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Tech Forum Questions

Wireless Data To PC September/October 2018

I have a number of wireless temperature/humidity sensors made by Oregon Scientific for use as weather station sensors. I would like to use them for a data acquisition and monitoring application. Does anyone know what frequency and mode they operate on and how I might use them to send data to a PC to record seasonal trends? Would I need a microcontroller to interpret the output or could it be read directly by the PC and then logged and displayed using software such as MakerPlot?

Jai Hooley
Edmonton, AB

Something Strange With The Range September/October 2018

The effective range of the key FOB for my Ford F150 seems to have decreased over recent months. While it used to work from distances of 75-100 yards easy, now I need to be nearly right in front (10-20 yards) of my vehicle to operate the systems. I changed to a fresh battery, but no difference. Is it likely there is an antenna problem? If so, where is it located and is there a test procedure?

Les Waldroup
Charlotte, NC

Resonator Frequency September/October 2018

I have in my junkbox what I believe to be ceramic resonators. What is the best way to test them to determine their frequency?

Donald Frazier
Geneva, IL

Scoping Out Some Advise September/October 2018

Certain techs and engineers still have a need to sample and view composite/NTSC video in its three axes form, that is: X axis (horizontal), Y axis (vertical), and Z axis (intensity).

In the past, this was accomplished using a CRT o’scope: horiz sweep to H input; vert sweep to V input; and video to Z, or intensity modulation input. Since CRT scopes are bulky, heavy, and, in most cases, not battery powered, a PC/digital scope with capture capability would be very handy for field use.

None of the scope ads I’ve seen list these features. Can anyone make some recommendations?

Paul Dendrenos
Barstow, CA

PCBs With PTHs July/August 2018

Is there a hobbyist method for making circuit boards with plated through holes at home? Also curious by what is meant by multilayer boards. I'm just getting started, so forgive if this is an obvious newbie question.

Leonelo Márquez
Maplewood, MN

Show Me The Power July/August 2018

When renovating houses as a hobby, I need to locate powerlines inside the walls and also underground. What curcuit can I build that would be useful? Also, what is the theory of how such a detector would work?

Denzel Meier
Winnsboro, SC

Phone Line Intercom July/August 2018

We use our cell phones as our main phone service, so our “land line” is no longer being used. Is there a way to use the house phone wiring along with the old phones as a whole house intercom system? I have a detached garage/workshop away from the house where this would be especially useful, not to mention almost every room is wired. Would I need to disconnect from the phone company "grid" to do this?

Jay Bousquet
Lexington, NC

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

Smooth LEDs? July/August 2018

What makes some LED replacement bulbs dimmable, while others are not? Some of the replacements I have purchased do not dim very smoothly. They seem to dim in steps and then just turn off before lowering to where I need them to be. The previous incandesants dimmed much more smoothly down to a soft glow. Is there a different, (maybe more expensive) type/technology that  would more closely emulate the incandesants? I'm using an X10 controller for the dimming, could this be the problem?

Everett Barham
Elvaston, IL


There is no such thing as a dimmable LED. The way we get around this is by using Pulse Width Modulation (PWM), which means the LED is turned on for shorter and shorter periods of time within a time frame. You can see that if the LED is turned on for 100% of the time, say 1/100th of a second, then it is on for 10,000 1/10,000ths of a second, the LED will be only 1/100th as "bright" if it is turned down to being on only 1/10,000th of a second for every 1/100th of a second.

I used 1/100th of a second as the time frame because the human eye can't see flickering when the flicker rate is around 1/50th of a second, this is why old analog TV's in the US used 1/60th of a second to build a picture frame, and in Europe they used 1/50th of a second for their TVs.

You can also think about this using a 1 second time frame if you want. In this case when the LED is on only 1/100th of a second for every second of time frame it would appear only 1/100th as bright. The problem here is that we would see the LED turning on and off, so we would lose the appearance of the LED dimming. We have to use a time frame that the human eye can't "see" such as a time frame of 0.01 (1/100th) seconds.

Phil Karras, KE3FL
via email

A LED is a beautiful dimmable device; lower the current, and the output brightness goes down. This can be done through simple resistors, or through PWM (Pulse Width Modulation). That is the good news.

In order to use a LED as a house incandescent lamp replacement, something needs to be done to make that possible. First of all, the voltage is WAY to high, it is AC (the LED prefers DC), and we need to feed it the correct current. So a power supply module is included to convert the 120 (or 220) Volt AC to some small DC current appropriate for the LED used.

Herein lies the problem. This PS module is designed to deliver x mA to the LED more or less regardless of the input. When you adjust the input, the PS module tries to compensate, to the point where it no longer can, often resulting in a flickering light. These are the now non-dimmable LED bulbs.

To make this “house” LED bulb dimmable, a radically different PS module must be built. It must sense that the input has changed (due to the dimmer setting), and adjust the LED current accordingly. Not easy to do! and of course, more expensive.

Several designs have been marketed, some better than others. In most cases, even with a “dimmable” LED bulb, a special dimmer is required. It is a mess, and most still don’t work perfectly.

Bill van Dijk
Carp, Ontario

I can’t answer specifically what is different about dimmable LED lamps, but will note that the type of dimmer is important. You need to use a dimmer specifically designed for LED lamps.

I have had good luck with Lutron CL line of dimmers that are made specifically for LED lamps. These are not X10, I have not seen any X10 dimmers for LED lamps. Also note that X10 light controls generally only work for incandescent lamps, they need a bit of current flowing through the filament to work correctly.

For LED lamps I have used the relay based X10 switch WS13A with good success, it also works with CFLs.

Dan Koellen AI6XG

Different bulbs behave differently so you have try some options. There are some test results on the X10 Forum that you might find useful. X10 dimmers dim in 16 levels so some bulbs may not dim smoothly.

Bruce Robin

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:

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

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