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TV Image Capture July/August 2018

What’s the best way to capture images from my TV? I’m looking to do something like a computer “print screen.”

Sharon Fitzgerald
Chattanooga, TN


There are two “inexpensive” ways to do a print screen from a TV screen:

1. Put a digital camera on a tripod and set it up in front of the screen — ensure the TV screen fills the viewfinder and ensure the flash is off. When you want to capture something, just take a picture of the screen.

After that, connect the camera to your PC via USB cable and download the screen capture. Be aware: the resolution of the photographed image will be limited by the screen resolution of the TV screen AND don’t be surprised if you get screen bars, etc. due to the timing between your camera’s shutter and the TV’s screen image refresh.

2. Install a stand-alone DVD recorder between your cable box and TV, or connect the recorder to your TV’s video out jack using a suitable interface cable. What you’ll do is record the program to the DVD (use DVD-R discs) and use the machine to finalize the recorded DVD (make it playable on other DVD drives) when the recording is complete.

Then, play that DVD on your computer’s DVD drive, use the computer’s playback program (i.e., Media Player for Windows) to get to the screen to capture, then PAUSE the playback. Use the image capture feature of the player to take a snapshot of the video image, then save it to your disc.  Be aware: the resolution of the DVD-recorded material will be no better than the video signal (Composite, S-video, HDMI) fed to the recorder.

After you’ve screen captured your images to your computer, use your Image Editing utility to clean up, etc. the images for your use.

Now then, there’s a more expensive way to do it: it involves installing a video capture card into your PC. Like the DVD recorder option, you’ll have to patch the video signal from the cable box or TV’s Video Out into your video capture card.

This way, you can watch TV using your PC and, using the Video Capture Application Software, capture a snapshot in (more or less) real time. Also, you can record the program, while you’re watching it, to your PC and edit/manipulate the recorded material at your leisure.

The big advantage is your captured images will (typically) be the screen resolution of your computer’s display (or at least much better than the inexpensive options above).

Ken SImmons
Auburn, WA

This would be most easily accomplished using an HDMI splitter and an HDMI-capture device used with your computer.

HDMI-capture devices can be external (to the computer) for PC or Apple machines, or can be implemented as a pluggable card for use with a PC desktop machine (assuming that the machine has an unassigned motherboard PCIe connector available).

Connect the splitter to the television program source – e.g., a cable box. Using an HDMI cable, connect one of the splitter outputs to the television set. Using a second HDMI cable, connect the remaining splitter output to the HDMI-capture device.

Record the program material using the software provided with the HDMI-capture device. See Newegg et al for available devices.

Peter A. Goodwin
Rockport, MA

Measuring Heat With Crystal Diodes May/June 2018

I have a bunch of crystal diodes in my junkbox that I want to use to measure heat. I seem to remember a circuit for it some years back that I’m trying to reconstruct from memory. It’s not working very well, so I must be missing something. Anyone have a simple circuit or explanation of how it should work?

John Marion
Bend, OR


Texas instruments always encouraged its engineers to use a diode in the emmiter of thier transistors. Why, because the reverse temp curve, cancelled the xistor temp curve. So we built several little circuits to monitor the temp. So, a series circuit that lets a low current through your diode, monitored with a sensitive meter, will give a value for different temps.

Caution, some diodes are sensitive to light, so take that into account. The 1n914 and 4148 were a common device. Never messed with the gallium devices, just be cautious they are easy to let the smoke out. Luck in your endeavors.

Tom Sides
Phoenix, AZ

I’m not certain what you mean by “crystal diodes” but most modern semiconductor diodes can be used as temperature sensors. A standard silicon diode will have a forward voltage drop, usually stated as “0.6V” or “0.7V.” However, it varies with temperature. Typically it will drop about 2 millivolts per degree C. By measuring that drop and calibrating at two extreme temperatures, the current temp can be determined.

The diode should have its cathode connected to ground and the anode should have a resistor to the positive supply. The voltage measured between the junction of the resistor /anode and ground will be the forward voltage to measure.

