this right), the arrangement of their molecules is such that the electron orbits of the individual atoms are oriented such that the fields generated add together. The relative strength of a magnetic material is based on how well they are aligned. Magnet discussion usually describes magnetic domains within the material. (Magnetizing a material means that you apply a magnetic field to align the domains.)
  4. Non magnetic materials don't feature this alignment. Electron orbits are in random orientation and the fields cancel.
  5. Magnetic fields are constant for DC current and non moving magnets.
  6. Time varying fields are generated from AC current and moving magnets.
  7. Time varying fields do the following: A. If they pass through a wire they will cause the electrons to move — thus a transformer. B. If they encounter another magnetic field they will cause either an attractive or repulsive force — thus a motor. (This force is also present in static field interactions, but that won't make a motor.)
  8. Coils of wire are used in transformers and motors because the fields add and compact devices can be made.
  9. The field descriptions are always confusing. What I got out of it was that the B field is the description for the field you would find due to the current or magnetic material. The resultant field that you can measure depends on the material that the field is in.

Exmple: If you have a long solenoid, the field inside will be fairly constant if you're not too close to the ends. If you put a piece of ferrous material inside, there will be an increased field through the metal because it's easier for the 'flux' to go through this material. (This also affects the input current to some extent, in the same way that a a lower resistance load affects an electric circuit.)

  10. There seems to be no end of magnetic units — just like farenheit, centigrade, and Kelvin for temperature.
  11. Maxwell hopefully understood his equations, along with some other smart people.

The math that you see everywhere generally shows what fields you can expect due to different circumstances. It is very difficult, and I certainly don't understand it. Happily, computers are now available that are powerful enough to avoid a lot of it. They use a method called finite element analysis. Basically they calculate fields based on the sum of tiny elements at each point of interest in a field.

Harold Johnson
via email

Diesel Computer Mods Answered 2019 Issue-5

There are several entities that will modify the settings on the engine control computer for a diesel Chevy pickup to increase power. I assume they are adjusting the timing curves and other parameters. Is there an adapter and software available for me to experiment with this myself?

Karel Dostál
Covina, CA


I really hate to be a wet towel, but this is really not a good idea. Arbitrarily adjusting things like fuel and timing is most likely to result in a smoking, noisy engine. If run to long the engine is more likely to self-destruct than increase power. Not to mention tailpipe pollution. Even "professionally" reprogrammed engines emit excess pollution, and shorter lifespan.

Bill van Dijk

Car Detection Answered 2019 Issue-5

I’m looking for help designing a circuit to detect cars driving up my private road. I have in mind something like the wire loop embedded in roads to sense cars at a traffic light. Can you explain the principle behind this method and how a basic DIY version might be implemented?

Ulrike Krüger
Laconia, NH


A Passive Infrared (PIR) detector is much less expensive and easier to use. There are many available for outdoor use, with battery operation and remote monitoring by Wifi or other means. I don’t know of any DIY road loop circuits, and they are being phased out due to high installation and maintenance costs in favor of cameras, which are another choice.

I once bought a little board from a guy in Australia which monitored standard NTSC video and detected movement. Worked pretty well.

Richard Cox
Thousand Oaks, CA

Guitar Tuner Answered 2019 Issue-5

I’m trying to build an electronic guitar tuner. Is there an IC available to generate the proper tones?

Hamish Morisset
Norcross, GA


I would suggest an ebay search for guitar tune”. There are several listed in the ten dollar range with free shipping. The tuners don’t generate a tone but receive a tone and show the note on the lcd. I think parts cost for self build would be higher than purchase.

Steve Benson
New Castle, IN

Dish Projects Wanted Answered 2019 Issue-5

I have a couple of small satellite dishes (Direct TV I think) with receivers that have been sitting in the garage for a while. I hate to throw out technology that could be repurposed. Anyone have an idea for a cool DIY project I could reuse them for?

Devin Martel
Durham, NC


Make a long distance microphone like they use at football games. Mount a microphone at the focal point and connect to an audio amp and headphones.

Richard Cox
Thousand Oaks, CA

Solar Panels And Lightning 2019 Issue-4

Are solar panels suceptable to damage by nearby lightning strikes and would they provide any margin of safety to electronic equipment in a grid-tied or off-grid system?

Alex Freeleagus
Tigard, OR


While a direct hit will wipe out just about anything, a nearby hit won't do much damage to a solar panel. If something were close enough to get a shock, electronic gear would be the least of your trouble! Just use proper grounding per NEC codes. (and fuses)

Robbie J
Green River, WY

NFL Magic Lines 2019 Issue-4

Can someone explain how the first down and scrimage lines are generated on screen in televised NFL games? Obviously, it's done by computer, but I can't figure out how the angle and aspect is able to change and keep up with the constantly changing camera angles. Also, the lines are seemingly underneath the players, as if they were actually drawn on the ground.

