Tech Docs - Sine Systems, Inc.

The FCC requires tower obstruction lighting to be checked once each day for proper operation and written record to be kept of this check. The FAA must be notified when a problem is detected and again when repairs are completed. The Sine Systems AC Current Monitor provides a means to accomplish this in conjunction with a remote control system such as the Sine Systems RFC-1.

Because of the potentially serious consequences of obstruction lighting failure on a tower, some stations want to go beyond the FCC requirement and provide an automatic means to continuously monitor the obstruction lighting and to be notified immediately of a failure. However, the lighting is not normally used during daylight hours and the monitoring system does not usually know that this is a normal condition.

One way to work around this problem when using the RFC-1 Dial-Up Remote Facilities Controller is to "lock out" the tower lights as a telemetry alarm source during daylight hours using the internal clock. For example, the tower lights could be inhibited from issuing an alarm from 5:00 am to 8:00 pm each day. If the lights were not on at other times, an alarm would be issued.

The problem with this approach is that for the time lockout to allow for the daylight hours during the summer months, the lights are not monitored for several hours of darkness during the winter. This effect is more pronounced at the more northern longitudes.

Another way to monitor for obstruction lighting failure only at night is to automatically disable the ACM-1 telemetry during daylight with a photosensor circuit. The circuit shown below can accomplish this and costs less than $25 to build.

The idea behind this circuit is to force a DC voltage at the input of the RP-8 telemetry input terminals during daylight to prevent the telemetry alarm from occurring. At night, the forced voltage is removed and the voltage from the ACM-1 is monitored. This circuit is "failsafe" in the sense that if power is disconnected, the ACM-1 will not be disabled.

The entire circuit can be built on a small piece of pre-punched PC board. The light sensor (Q1) should be pointed toward the sky. The southern sky is preferable and light sources such as street lights or building lights should be avoided. A light shield can be placed around the sensor if necessary. A short length of heat-shrinkable tubing works fine. The sensor can be located away from the rest of the circuit but since it is a high impedance device and potentially sensitive to RF, it is usually better to keep the sensor and the circuit together. Since the circuit is small, this is typically not a problem.

If nothing more elegant is available to weatherproof the circuit, an inverted glass baby food jar can be used with a dab of RTV (silicon seal) to seal the hole in the lid where the cable passes through it. A foil-shielded cable such as Belden 8451 is recommended. D2 is an optional LED that illuminates during daylight.

The sensitivity of the photosensor is controlled by R1. The value of 33K is typical but it might need to be adjusted. Lower values of resistance decrease the sensitivity. The LED (D2) should indicate daylight at least up until the time the tower lights are turned on (by their photoelectric control). In any event, do not make R1 lower than 1.5K.

After the photosensor is installed, the telemetry for the tower lights will read normally at night (usually 100.0%) and will read a much higher value during the day--usually full scale. This may be 204.0% or "Status: On" depending on how the telemetry channel is setup on the RFC-1. With this arrangement it is generally best to use Trigger Rule 7 for the Telemetry Alarm System. Set the upper limit to 9999 and the lower limit to whatever is desired for an alarm point; 0900, for example, if an alarm is desired when the telemetry falls below 90.0%.