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Tech Forum Questions

Arduino Clone - Yes Or No? 2022 Issue-1

My daughter is preparing for a science fair. Each participant is limited to a total of $50 for parts and supplies. Central to her project is an Arduino Uno. I’m tempted to order a half-priced Chinese clone from eBay vs. an authentic model from one of the domestic supply houses. Is there a downside to using one of these inexpensive clones or are they identical to the real thing?

Diogo Alves
Saint Charles, MO

Assistance Against Cyber Attack 2020 Issue-5

What materials make a good phased array antenna (i.e., efficient transmission and reception and the shape of the individual components)? What frequencies go through earth and seawater above 10 GHz also?

William Zimmerman
via email


I’m curious about your impression of phased array antenna systems. What is it that you want to do?

Phased arrays are most commonly used in the AM broadcast band through the shortwave spectrum (500 kHz to 30 MHz). These arrays consist of a reference radiator and a number of phased radiators determined by the desired pattern. As frequency increase above 30 MHz different antennas types work better such as YAGI antennas or parabolic ‘dish’ antennas. Usually as the frequency goes higher, phased arrays are not practical.

As to your last question, water and water vapor ABSORBS radio frequency energy. The only frequencies that will penetrate water are in the VLF (very low frequency) band. WWVB (60 kHz) may be able to penetrate water, but this is about the highest frequency that possibly can.

You need to elaborate on your question. BTW, metal is always the best conductor for an antenna system. The efficiency of a system is determined by a number of factors. One primary factor is cable (coaxial) loss as the frequency increases (loss increases).

Douglas Hall, CPBE
Vilas, NC

Sump Pump Monitor 2020 Issue-6

My basement is prone to seepage. When the sump pump runs so often that there is only a 15 second off-time the seepage is eminent. I am looking for a stand-alone circuit that gives a digital read-out of the off time in seconds. An adjustable alarm output would be a big plus.

I am not concerned with sensing the pump power status, that I think I can do. The off time range of interest would fall between 120 and 2 seconds. I am sure this can be easily accomplished with an Arduino or Raspberry Pi circuit but I have no experience with either. Thank you.

Allen Goodcase
Brookfield, IL

VSWR Value Change 2020 Issue-6

Last night I was testing my 145MHz Yagi antenna for VSWR value when I realized something. When I kept the antenna close to ground, the VSWR value would change. Also, when I stood in front of the antenna the same thing happened.

I searched about it on the internet but I'm not satisfied. If any one knows the reason for this, could you explain it to me please. It will be really appreciated.

Emre Eroglu

In-Line speaker Amp 2020 Issue-5

I recently moved into a home that has in-ceiling speakers. I have them connected to an A/V receiver and in one room they work great. In the other room, the sound is very muted. There’s a volume control in that room which I’ve replaced and checked. I’m looking for some kind of amplifier that I can purchase or build that can just increase the volume level on that pair of speakers (there’s a pair leaving the receiver which goes into the volume control and then splits into four speakers). I have checked obvious issues and swapped the A/B pairs just to make sure my receiver hasn’t failed.

Michaeljon Miller
Trabuco Canyon, CA


It sounds like your problem is an impedance mismatch in the speaker system. Maximum power transfer occurs when the source impedance (output of your amplifier) matches the load impedance (speaker). The gauge of the speaker cable may contribute to the problem. The smaller the gauge the higher the IR loss in the wire. And NO... Monster Cable is NOT significantly better and definitely NOT worth the extra cost.

A better solution is an additional amplifier for those ‘other room’ speakers... defining another ZONE. That amplifier should be fed by a low level output from your receiver My guess is that the previous homeowner had a system with an amplifier per zone and a low level signal distribution system to feed the amplifiers. This can get a bit complicated in design but may translate into a more versatile system.

Douglas Hall, CPBE
Vilas, NC

Because you swapped the A/B speaker leads and got the same audio results, the culprit might be speaker-impedance mismatch. Check the output impedance of your A/V receiver and of the low-volume speakers. The receiver manual should specify an impedance, which in most cases comes to, 4, 8, or 16 ohms. If not in the manual, check for a label at the outputs. 

Also, find the impedance of your speakers in the manufacturer's information or on a speaker label.  You want the same impedance at both ends.  A mismatch can cause reduced volume and even distortion. If you want to measure impedance, here's a link to a helpful article: https://www.wikihow.com/Measure-Speaker-Impedance. If all else fails, look for an impedance-matching transformer. More information here: https://www.electronics-tutorials.ws/transformer/audio-transformer.html.