If by crystal diode you mean the old “cat’s whisker” crystals (e.g. galena) I suspect they will have much lower voltage drop. However, it will probably still show a temperature sensitivity, so may work. Silicon diodes work pretty well.

For an in depth explanation, with lots of math and theory, look here: https://www.eetimes.com/document.asp?doc_id=1279718.

William Cooke
Clarksville, TN

Theoretically, the forward-biased voltage drop is about -2.2 mv/deg C at reasonable currents like a few milliamps. This works pretty well for silicon diodes like the 1N400x and the 1N914. It also works well for the base-emitter junction of a silicon planar transistor (almost anything in a TO-92), especially if you short out the base-collector junction. I have not tried it, but I think it does not work as well for point-contact germanium diodes like a 1N34 (which might include your "crystal" diodes).

Joseph Feng
San Jose, CA

New Life For Magnets May/June 2018

Has anyone tried to revive permanent magnets that have weakened over time? What is the best method? Can they be made “good as new” or is it better to just replace them?

Petrina Martinec


I am not sure that time is the cause of loss of magnetism of permanent magnets. They normally lose their magnetism by three methods.

  1. Heat.
  2. Physical shock.
  3. Exposure to opposing magnetic field.

I have two large toroidal magnets which sit on a tube with opposite poles facing each other. The top magnet ‘floats’ about 2cm above the bottom one. They have been like this on my workbench for at least 10 years and there is no discernible difference in the distance between them so I don’t believe that they have lost any of their magnetism.

The magnets weigh about 100g so the force required to hold the floating magnet against gravity is about 1 newton. I have no idea where the energy to do this for 10 years has come from and I have not found a physicist who can provide a satisfactory answer. I will keep asking!

via Internet

Unless there’s something special about the old magnet it’s usually easier and cheaper to replace them. A case I’m familiar with where it’s worth “recharging” old magnets is in old motorcycle magnetos.

In this case we put a coil with a core inside the magnets and then pass a high current for a short period of time. The field from the coil has to be much stronger than the original magnet but doesn’t need to be maintained for very long. Discharging a capacitor into the coil is a good way to achieve this.

To little field and nothing happens, to high a field does no harm. Make sure the field you generate is in the same direction as the magnet or you’ll wipe out any remaining magnetism and possibly reverse it’s polarity.

Robin Hartley
New Zealand

There are a few ways to partially revive some types of magnets. However, the more powerful the magnet was originally, the more “brittle” is the arrangement of domains, and the harder it is to realign them.

For that reason, remagnetization is most useful for antiques, ornaments or teaching devices, made of materials such as carbon steel. Don’t expect a ferrite or neodymium magnet to gain back much strength.

First, determine the alignment of the original magnet. Rubberized magnetic backing, for example, often has stripes of opposite polarity on one side; these would be impossible to repair. A horseshoe magnet, often found on hand-cranked generators, or a bar magnet, is easier to fix.

Simplest is to put the old magnet in contact with a strong permanent magnet, e.g. a large rare-earth magnet, letting it align by greatest attraction (North touching South). Tap the old magnet a few times with a hammer to help rearrange domains. More effective is to wind wire around the magnet and pass a large pulse of DC current through the coil. Capacitive discharge magnetizers are simple to build, but involve high currents and voltages and present some safety hazards. See https://www.rcgroups.com/forums/showthread.php?307133-Inexpensive-magnetic-flux-(gauss)-meter/page3&highlight=capacitor%20discharge%20magnetizer for more details.

Dr Moishe P
via Internet

I would try making and using an electromagnet to re-magnetize your magnets. Works well for bar magnets, for other shapes it might be a bit more involved.

Since many “standard” type magnets are made from magnetic metal placed into a magnetic field to begin with I would say if this can be done for your magnets you stand a good chance of making them every bit as good as they were originally.

