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Tech Forum

Tuning The Music November/December 2018

Is there a way to vary the speed of a music CD player? I play my instruments along with the recordings and need to vary the pitch slightly to match the tuning of my instrument. We used to do this easily with vinyl by varying the speed of the turntable.

Klaus Herman
Raleigh, NC

About Resistors November/December 2018

Could someone explain what pull-up and pull-down resistors are, when and how they’re used, and how to calculate their values?

Andy Dietrich
Dallas, TX

Reflow Oven Opinions Needed November/December 2018

Anybody doing SMT reflow soldering at home? Trying to make the switch from TH to SMT. I was wondering what homebrew oven setups people are using and any tips and advice for a newbie.

Joey Dampier
Reston, VA

PC Joystick Interfacing November/December 2018

Is it possible to connect a joystick to a microcontroller? PIC, PICAXE, etc. I don't want to have to take apart and hack the joystick. If so, what is required?

Nick Hulst
Cedar Rapids, IA

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

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

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


Yes, you can use your landline phone as an intercom.

1st option: In our part of Canada, for example, if you dial your own phone number, then hangup when you hear a short “Beep” on the line, the phone company’s automated CO equipment will call you back, and ring all the phones on your line. When you pickup the phone, you will hear the same short “Beeps” on the line, BUT, you can also talk on the line to anyone else in the house/shop, that has also picked up the line.

Try it, it might work in your part of the world too.

2nd option: Most if not all house wiring has 4 conductors in the phone cable, but only 2 wires are used for the actual phone line (usually colour coded Red and Green), the other two (usually black and yellow) are spare. You can use the other two wires as a signalling pair to ring buzzers at other phones.

In the past, I’ve used a low voltage AC source in series with 2 diodes and two buzzers to make a signalling circuit ON THE SPARE PAIR (BLACK AND YELLOW wires).

The AC source was placed at a convenient mid point location, in series with one of the signalling wires (yellow or black). At each phone I installed a diode, in series with a DC buzzer, then placed a momentary push switch across the diode. Note, that the cathodes of each diode must be facing the AC source.

The diodes block any current flow through both buzzers, but if one diode is bypassed by the switch being pushed, the current (now DC) will flow through the both buzzers, signalling attention.

By picking up the phone and dialing any single digit (except 0), you turn off the dial tone, and have about 15 seconds of talk time before a warning tone comes on to signal you do do something, like dial another number. This is as simple as it gets, but there are many more options.

Back in the last century, phone companies didn’t like you messing with their equipment, that probably still applies today.

Using Google you can find many articles on using old style phones as intercoms (

A phone works just fine on 12 to 18 volts DC (my magneto phone operates on two 1.5 volt batteries), but then, you MUST, be disconnected from the phone company’s service.

Yar Nirodac
Vancouver, BC

You could use existing phones but you’d need a way to generate a ring signal. Hand-crank ring generators from old phones are for sale on eBay, but an electronic intercom costs less per station. I suggest you use your phone wires for a commercial wired intercom, or try a wireless unit instead.

Jon Titus
Herriman, UT

After disconnecting the line from the telco at the demarc (box most likely on the side of your house), you can do it with a telephone line simulator but for the same money you can just buy a cordless phone system with intercom built in.

As a bonus you can buy models that have bluetooth so you can connect two cellphones and have them ring throughout the house while they sit in their chargers.

Bruce Robin

Unless you are willing to set up your own switchboard and 24 volt power supply, you cannot reuse the phone for your intercom. It is perfectly possible, however, to use the wiring to connect your own wired intercoms which I have done for years. You should have 2 pairs in your home’s phone lines. YES, you must disconnect the pair you are repurposing from the phone system. You can leave one pair connected to your existing phones as a backup.

M. Herman

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


If you can ensure that the power is OFF (e.g., the main fuse or circuit breaker is open) you could just connect a battery at the end of the power line to be traced, and use a buzzer or test lamp to discover the power line(s) that connect to it.

But... It’s so much simpler (and a lot safer) to just buy a wiring circuit tracer. The device consists of two parts — a radio-frequency (RF) generator that is clipped to one end of the wiring to be traced, and a second instrument that will sniff out the wiring that carries the RF test signal.

Check the Internet for recommendations and merchants.

Peter A. Goodwin
Rockport, MA

The two main ways to trace wires is by inductive or capacitive pickup and by using a metal detector.

If the line is active and carries AC current, then an open inductor (coil of wire) connected to a simple audio amplifier and headphones or speaker will create an audible 50 or 60 Hz buzz near the current-carrying wires. A bulb with a blinker placed as a load on a circuit helps identify a particular line. There are also high-frequency modulated loads to help tracing, e.g.

Capacitive sensors, e.g., can detect energized lines with no current flowing, which may be useful to identify an unused line inside a wall.

Finally, metal detectors may help to trace wiring inside metallic conduit.

DrM Pippik
via internet

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

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