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Recent Questions

October 2017

Circular Polarizating Antennas

I’m an avid VHF/UHF listener, and have always used a vertical non-directional antenna for receiving signals. However, after buying a circular polarizing filter for my Nikon, I realized that RF can be circularly polarized as well. How can I tell whether a signal is vertically or circularly polarized, without investing in a circular antenna? Do you know of any resources for how to construct circular polarized antennas?

Ed Moreno
Schaumburg, IN

Camera Security

My roommate keeps a piece of masking tape over the lens of the camera on his laptop. I think he’s paranoid; he says he’s being safe. I’d love opinions about which one of us is right.

Neil Nelson
Elkhart, IN

Temperature Rise Of A Heatsink In An Amateur Radio Transceiver

I recently did a “tear-down” of a Baofeng BL-8 battery eliminator (for use with a UV82 dual-band tranceiver; see, articles). The tear-down revealed that the eight volt battery eliminator actually used 2X L7808CV TO-220 voltage regulators connected pin for pin in parallel, which is somewhat unconventional.

Both regularors were mounted to a common small (20 x 34 x 1.8 mm) aluminum heatsink and all enclosed within the plastic housing which was sized to resemble the battery it was replacing.

In the ideal case, each regulator would carry 1/2 the total maximum current taken by the transceiver, which is approx. 0.775 amps. Each regulator produces a nominal 8.0 volt DC output and has to dissipate approx 4.6 watts of heat. The thermal resistance of the TO220 package is 5°C per watt, junction to case (or mounting base).

They are mounted dry, which adds another 1°C per watt for mounting the base to the heatsink.
  Q1: What is the temperature rise in the aluminum plate?
  Q2: Since there is no direct path to air, what further temperature rise may take place inside the plastic housing?

Don Dorward
Pickering, ONT Canada

Should I Switch?

I’m torn between using 3.3 and 5.0 volt components on my microcontroller projects. Many people seem to be moving to 3.3V, but components don’t seem to be as readily available as 5.0V components.  What are the advantages and disadvantages of each?

Tony Sigler
New Brunswick, NJ

September 2017

Door Chime Protection Circuit

I need a way to protect my hardwired door chimes from being accidentally “burnt up” when the doorbell button gets stuck.

A couple of times when the button got stuck, I was home and able to fix it before damage occurred. The last time it happened, however, nobody was home. Someone came to the door, and we arrived home to the smell of burnt electronics.
I’ve replaced the button more times then I can count. I guess the way the weather hits it — eventually — it becomes stuck. I am now on my third set of replacement chimes.

I have TWO 16 volt chimes (10VA) wired to a single button (LED lamp) on a 16V 30VA transformer. The transformer is much larger than normal to accommodate the two chimes.

I would like to protect the chimes in one of two ways:

  1) Simple protection — Inline fuse that would blow if the chimes were pulling current for more than (let's say) five seconds. I figure a “slow-blow” fuse would be ideal, but not sure on the rating.
  2) More complex protection — A “time out” circuit that would cut power to the chimes if the circuit was live for more than five seconds, self-resetting either after the button was released or after 60 seconds (chime would activate again, indicating a stuck button).

Eric D. Bailey
Cecilton, MD

Audio Leveler

My wife and I are hearing impaired. We need an AVC amplifier for TV audio to keep the level constant going into WiFi to hearing aids. Commercial or build-it-yourself (no surface mount); Analog not digital.

Fred Imm
Kirtland, OH

Electromagnetics: Transformers, Generators, Motors and Other AC Machinery.

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 of 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

What’s In A Board?

There are plenty of high quality, double-sided printed prototyping boards readily available on eBay. They are perfect if used as they are, unless — as it often happens — the need arises to trim, grind, or otherwise shape them.

Here is my problem: I am deeply concerned that these boards are made with asbestos. I know that for any grinding job a facial mask is a must, but if the dust created in the process could contain asbestos particles, much stronger protection measures would be warranted. Or, perhaps such operations should be entirely avoided.

I imagine that many electronics enthusiasts cut or even grind these items, completely unaware of the danger they may pose. I tried to gather information from the Web about these products, with little success.

I would like to know — preferably from an authoritative source — what materials are used in production of these (mostly Chinese in origin) prototyping boards.

Jan Raj
Philadelphia, PA

August 2017

Laser Color Choice

I want to convert a 3D printer to a laser burning station to work with wood veneers and leather goods.

Any advice on the best type and color of laser to get? I’ve heard brown wood and leather reflect more red light and so I should use a blue laser.

Kyle Hoyos
Walkerton, IN

Oil Filled Caps

I’m trying to bring an old tube transmitter back to life. The power supply uses oil filled capacitors, which don't seem to have handy replacements. Assuming that I can change the oil to reduce the leakage current, what kind of oil is used?

Will motor oil work, for example?

William Helm
Eugene, OR

More Questions