Phil Karras, KE3FL
Mount Airy, MD

No Wi-Fi For The Pi May/June 2018

How can a Raspberry Pi be made to talk to IoT devices over low power FM rather than Wi-Fi? Looking for distances under 100 ft.

Durriyah Shamoon
Eau Claire, WI


There are many low power radio modules in the market, which can be interfaced via the RPi’s serial port and/or GPIO pins, and which will allow point to point communications over short range (have a look at www.radiometrix.com; other suppliers are available). But, so far as communicating with existing IoT devices, you may have some trouble, since you will need a communication device compatible with whatever you are trying to communicate with.

Myk Dormer

Proper Motor Selection May/June 2018

I need some tips on selecting motors for an art project that continously rotates five different circular platforms (10” diameter, 15 lbs) 360° in one direction, then in the reverse direction. The complete rotation of each platform should take approx 30-40 seconds (not critical). A direct drive approach would be preferred (seems simpler), but I could use a gear reduction scheme if it were more cost-effective.

Caitlin Russell
Stony Creek, VA


Because you say the load is 35 lbs, you probably need a heavy duty drive with bearings to support it. While the unit listed below is expensive, $180 USD, it will do the job.

  1. Speed can be slowed by reducing the voltage.
  2. Reversing is done by changing the polarity.



  • RPM 25 RPM @ 99 VDC
  • Voltage 99 VDC nominal
  • Amperage 2.1 Amps @ 99 VDC, No load
  • Gear Ratio 108:1
  • Rotation Reversible
  • Enclosure Totally Enclosed
  • Duty Intermittent

This cheaper motor might also work:  https://www.surpluscenter.com/Brands/Parvalux/30-RPM-50-Volt-DC-Parvalux-Gearmotor-Single-Shaft-5-1549.axd

Jim Faulk
Longmont, CO

PS2 Emulation May/June 2018

My Sony CD changer has a PS2 keyboard port to enter the CD title, details, etc. Instead of typing, is there a way to emulate a PS2 interface on a PC or processor board to automate the key presses?



I have successfully plugged an old PC keyboard into that CD changer to enter data. However, what is it that you want to automate? Every entry i.e. artist, titles, etc.; are different?

M. Herman

Yes, you can emulate a PS-2 keyboard. An SPI-master port will do the job as long as it can handle 11 bits, or two 8-bit values in rapid succession. Not all microcontrollers can. (If you lack a good SPI port, emulate it in software.) Use the MOSI signal for the key-code transmissions and use the SCLK signal to emulate the keyboard’s clock output. The bits transmitted when you press a key look much like those put out by a standard UART: a start bit, eight data bits, an odd-parity bit, and a stop bit.

The keyboard’s clock output produces a logic-1 in its idle state and created a positive edge for each of the 11 bits. An SPI port configured as CPOL=1, CPHA=1, provides the proper timing. Note: The least-significant bit of an 8-bit key value gets sent first. My old PS-2 keyboard produces clock signals at 13.7 kHz. That’s a good frequency to start with for the SPI port.

A PS-2 keyboard assigns a unique non-ASCII code to every key. Find a list of the keys and assigned codes here: https://techdocs.altium.com/display/FPGA/PS2+Keyboard+Scan+Codes. When you press the “A” key, for example, the keyboard’s circuit produces the hex code 1C. When you release the “A” key, the keyboard transmits a key-release code F0, followed by the key’s code again.

The shift key works the same way. To send an uppercase “A” your microcontroller or processor board would send the SHIFT-key code, the A key code (then a short delay) followed by the key release code and the A key code again. Then it would send the key release code followed by the shift-key code.

Remember, your code must calculate an odd-parity bit and insert it in the 11-bit value to send. You can find parity-generator code on the Internet. Also, ensure you send the key and release values in bit-reversed form. Thus, the 1C code (0001 1100) for “A” must get transmitted as 0011 1000. You can simply set up your code so it uses the “reverse-bit” values to start with.

