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Tech Forum

Neutralizing Battery Corrosion February 2018

What’s the best way to neutralize battery corrosion? I inherited an old, but expensive quartz watch from my grandfather and found a heavily corroded coin battery inside the watch case. I need to neutralize the corrosion without damaging the movement.

Clarence Dugan
Quitman, TX

Measuring Breathing Rate February 2018

I’m building a lie detector based on heart rate, breath rate, and galvanic skin response. I have it all figured out except for breath rate. What sort of sensor should I use to detect breath?

Jeanne Villeneuve
Vidalia, GA

Legacy Printer Port (LPT1) February 2018

I’m doing some work with legacy computer equipment and have written Basic software to control time and temperature in a high temperature kiln. I’ve used an old Compaq laptop (LTE Elite 4/40CX). I use the COM 1 port to control temperature and the LPT1 port to cycle the kiln on/off via an SCR.

Everything works great. I’m able to toggle the printer port pins via OUT commands. When trying other legacy computers, everything works except the printer ports don't respond to the OUT commands and remain in a high state. However, they DO operate the dot matrix printer indicating they are functional. My question is how do I gain control of some of the data pins and be able to toggle them high and low?

Jerry Sobel, R.Ph.
Las Vegas, NV

Sharing A Monitor With Two PCs January 2018

I have two desktop computers and three 32" HP Pavillion 32Q monitors being used by two people at the same time. One of the monitors is assigned to each computer and depending on the applications being used, the third one goes to whomever benefits most by having two screens. I have to physically disconnect and reconnect the third screen when it needs to be moved to the other PC.

How can I do this without having to crawl uder the table to make the switch? I've looked at KVM switches, but I only need to share the third monitor, not the keyboard, mouse, etc. The monitors use the display port input and run at 2560 x 1440. I would just add another monitor to run two with each PC, but there isn't enough room in the work area.

Vicente Benedetti
Athens, TX

Converting S-Video to Composite Video January 2018

I have a newer HD LED television that does NOT have an S-Video input jack! My old VCR and tapes are all S-VHS, so I need a schematic for building a converter for S-Video to Composite if I want to watch my old family movies. Please help!

Henry Vaden
Whitestone, NY


VHS tapes deteriorate and fail. The machine is unlikely to be repairable due to parts availability and even replacing the machine is major dollars. For family videos, you really want to keep, I would suggest converting the tapes to a more modern format say DVD or a standard video file format.

Modern PCs have an HDMI port that connects to your TV. You can buy hardware you can use to convert them to PC format yourself or contract a service to do it for you. You can buy an S-Video to HDMI converter if you want to go that way. I will say that my HD TV had issues with playing the tapes due to slight variations in the timing signals from the tape which caused blackouts and breakups in the picture. You may have better luck with your TV.

East Hartford, Ct

Low-Cost 3D Printers January 2018

I’ve been eyeing 3D printers for a while, and there are now basic units in the $100 range.

However, it appears that software can cost between free and several hundred dollars depending on features and capabilities. I like free. Is anyone using free software to create and print using one of these low-cost printers? If so, which software and printer are you using that you would recommend? Any caveats?

Brenda Spellman
Green Bay, WI

Chip Replacement January 2018

I have an old guitar “echo” pedal that is dead. After some troubleshooting, it appears the SAD1024 chip in the unit has given up the ghost and I have been unable to find a replacement chip.

Is there a substitute for it or a source for replacing it?

Mike Styles
Pennsauken, NJ

Solid-State Tube Replacement December 2017

I’m refurbishing a tube-type Hallicrafters shortwave receiver. I was planning on using a solid-state plug-in replacement for the rectifier tube; mostly because I can’t find the tube.

I’ve been told that a solid-state rectifier could result in higher voltage, and may blow the filter capacitors and run the tubes at a non-linear part of their operational curves. Can someone confirm or explain if this is a good idea or not?

Matthew Stiefel
Steelville, MO

AMP Clamp December 2017

Could someone explain in simple terms how an AMP clamp works? Does it have a transformer in it or Hall-effect sensors or similar?

Kevin Champion
Cleveland, OH


There are two major types of clamp on current meters. The AC type is a transformer; the wire in the clamp is the primary, the secondary is internal and is normally 1000 turns. If you are measuring 100 amps the secondary output will be 100 mA. The other type measures DC and could be calibrated for AC. The DC type uses a hall-effect or similar sensor. Amazon lists an AC/DC meter for $39.

Russell Kincaid
Milford, NH

Most clamp-on ammeters use a transformer located inside the body of the meter to step-up the measured current to a value which the unit can use.