Ryan Johnson
Jackson, MS


If you’re in Alexandria VA, you can go to the National Inventor’s Hall of Fame museum and they have an interactive exhibit about it: https://www.invent.org/museum/exhibits

“1st & Ten Line Stadium
Get in the game with the 1st & Ten Line Stadium exhibit, inspired by several game-changing NIHF Inductees. Visitors can step onto the turf to experience the sounds and visuals of a real football game. This hands-on exhibit puts you on the big screen, as you control the placement of the Virtual Yellow 1st & Ten® line, invented by Inductee Stan Honey.”

ESPN Front Row has an article about it that has additional links to the history of the system:


I hope that helps give Mr. Johnson some resources to read.

Phil Gates
via email

The short answer is the TV broadcaster maps out the stadium and instruments the cameras to report exactly where it is focused (tilt, pan, zoom, and focus). Computer processing then overlays the first-down (yellow) line onto each frame of the video. There is a good write-up on the technology here: https://entertainment.howstuffworks.com/first-down-line1.htm.

Kerry Imming
Rochester, MN

Ethernet Port Expansion 2019 Issue-3

My Internet router is located near our entertainment center in the family room. There are only four Ethernet ports on the router and I have five pieces of equipment that need to plug in. I also run a cable from the router to my home office (in the back of the house) that connects to an Ethernet switch that provides Internet to multiple computers, VOIP phones, as well as network connections for various printers.

At the router, I have to plug and unplug whatever equipment I want to use, since I don’t have enough ports to keep it all plugged in. Some devices can use the Wi-Fi, but performance is better and more reliable on the wired connections.


  1. Can I add another switch at the router to expand the number of ports?
  2. What are the limitations/drawbacks on adding more switches to the network? (I currently have two in the office.)
  3. Is there a better way to do this?

Byron Rochefort
Fort Wayne, IN


The easiest way to get more ports is to just buy a new router or switch with more ports than the one you currently have! It sounds like you have a 5-port router driving your Entertainment Center, so I suggest replacing the 5-port router with an 8-port Router (i.e., https://www.amazon.com/D-Link-Gigabit-Dynamic-Filtering-DSR-250/dp/B008021NSI/ref=sr_1_5?keywords=8+port+internet+router&qid=1566511970&s=gateway&sr=8-5). Because routers intelligently manage the output ports to ensure that each port will have maximum bandwidth and manage Network addressing (via DHCP, etc.), they’re always more expensive than unmanaged switches.

Unmanaged Switches, OTOH, simply connect to a source port (i.e., one network address) and simply devide the incoming bandwidth to each active device on the other ports, depending on how many connected devices are active, relying on the source port (usually a modem) to do all the port management (DHCP routing). In other words, if you have a 1000 megabit source (i.e., cable modem feed) connected to one port of a 5-port switch and 4 output (i.e., smart TV, game console) devices connected to the other 4 ports, each output port will only see a maximum of 250 megabits of bandwidth (1000 megabits/4 active outputs) IF all 4 output devices are simultaneously active. As the number of active devices decreases, the per-port bandwidth naturally increases. Only ONE active output device will receive full bandwidth from the input source. This is why unmanaged switches are much cheaper than routers, because they use something else to take care of address, etc. management.

Therefore, these are your choices:

  1. If you want maximum throughput to all your internet devices, invest in a router that has lots of output ports. This is the mo$t expen$ive option as more ports = more $$$. However each active output port will see a pretty high bandwidth due to the router’s intelligence.
  2. If per-device throughput isn’t an issue, invest in HIgh-Bandwidth (Gigabit) unmanaged switches that have more than five ports each (i.e., 8-port, 16-port, 24-port). While the more ports = more $$$ formula still applies, their cost is still proportionately lower than routers with similar port configurations.

When dealing with Area Networks (home or business), It’s always wise to have more available ports (router and/or switch) at all stations (Home Office, Entertainment Center, etc.) than there are active network devices so you have expansion capability if you desire (i.e., extra printer, extra computer, etc.).

Ken Simmons
Auburn, WA

Adding a switch to a router port is OK. The router will only send the packets that are addressed to the devices on the switch. The negative is that the devices on the switch have to share the bandwith of that one port. Put devices that have low usage or not used at the same time on the switch. The alternative (better way) is buy an eight port router. The switches also come in larger number of ports.

