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When Your Silence Sensor … Doesn’t

Paul Lyons takes us to the pilot, building a simple circuit to solve a problem

Fig. 1: The 19 kHz pilot appeared at the receiver’s audio output. Loss of audio on your FM stereo station fails to trip your silence sensor. What’s going on?

Clear Channel Dayton Senior Broadcast Technician Paul “Hitchhiker” Lyons encountered this interesting problem. Yet when he unhooked the tuner audio from the silence sensor input, the device tripped normally.

It turns out that the tuner Paul was using to feed the sensor was passing the 19 kHz stereo pilot through to the audio output, seen in Fig 1. This caused the silence sense to remain untripped even when program audio disappeared.

Paul came up with a simple fix: a 19 kHz notch filter, paralleled with the audio output. The schematic is shown in Fig. 2.

He chose to build the circuit on a perfboard and mount it inside the tuner, but the filter could just as easily be placed in line, external to the tuner.

The schematic shows values for the coil and capacitor, and is, of course, only one channel. If the silence sensor you use has a stereo input, you’ll need one for each channel. Paul suggests using a slug­tuned coil for fine-tuning of the circuit.

The oscilloscope shows about a 60 percent reduction in the level of the pilot signal at the audio output of the tuner. After installation, the silence sensor worked perfectly. Fig. 4 shows the reduction in pilot level.

Paul has been involved with broadcasting since he was a teenager, following in his dad’s footsteps. His father was an electronic technician and chief engineer.

* * *

Fig. 2: A 19 kHz notch filter schematic. You may recall a suggestion from Frank Hertel about panel labeling in my March 1 column (“Your Panel Lettering Made Easy”).

Engineer Leon Amstutz, CBRTE, offers a method that he has found to be simpler, with less mess, than gluing paper-based labels to equipment.

A couple years ago, Leon ran across a product from Avery-Dennison Office Products. Avery #6575 is a package of 50 sheets of 8.5-by-11-inch white durable multipurpose labels for laser printers. These full-page labels can be laid out and printed, as Frank mentioned. Leon, too, uses the OpenOffice/LibreOffice draw program for best results.

These Avery labels have a peel-away self-adhesive backing, which eliminates the mess of glue. Another benefit is that it incorporates “TrueBlock” technology, which appears to be a thin layer of foil embedded within the label. This helps mask what the label is covering, so that previous printing or markings on the surface to which the label is applied will not bleed through.

Fig. 3: Paul built the circuit on a perfboard and mounted it inside the receiver.

Finally, the label is specially coated to hold the laser toner well, and resists rubbing off — hence the “durable” aspect of the product’s name. It is resistant to scuffs, smudges and moisture, so it doesn’t need to be coated with varnish or clear coat.

Leon has used this on everything from mobile remote broadcast rack cabinets with the station logo to property ID labels on tools and parts bins. The labels hold up well, even for areas that are printed with a solid dark color on the white label surface.

Fig. 4: With pilot level greatly reduced, the silence sensor works perfectly. The labels cost about $1 per sheet, but Leon finds that he can usually combine several jobs on one page, and even use the “waste” areas to print small wire identification numbers or terminal labels. Avery also makes this product in pre-cut smaller size labels, if you don’t want to cut them from a whole sheet of paper.

Check your office supply vendor for these labels, or visit, keyword 6575.

(I fell in love with Avery years ago, when they started making an adhesive file folder type label that could be used to label audio tape cartridges. What made these labels so great was the label held secure to the cartridge plastic yet could be removed easily, putting an end to the gummy paper and glue residue caused by the typical file folder-type label.)

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