Jeffrey Massey

I am no expert. But you are correct in your assumptions. I found this great article at Wikipedia on the subject:

Fernando Cordero

The common AC current meters use a transformer to measure the current. The “clamp” is the core for that transformer, and it is opened to go around the AC wire as it becomes the one-turn primary. There are simple components to scale the multi-turn secondary current down for the correct reading, either analog or digital. The digital part uses its own circuit to convert the measurement to its display.

A DC current meter cannot use the same transformer method, so it uses the Hall device that converts a magnetic field to a resistance, and the internal components convert that resistance to a reading, analog or digital.

Raymond Ramirez
Bayamon, PR

You are on the right track. The ones that measure AC use a pickup coil, and the ones that measure DC use a Hall effect sensor.

Chip Veres
Miami, FL

An AMP clamp is a transformer having a one-turn primary (the conductor carrying the current to be measured) and a multi-turn secondary (to feed the measuring instrument).

Being a transformer, it can only be used to measure alternating current. Be aware of its measurement rating, because too much current can saturate the clamp, resulting in inaccurate readings.

The clamp is rated for sinusoidal currents: measurements of non-sinusoidal current is best done with a true-RMS instrument connected to the clamp secondary winding.

Currents small relative to the capability of the clamp can be measured by passing the conductor through the clamp multiple times and dividing the measured value by the number of turns.

Peter A. Goodwin
Rockport, MA

AC amp clamps use a current transformer. The clamp is a laminated iron ring with a coil of wire wound around it. The alternating current in a powered AC line induces a magnetic flux in the clamp’s ring which the attached coil picks up via electromagnetic induction. That induced signal is fed to an amplifier who’s output is rectified and filtered and the resulting averaged (i.e. RMS) DC voltage is fed to the meter’s measuring circuit. The typical conversion is 1A AC = 1V DC on the meter.

DC clamps use Hall Effect sensors. As I understand them, the construction is similar to AC clamps, except an exciter coil is attached to the iron ring to pre-energize the ring with a specific frequency/voltage, which the Hall Effect circuitry sees as a zero level. DC current flowing through the clamp creates a magnetic field which alters that excite signal, which in turn changes the signal the Hall Effect sensor sees, which causes the Hall Effect detector circuit to output a voltage (positive or negative) proportionate to the current flowing in the measured conductor. Like the AC clamp, that detected signal is fed to an amplifier which outputs a DC level which is fed to the meter’s measuring circuit with a similar conversion scale (i.e. 1A DC = 1V DC on the meter).

As you’re aware, amp clamps are an EXTREMELY SAFE method of measuring high current (i.e. 10’s and 100’s of amps) in powered circuits because you don’t have to break any wires for making the measurement. HOWEVER, because their resolution is typically between 100 mA (0.1A) and 1A (due to the limitations of the magnetic detection circuitry), amp clamp’s use is limited to measuring power lines connected to large devices (i.e. pump motors) that naturally consume large amounts of current in operation.

Ken Simmons
Auburn, WA

VGA To LCD December 2017

I would like to be able to drive a 4x20 line LCD with a VGA output from an old computer. Is there a simple interface to do this?

Evan Lee
Elizabethtown, IL


The short answer is no. VGA is an analog interface, LCDs use a digital interface. You would need a 3 channel ADC to start with, but even that’s not enough. It would be very difficult to extract alphanumeric characters for the LCD from that data. Plus the 4x20 LCD is so small compared to the VGA display that there’s no way it can hold all the information shown on VGA.

You would be FAR better off rigging up some kind of serial port interface to the PC with the VGA output and selectively sending whatever data is of interest to the LCD.

Ralph Hipps

In all honesty, I believe this would be a fruitless exercise. The interface would be far from simple, and the resulting image on the 4x20 LCD display would most likely not be recognizable from the original VGA image.

VGA has a resolution of 640 x 480 and the LCD display would be 100 x 28, assuming 5 x 7 matrix for each character. Keep in mind the LCD display consists of a 4 rows of 20 characters each. Each character is formed by a 5 x 7 dot matrix, and the characters are separated by approximately a pixel width and 0.2 pixel height. So you are going from an aspect ratio of 4:3 to 25:7 or 12.5: 3.5, a significant difference.

In addition the pixels in the VGA display can vary in color and brightness, where the LCD display is only black and white, and are not individually adjustable in brightness.

And finally, the electronic interfaces to drive each type of display are significantly different both in hardware and software, and would require a very complex interface to even come close.

Ira Wexler
Owings Mills, MD

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