Jon Titus
Herriman, UT

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


Sorry Clarence but I suspect this is going to be difficult to impossible. I have used white vinegar to neutralize and remove the deposits and corrosion but:

  1.  The corrosion/battery acid and possibly the addition of the vinegar acid end up removing the coatings on metal parts which makes them less robust.
  2.  A watch has extremely small parts and the connections may be ruined already. You need to first take the watch completely apart to see the extent of the damage inside it. If the damage is confined to the battery port you may be in luck.

Use a Q-tip cotton swab with some vinegar on it; place it on the residue/corrosion and keep it there long enough to soak into the residue. Then twist the swab to remove the residue. If more removal is needed repeat this process. You can also use dry swabs to remove more residue if no more vinegar is needed.

Once all the residue is removed, you should use a contact cleaner/lubricator/protector fluid to coat the battery contacts.

Phil Karras, KE3FL
Mount Airy, MD

If the watch is really expensive, I would bring it to a watch maker shop and pay for the cleaning. At home, for my electronics, I use dish washing liquid — pour a few drops onto the corrosion and let it soak overnight. This usually dissolves the acidic corrosion. You can still clean with a Q-tip, then rinse and carefully dry the watch.

via Internet

Baking soda is a good acid neutralizer. It will stop further damage, but does nothing to restore damage done. The oxide already deposited will need to be removed with a scraping tool of some kind, and will without doubt, reveal damage.

Bill van Dijk

First, use a Q-tip and 90%+ isopropyl alcohol to clean the corrosion — do NOT immerse or flood the watch with alcohol! When you’re done with this step, CAREFULLY use canned air to GENTLY dry the cleaned area.

Then, apply a paste made from baking soda and water to the cleaned areas and let it sit for 20-30 min. The baking soda will stop any corrosive action by any corrosion not removed by the first alcohol cleaning.

Finally, use a dry q-tip to remove as much of the baking soda paste as possible, then finish cleaning any residue with q-tips and alcohol. Again, use the canned air to dry the cleaned area and ensure you don’t get any of the paste blown into the watch.

Take your time in cleaning and pasting — the leaked corrosion tends to be very tenacious, etc. and sometimes difficult to remove. Be aware that you’ll use lots of q-tips as you’ll want fresh ones when they get dirty and ratty.

Ken Simmons
Auburn, MI

Legacy Printer Port (LPT1) Answered 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


Very old Legacy computers used different physical addresses for the LPT port. Try some of the other addresses. The most common ones:

  • Logical parallel port 1: I/O port 0x3BC, IRQ 7 (usually in monochrome graphics adapters)
  • Logical parallel port 2: I/O port 0x378, IRQ 7 (Addon I/O cards or using a controller built into the mainboard)
  • Logical parallel port 3: I/O port 0x278, IRQ 5 (Addon I/O cards or using a controller built into the mainboard)

Note: More modern computers can require interrogating the OS or an interrupt call. Also a device driver may be required.

East Hartford, CT

This is great! The humble printer port lives! Here’s my guess as to what is happening in your situation.

You are using the OUT command and the syntax is OUT Port#, Data where Port# is the address of the parallel port, typically &H3BC, &H378 or &H278. You need to find the address for your particular configuration. In the older machines, the Parallel Port was on a separate plug-in card and there should be jumpers on the card for setting the address.

The reason why the dot matrix printer works is because the print routine in use calls for the data to be sent to LPT1: instead of the port address. The computer’s BIOS translates the LPT1: to your port’s address. You are bypassing this function by going directly to the port via the OUT command. You should be able to go into the PC’s BIOS setup and find the port address assigned to LPT1:, then use that address in your OUT command.

Or open up the PC, find the parallel port setup jumpers and set them so the port address matches the address on your Compaq. Note that if you do change the card’s address, you’ll probably have to go into the BIOS and make the change there too.

May I suggest getting hold of the book Parallel Port Complete by Jan Axelson? A veritable boatload of information, including an example program for identifying what ports are installed in Chapter 4.

