C'mon Baby Light My Fire (Alarm)

Although this sensor doesn't take the place of smoke and fire detectors, it can provide helpful feedback via remote control, when used to monitor spark gaps, RF contactors, AC contactors and even dummy loads.
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(click thumbnail)Fig. 1: Rick Levy 'flicks his Bic' to test Burk's Arc and Flame Detector.No, Rick Levy of Boston's Broadcast Signal Lab is no fire bug — but it sure looked that way to restaurant patrons as he lit the flame of a butane lighter to test out Burk Technology's AFD-1 Arc and Flame Detector.

Rick's company (visit www.broadcastsignallab.com) coordinates a regular luncheon meeting of Boston broadcast engineers. For the demo, we placed Rick and the flame about 20 feet away from the device, shown on a table in Fig. 2. The blue LED lights when an arc or flame is detected. A Fluke DVM was set up connected to the output of the device, sounding an "aural" alarm when it sensed the flame.

Sure enough, the blue LED lit and the DVM sounded almost immediately when Rick lit the lighter. As he walked around the dining area, the AFD-1 sounded each time the flame appeared.

(click thumbnail)Fig. 2: The AFD-1 senses the flame from across the room to trigger an alarm.
Setup of the device is simple, using a five-pin Phoenix connector. A selector switch adjusts sensitivity, to cancel interfering background levels. The device comes with an industrial-strength Velcro-brand hook and fastener square for easy mounting to most clean surfaces, without needing to drill mounting holes.

Although this sensor doesn't take the place of smoke and fire detectors, it can provide helpful feedback via remote control, when used to monitor spark gaps, RF contactors, AC contactors and even dummy loads.

You can get more information by heading to Burk's Web site www.burk.com. The product won a "Cool Stuff" Award last year; you can watch a Coolcast video demo at radioworld.com/coolcasts/archive/2007/.

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I received a number of comments about Harry Bingaman's find of an LED cap courtesy of L.L. Bean.

Edd Monskie, vice president of engineering for Hall Communications, uses one during hunting season, walking in the woods before sunup. So they're not just good for entering a transmitter building during a power failure.

Edd offers another suggestion. If you happen to have a favorite hat, you can use a clip-on light assembly that fits any hat brim. This assembly works the same as the built-in system, but the assembly clips on and off as needed. You can find them at any hunting or fishing supply store, even the outdoor recreation department at Wal-Mart.

* * *

Since Harry Bingaman scored such a home run with the LED cap, I thought I'd include one of his favorite Web sites; and it is intriguing.

Harry's always been fascinated by advancements made in robotics; the video shown at the Boston Dynamics site will blow you away.

Visit this site for an introduction to The Big Dog: tiny.cc/ExtZL.

Isn't electronics great? Even on ice, the Big Dog slipped a little, but never fell down. Can you imagine that computer running overtime to keep the Big Dog upright? And we think we have challenges with computers. Also browse that site for other videos.

Thanks, Harry, for sharing such a neat site. Harry is chief engineer for the Sunbury Broadcasting Corp. in Pennsylvania.

* * *

David Matthews, president of Bradley Broadcast Sales (www.bradleybroadcast.com), offers thoughts on the Aug. 1 Workbench column mentioning a solution for excessive B+ and filament voltages in a prototype tube-type processing amplifier.

To reduce the B+ voltage significantly, change the power supply design from capacitor input to choke input. A supply with a capacitor-input filter will give you a B+ of about 1.4 times the RMS voltage on the secondary winding. A choke-input filter will give you about 0.9 times RMS voltage.

So in this case, (0.9/1.414) = 0.637 of the original 550 volts DC, which brings it down to 350 volts. By doing this, you also gain very tight dynamic regulation of the B+ supply — enough so, that in many tube audio amplifiers you can hear an obvious improvement.

You can also take the capacitor previously being used in front of the choke (or resistor), and parallel it with the one behind the choke, giving you better peak current capability.

Many people forget that the only reason capacitor input filters gained widespread use was simply for cost/size/weight reasons, David says. It meant that you could get higher DC voltage without a bigger transformer, and you could substitute a cheap power resistor for the series choke. This expedient was economical, but not at all the best design choice from an audio quality standpoint.

On the filament supply, you may notice that once the secondaries (both HV and LV) are under actual operating load conditions, the filament voltage may drop quite a bit on its own.

Also, remember that the filament voltage rating is in RMS volts. If your AC voltmeter reads in peak volts, your 6 Volt RMS winding would be expected to measure as (6 x 1.414 = 8.48 volts), so you might already be in the right neighborhood.

If it turns out that you need to drop the filament voltage a bit, you could get two silicon rectifier diodes, wiring the cathode of one to the anode of the other and vice versa, then place that combination in series with the filament line. For each pair of diodes like this, the voltage drops around 0.6V.

Remember to size the diodes for several times the expected filament current (to account for the high current inrush at cold start), and/or make part of your voltage dropping solution a series power resistor.

David wraps up with an important safety note. If you are using the 24V buck/boost solution suggested in the original Workbench article, be aware that the insulation on the 24V winding may not be adequate to safely make this winding part of the primary circuit connected to the AC mains.

One good spike on the AC input, the insulation fails, and you could end up with an unwanted surprise — perhaps line voltage on the frame of the transformer and thus on the cabinet of your amplifier. Not a happy ending.

Thanks, David, for the suggestions and tutorial.

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