Field Report: FM Services TLM-1

March 1, 2009

It was inevitable, with the changes in technology over the past years, that items started shifting to a microprocessor-based design. From studio automation systems to remote control units and transmitters, those pesky, rectangular ICs with all the leads cropped up everywhere. I could hardly believe my eyes when I saw an ad for the FM Services TLM-1, a microprocessor-controlled tower light monitor. What really caught my attention was the claim that multiple beacons and sidelights could be monitored by sensing one wire. Over the years I have used other types that required each beacon level and each sidelight level to be monitored by a separate wire going up the tower to that level; a condition that didn't always exist either by design or tower wiring changes over the years. I have built several of my own basic design tower light monitors using a toroid core to sense the current and then rectify and smooth the flash transitions for a remote control to sense. However, the idea of being able to monitor the whole tower with a single current sense was intriguing.

Performance at a glance
Senses current on power supply lines
Multiple alarm outputs
Opto-isolater alarm and status outputs
Remote alarm reset
Single-phase or three-phase monitoring

With the TLM-1, installation is very easy and straightforward if your system uses single-phase power and either 120V or 240V supply. With a 120V feed system, the hot wire from the breaker panel is fed through the current-sense transformer so the total current — beacon and sidelights — is sampled by the TLM-1. In the case of a 240V feed, where two hot wires are used to feed the tower light system, both are fed through the current sense transformer, but one lead is inserted in a reverse direction so the currents, which are 180 degrees out of phase, will be additive through the current transformer. Sampling of a tower light system fed by three-phase power will require three of the TLM-1 units. Single-phase power was used in both locations where this unit was tested.

FM Services

Once the electrical feed to the tower light system is fed through the current sense transformer, with the lights all operational in a normal mode, the unit is calibrated by pressing a button. This tells it what normal current flow is for beacons, sidelights, and even if you have a steady burning bulb in the tower light circuit. This is a handy feature if you leave a light on constantly for humidity control. It is now ready to provide status alarms for changes it senses as abnormal operation.

System status

The TLM-1's status outputs indicate problems:

  • Photocell circuit failure
    If the status of the lights does not change in 20 hours, it is assumed there may be a problem with the photocell circuit. Instances where the tower lights are on 24 hours each day, this function can be disabled by a dip switch.
  • Flasher failure
    If beacon current is constantly on or off, or if the flash rate and duration are not according to FAA specs, then this status will alarm. Note: the beacon on vs. off time is monitored for FAA specifications.
  • Beacon failure
    Activated if a beacon bulb fails or flasher is in constant off state.
  • Marker failure
    Activates with a failure of any sidelight bulb.
  • Summary Alarm
    Any of the four above alarm conditions causes a summary alarm. This alarm may be reset locally or remotely and the unit is ready to monitor for an additional lamp or condition to fail.

  • More indicators

    The status outputs can be programmed to either high level or low level on alarm. However, I noticed that no alarm indication will be present if all power is lost to the system, unless the unit is programmed for alarm condition to be active high status output. An example would be if the site was without primary electrical power. Assuming the remote control system was on a UPS and could alert the operator with a status change to open for alarm when a power failure occurs, the site alarm will be sent. The summary alarm channel could be monitored by the remote control, and each of the status outputs can be monitored to determine the reason for a summary alarm.

    A feature I like about the TLM-1 is the ease in which the unit can be reset for a change in the “normal” status. For example, assume one of the beacon lights has a failure of one bulb. This will trigger an alarm so the remote operator knows an abnormal situation exists. When a visual check confirms that all beacon levels are still flashing, the unit may be reset and it will be ready to alarm if an additional bulb, either a beacon or sidelight, goes out. This allows time to schedule a tower crew for relamping, but still be fully compliant in monitoring for additional outages.

    I first evaluated this unit at an FM tower running analog and digital transmitters. The site had one side light bulb outage and also one beacon bulb outage, so it allowed me to evaluate each condition. The unit found these discrepancies within seconds of having calibrated for normal current operation. The second evaluation site was a directional AM, with transmitter power of 10kW at 750kHz. I've previously had sensor problems at this site with other brands of sensors due to the high RF levels. The TLM-1 operated flawlessly at this site also, although the only simulation I made this time was the loss of one sidelight. I'm sure the loss of a single beacon lamp would have also been detected. Initially, I was concerned since the unit is shipped from the factory in a plastic housing, but the circuit seems to be immune to AM radiation, at least to the level present at our 10kW transmitter site.

    The status outputs and reset command input are all fed through opto-isolators, a good design feature. Personally, I would install the unit with ferrite beads on all wires connecting to the unit, but for this evaluation no RF suppression was used, and the unit performed well. The TLM-1 is manufactured by FM Services in Wilkesboro, NC, and questions about installation and operation were readily answered by telephone. I would recommend you consider this when the need for a tower light monitor arises, especially if you have a tower light circuit where a separate wire is not available for each sidelight and beacon level.

    Mintzmyer is the president of The Praise Network, with AM/FM stations in Nebraska, Kansas, Colorado and South Dakota.

    Editor's note: Field Reports are an exclusive Radio magazine feature for radio broadcasters. Each report is prepared by well-qualified staff at a radio station, production facility or consulting company.

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