Jerry McCarty
Jackson, MI

Do a Google search for partester by Kellyware (freeware). The information displayed and ability to toggle the outputs will give you the troubleshooting information you need. The program was created in support of the CNC program Kcam that uses the parallel port to control these machines.

Steve Benson
New Castle, IN

Depending on your computer, LPT1 may have one of two addresses. LPT1= 03BC hex, DATABYTE 956, STATUSBYTE 957, CONTROLBYTE 958 or LPT1= 0378 hex, DATABYTE 888, STATUSBYTE 889, CONTROLBYTE 890. Whichever one you are using, try the other one.

Dave Hogan
N Palm Beach FL

I would check the printer port settings in the BIOS. The LPT port should be set up for bi-directional communication. “Standard parallel” will not work. ECP should work. Different BIOSs have different options for the port settings. P. S. My dad and my brother were both pharmacists.

Bob Kottas
Omaha, NE

There are 3 types of printer ports for PCs, Standard, EPP and EC. Check out the Wikipedia article at https://en.wikipedia.org/wiki/Parallel_port There may be an issue with LPT1 not being at the address expected, see the Interfaces and Access sections. It may also be possible to use one of the DSR or DTR lines from the serial port to control the SCR.

Philip Martel
Salem, NH

Although the early BASIC language gave instructions for direct access to hardware such as LPT and COM ports, operating system and BIOS variations can interfere and limit direct control.

For an excellent source of parallel-port information, get a copy of the 2006 edition of Jan Axelson’s book, Parallel Port Complete. You’ll find much helpful information therein. Also find guidance here: [url=http://retired.beyondlogic.org/spp/parallel.htm]http://retired.beyondlogic.org/spp/parallel.htm[/url].

Jon Titus
Herriman, UT

Many eons ago I did something similar, using 80286 vintage computers to control a weather station via LPT1. The BASIC routine needed to know the address of the LPT port, which on my machines was always 888 (decimal). I do remember that Compaq machines used a different address, but unfortunately I can’t recall what it was.

I suggest you consider migrating your control system to an Arduino or Raspberry Pi platform. I have done this myself, and never looked back. The new boards are so much smaller, cheaper, less power hungry, more user friendly from the standpoint of hardware control, and there is exhaustive online support.

There is no need to buy a chip programmer or use assembly language. I have not been so excited about electronics since discovering solderless prototpying and integrated circuits 30 years ago.

Steve Turner
Labadie, MO

Measuring Breathing Rate Answered 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


This makes me think of the automotive MAF (Mass Air Flow) sensor.

Essentially it is a small diameter resistive wire suspended in the air duct. A small current flows through the wire, and is warmed up that way. The wire resistance will change (due to the cooling effect) of air flowing over it. So at a specific regulated voltage, and used in a breathing tube, the current will change depending on breathing.

It could possibly even distinguish between breathing in, and out (due to assumption that air going in would be cooler than air going out).

Bill van Dijk

You’ll need a strain gauge band to wrap around your subject’s chest and a detector that’ll take the (very small!) varying voltage generated by the strain gauge and turn it into something useful.

Ken Simmons
Auburn, MI

Amplifying Plant Signals February 2018

I’m trying to duplicate an ESP experiment described in the book “What A Plant Knows,“ by Chamovitz. Basically, I need an amplifier and sensor to read signals from stems and roots. What frequency response does the amp need? My guess is from DC to maybe 20 Hz? If this is the case, then what sort of amp configuration do I need?

Carlos Dixon
Flint, MI


Mr. Dixon desires to amplify electrical signals originated in plant stems and roots. This is a perfect application for an instrumentation amplifier circuit. Instrumentation amplifiers amplify the voltage difference appearing between two sources, neither of which is referenced to ground.

Basic information on instrumentation amplifiers and lots of applications bulletins from IC manufacturers such as Analog Devices, Texas Instruments and Intersil can be found if the phrase “instrumentation amplifier applications” is used as an internet search argument.

Peter A. Goodwin
Rockport, MA

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