Jon Titus
Herriman, UT

Battery Woes 2020 Issue-4

I have some brand new lead-acid batteries that have never been used. They have been stored in my garage for a while (1-1/2 to 2 yrs). My smart charger errors and won’t charge them. Why is this and is there anything that can be done to revive them?

Reva Pino
Charlotte, NC


Most smart chargers are designed to not put out current if the voltage on the load is too low. This protects the charger against short circuits and any load that isn’t a short but would overheat the charger.

Batteries that have sat that long may or may not be rescuable. The best thing to try is a “dumb” charger with external current limiting. For a car battery, the external current limiter can be a sealed beam or H4 halogen headlamp bulb; the charger should be 6 amp or bigger. For motorcycle batteries, the same rig but with a tail lamp bulb.

Once the battery has some voltage on it, you can switch over to the smart charger. If the battery voltage is above the smart charger’s go/no go threshold, it will charge the battery. A field expedient to the dumb charger and bulb limiter is to use another battery of the same voltage which has charge in it, plus the bulb limiter, across the discharged battery to bring its voltage up. A rescuable battery will have the bulb glow brightly and then gradually dim as the dead battery voltage rises.

Jonathan Wexler
Los Angeles, CA

Lead acid (PbA) batteries have a high self discharge rate. They will go dead just sitting. They also have a short calendar life, more than 3 years old you can expect to have problems. A smart charger will see the voltage is to low and abort charging. The longer the cell voltage remains below 1.5V the more damage is done shortening their life and capacity. To extend PbA battery life requires a float charger like the 'Battery Tender'.

From the 1.5 to 2 years of storage mentioned I'd say you now have paper weights. A slow trickle charge done with a bench power supply at a low current to about 2V per cell, may bring them up. But how useful they'll be remains to be seen. Then you can hook up your smart charger and see what happens. Don't leave the charging unattended, you can stop it over night by disconnecting one terminal. Some power supplies can be back fed from the battery when turned off. You have to watch out with batteries of unknown condition. Fires can happen. This applies to PbA as well as lithium based batteries. An alarm is useless when there's no one around to take action.

Randy Carter
Kent, WA

Capacitor P&Cs 2020 Issue-4

What are the pros and cons for using electrolytic capacitors in a voltage divider circuit to provide about 24 volts AC to a heater cable from the 120 volt AC line?

Is there a possibility of having a capacitor explode from overheating? If so, could that be prevented by stringing several capacitors in parallel to provide for additional heat dissipation?

Robert Gotts
Madison, IN


Electrolytic capacitors are polarized and every half cycle of the powerline their polarity will be reversed. Depending on the values and types, they may get hot, or more exciting, blow out their pressure relief attended by a puff of smoke and fumes. In any case, their life is rapidly shortened if reversed. Non polarized caps are available but it all depends on the values needed. Most other capacitors are non-polpoarized and they should work for you.

Len Powell
Finksburg, MD

Mr. Gotts seeks information on employing the reactive property of a capacitor to reduce AC line voltage to 24 volts.

The short answer is “Don’t do it.”

Been there. Done that. Didn’t know any better. In my case, I had a small circuit comprising one vacuum tube having a 12-volt filament drawing 0.15 amperes. Dropping the voltage from 120 volts required a series impedance of 720 ohms. Like you, it occurred to me that the reactive impedance of a capacitor might provide the needed voltage drop, eliminating a large (and hot) series resistor. A capacitor of 3.7 uF at 60 Hz provided the necessary 720-ohm impedance.

The technique worked and nothing blew up. I was lucky. Seventy subsequent years of experience, however, lead me to consider the reasons NOT to use this technique:

  • Most importantly, no isolation from the AC line is obtained, so that the possibility of shock hazard always exists. Furthermore, because the reactance of a capacitor varies inversely as the frequency of the applied voltage, and lacking any line-to-load isolation, it follows that any high-frequency line-voltage transients will be coupled through to the load without attenuation.
  • The technique does not, in general, work where the characteristics of the load are unknown. If the load is resistive and the current drawn is constant, — as in my vacuum-tube example — the technique works. If the load varies, however, then the voltage across the load will vary accordingly.
  • In any application with this technique, phase shift of the load voltage occurs relative to the AC line voltage. This phase shift is unimportant to a resistive load. If, however, the load is inductive or capacitive, this phase shift may produce wild swings in load voltage due to oscillation. Heater control circuits usually employ relays, and relay coils are inductive loads.

One would never use an electrolytic capacitor for this job. Film-dielectric non-polarized motor-start and motor-run capacitors are available with operating voltage ratings suitable for the job. But the off-the-shelf tolerances of such devices is relatively large, running to 6% for motor-run capacitors and 10% for motor-start capacitors. For a 24-volt resistive load supplied through an off-the-shelf motor-run capacitor, the load voltage may be anything from 22 to 25 volts.

