Keep an Eye on the Heat at Your Site

Add this inexpensive temperature sensor to your remote control
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Our Radio World colleague Cris Alexander is, by day, Crawford Broadcasting director of engineering. He encourages his group’s engineers to share tips that have made their work lives easier and more efficient; over the years we’ve included several in this column.

At KCBC(AM) in Modesto, Calif., Chief Engineer Steve Minshall writes about the familiar task of monitoring transmitter parameters, antenna parameters and tower lighting via remote control systems. One item easy to overlook is the measurement of temperatures in the building.

Fig. 1 provides a simple, inexpensive way to monitor temperatures. Simply take 5 volts, usually available from the remote control system, and connect it to a thermistor. The other end of the thermistor connects to a metering channel on the remote control system.

Fig. 1: A simple thermistor circuit can provide temperature monitoring for your remote control.

Fig. 1: A simple thermistor circuit can provide temperature monitoring for your remote control.

The correct thermistor must be chosen, and it needs to have a negative temperature coefficient. This means that the resistance of the thermistor decreases with increases in temperature. As can be seen from Fig. 1, the thermistor and the internal load resistor of the remote control system create a voltage divider. When the temperature increases, the voltage applied to the remote control metering circuit also increases.

Steve chose a thermistor from Mouser. It’s their part number 527-NK123C5R2. Using this device along with the assumed 10K ohm internal load resistance of the remote control metering circuit (use an ohmmeter to check the input resistance of your remote control channel), one can expect acceptable performance for the purposes needed.

If the remote control system is calibrated at 88 degrees Fahrenheit with this thermistor, you can expect the temperature reading to indicate 36 degrees for an actual temperature of 32, and an indicated 116 degrees for an actual temperature of 120. This is plenty accurate for what we normally need when monitoring building temperature. If your remote control system permits an upper limit alarm, so much the better — you’ve got a warning of fan or air conditioner cooling failure.

For the purist, a 47K resistor can be placed across the input of the metering channel (assuming an internal load resistance of 10K ohms), which will bring the temperature error at the above extremes to within one degree. The target load resistance for this thermistor is about 8,000 ohms.

The thermistor is so small that it can be placed directly onto a 37-pin sub-miniature D connector, or placed on a terminal strip. The thermistor costs about a dollar. At this price, several could be placed at different locations in the building such as the transmitter exhaust air outlet, air conditioner ductwork, and even the outside of the stucture, though Steve has found that one for the building is sufficient. Steve has installed these at every site that he takes care of, and they have been 100-percent reliable. On a number of occasions, these little devices have given Steve early warning of temperature excursions, which could have caused major problems.

For those who like to do their own research and experimentation, Steve suggests a couple of websites: www.electronics-tutorials.ws/io/thermistors.html as well as www.electro-tech-online.com/tools/thermistor-resistance-calculator.php.

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So much good information can be gleaned from attending Society of Broadcast Engineers meetings. However, many engineers say they are just too busy to attend. If you’re a ham, you might want to consider checking out the SBE VHF/UHF Chapter 73 of the Air HAMnet, coordinated by Jack Roland.

Usually scheduled every Monday at 9 p.m. Eastern, the SBE VHF/UHF HAMnet is based in Denver on 449.625, pl 141.3 and 449.450 pl 103.5. The chapter meeting is also found on the ALLSTAR node 46079 and connected to the world via ECHOLINK KD0WHB-L node 985839.

See the latest edition of “The KE0VH Hamshack” for more information at www.ke0vh.com.

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Continuing our series exploring how engineers caught the radio bug: Allen Branch was 12 years old when he wanted to go see a radio station. When he called WKLS(FM) in Atlanta, the operator on duty encouraged Allen to come out. Allen’s aunt drove him out to a hill in Marietta, where there was a building housing the control room.

The control room featured tape machines and an automation system for the FM station, as well as a second automation set up for the background music SCA service.

What fascinated Allen the most was that everything was custom-built by the engineer. It was a sight to see, and led Allen to a career servicing radio stations, which he’s been doing since 1993.

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With more and more electronic parts stores like Radio Shack closing, you may wonder where you can find those little connector adaptors and utility items like colorful electrical tape, hook and loop fasteners, or heat shrink tubing.

Enter American Recorder Technologies, sold through Broadcasters General Store.

BGS has a multi-page catalog chock full of the items you used to find at your local electronics store. Two pages of the catalog are shown in Fig. 2. Call BGS at (352) 622-7700 for your copy of this reference guide.

Fig. 2: An assortment of American Recorders adaptors as seen in the BGS catalog.

Fig. 2: An assortment of American Recorders adaptors as seen in the BGS catalog.

Contribute to Workbench. You’ll help fellow engineers and qualify for SBE recertification credit. Send Workbench tips and high-resolution photos to johnpbisset@gmail.com. Fax to (603) 472-4944.

Author John Bisset has spent 48 years in the broadcasting industry and is still learning. He handles western U.S. radio sales for the Telos Alliance. He is SBE certified and is a past recipient of the SBE’s Educator of the Year Award.

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