Steve Benson
New Castle, IN

Let's take you questions one at a time.

  1. Can I add another switch at the router to expand the number of ports? — Yes, up to the theoretical maximum of 254 ports.
  2. What are the limitations/drawbacks on adding more switches to the network? — I've experienced no problems. I have about 45 Ethernet devices and 22 WiFi devices in my home. I have both 8 port and 24 port gigabit switches plugged into the router plus two 8 port switches plugged into the 24 port one. Additionally, I have 5 port and 8 port switches located in several rooms that go back to the 24 port one as well. With 9 HD IP cameras and streaming services running I found that using gigabit switches (10/100/1000) eliminated any issues.
  3. Is there a better way to do this? — With gigabit switches costing as little as $10 you'd be hard pressed to find an easier or more economical solution. With a total of seven switches in service I have a highly reliable network.

Bruce Robin
Naples, FL

The main limit with today's switches, even the cheapest, will be the total number of IPs used on your local network. This sets a practical limit to about 240 ports less the number of WiFi connected devices. I would consider an inexpensive Dlink / Netgear / TPLink / TrendNet "Green" 8 port gigabit switch at the router; this will give you five empty ports when finished. I'd also move all the connections off the router for the inside (the four you've got plugged in now) and run this switch off one of the router ports, and the two remote switches off of two of the other ports, leaving the non-switch items on the new switch.

Ralph Phillips
Bossier City, LA

1. Yes. I have the exact same problem, so I plugged a small 8-port Cisco switch (SG110D-08) into one of the ports on my router, and it works great.
2. There may be a practical limit to how many switches you can cascade, but I have another switch plugged into the first one to service some equipment in another location, and it works just fine. Ideally, to avoid any latency when going through multiple switches, you may want to plug all switches directly into the router if it has enough ports, but it will work fine if the switches are cascaded.
3. Replacing your router with one that has more ports would probably be the ideal solution, but the above is cheaper and works well.

Gary Rathbun

Yes, just add another ethernet switch with a short cat5/6 jumper to the switch on the back of the router to which you wish to expand the ports. The ports on the back of a router are really "switch" input/outputs.

William B Runyon Sr
Chesapeake, VA

PA system to POTS interface Answered 2019 Issue-4

I need to put the speaker-level output from a PA system on a POTS telephone line. I don’t need dialing capability, as I can connect a POTS phone to make the call, but once the call is established, I need a circuit that will:

  1. Hold the phone line open, as I’ll hang up the phone I used to make the call.
  2. Put the PA output onto the phone line, preferably with adjustable volume. I can attenuate the speaker-level PA output to line level if necessary.

If a reasonably-priced product with good sound quality (i.e., no hum, etc) is already available to do this, suggestions are welcome. I’ve found many products for recording phone conversations or putting them into a PA, but only quite expensive ones with way more features than I need that will do the simple job I want.

Thanks for any help/suggestions.

Gary Rathbun
Placerville, CA


All you really need is a “wet” 600 to 600 ohm transformer. The “wet” designation is for a transformer that will work properly at audio frequencies with up to about 100 mA of DC flowing through the POTS side winding.

Connect it directly across the telephone line through a switch. Then apply the audio attenuated down to a low level to the other side of the transformer. Make sure that you don’t apply very much audio, as there are FCC regulations regarding the maximum audio levels. See FCC part 68 for the particulars.

Richard Cox
Thousand Oaks, CA

You can find music on hold adapters that will do exactly what you want, like the On-Hold MOH 150 for about $15 on ebay. You can plug the PA line into the device and it will play on the phone by putting it on hold from the device.

Bruce Robin
Naples, FL

Image Reverse Answered 2019 Issue-3

I’m building a rear facing camera system for my car that displays the image on a 7” screen mounted on the dash. However, the image appears reversed horizontally. Is there a circuit to flip the image left to right so it would be correct from the perspective of the driver? Any ideas are appreciated.

Krystian Czarnecki
Lombard, IL


Your best bet is to buy a backup camera that reverses the image for you. Many models let you select which way you want the image displayed. Depending on the resolution and night vision features you want, the backup camera can be purchased for under $20 online.


You don't need any electronics. Instead of having the camera face backwards out your rear mirror, place it so it "looks" left or right. Then use a mirror at 45 degrees to the camera's optical axis (top view) and you'll get a left-to-right reversal of what your display shows. You could mount the camera and mirror on a plastic or plywood base and support the mirror with a small angle bracket that lets you adjust the angle so it suits your purpose. And you can move the base as necessary.

Jon Titus
Herriman, UT

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