A small transformer is less expensive than a motor-start or -run capacitor, it provides safety isolation from the AC line, and the load voltage won’t oscillate.

Peter A. Goodwin
Rockport, MA

That is not at all practical. Use a 24 volt transformer from a sprinkler timer or thermostat.

Richard Cox
Thousand Oaks, CA

Most aluminum electrolytic capacitors are not suited to having large amounts of AC voltage on them. Also they have to be used in pairs, to handle both polarities of voltage. And yes, you may have them overheat and "rapidly disassemble."

Also the heater cable will not be isolated from the AC line, which may be hazardous under fault conditions. You didn't say how much current you needed at 24VAC, but I assume it might be more than an ampere. My first choice would be a 120:24V transformer. You'd get decent efficiency and isolated power.

Jonathan Wexler
Los Angeles, CA

Temperature Sensing LEDs 2020 Issue-4

I’m looking for a temperature sensing circuit that will light each of three LEDs at approximately 80°F, 90°F, and 100°F.

Matthew Seltzer
via Internet

Fan Indicator 2020 Issue-4

Good day to all you experts!  I have a plywood basement floor that is suspended like any other floor in the house (bentonite soil in my area requires this construction). The actual dirt ground is about two feet below the wood floor, covered by a rubber tarp.

To prevent a build-up of mold and stale air, this space has a 6” duct vent fan that turns on via a humidity sensor rheostat. The supply side duct is on one side of my basement and the evacuation duct is on the other.

In the past, I could hear this fan running, so I knew when the bearings were wearing out. It was an easy job to buy a new duct fan and replace it. We just had our basement finished, putting drywall around the perimeter wall. Now I can no longer hear this fan when it kicks on.

Does anybody have a suggestion for some sort of sensor that detects when the fan is turned on by the humidity sensor but drawing too large of a current supply, so on the verge of bearing failure? Ideally, I would like some sort of an indicator light that I can make part of the access panel that is over the fan. Even an AC ammeter movement would be adequate.

At the location of the fan, I have both the switched 120 VAC power supply and a constant 120 VAC available if needed. I don’t have the specifications on this exact fan available, but a quick search online found several that had operating currents of 0.35-0.40 amps. I know the start-up amps would be a little higher but not too much because the motor is small and has very little inertia to overcome. Thank you for any suggestions!

Bill Young
Denver, CO


Possibly a simple answer might be to configure a small plastic flag attached to a micro switch and positioned somewhere in the air-flow. The switch activation could activate a remote lamp or indicator when the air-flow slows or stops.

Bob Lund
Elmhurst, IL

As a first thought a current sensor, watching the motor’s current draw, comes to mind. A current sensor is simply a single winding coil that one motor wire passes through. It is a basic transformer and the coil develops a voltage relative to the motor current. They are available commercially or can be salvaged from a junk box transformer.

OK, but that seems to be more bother than it’s worth since all you really need to know is if the fan is running not it’s actual current draw. So now it looks like an air flow switch is the best choice. Don’t go off the deep end here, they are quite simple. Many commercial airflow switches are nothing more than a lightweight paddle connected to a micro switch actuator arm. This is placed in the airflow, the air lifts the paddle and the switch operates. Just be sure the paddle falls freely without airflow and that the airflow raises the paddle high enough that it doesn’t dance or flutter on the air stream.

A common SPDT micro switch offers many options for alarm or indication connections without the fussiness of measuring the current sensor voltage and the circuitry required. Since the micro switch is isolated it could be connected to a line power, low voltage or even an alarm system! Low cost possibilities and reliable operation are unlimited.

No power, clogged duct work or fan assembly, bound up motor, a squirrel in the squirrel cage... all result in no airflow! Not having specific information about your fan or it’s installation, it would seem easy to cobble this together with common, and easy to find items. Hope this solves your problem.

Len Powell
Finksburg, MD

Judging by the current, the fan motor is probably a shaded pole motor. Bad bearings may not alter the current very much, until they freeze up, at which point, you'd have the motor's locked rotor current. I assume you'd like to know about motor trouble before it fails. Since you are able to tell by ear, why not use a cheap intercom to monitor the fan?

Jonathan Wexler
Los Angeles, CA

Component Footprints 2020 Issue-4

Can someone please explain the standards for the footprint of electronic components? I’m trying to figure out the the best way to lay things out on a circuit board. I’m new at this and any advice would be appreciated.

James Devera
Scandinavia, WI

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