3.13 Part Night Controls

Many Off-roadway applications do not need the lights ON all night. Parking lots, race tracks, back yard security lights, and anywhere light trespass is an issue, is a good place for a timer or Part Night Control. Further, there are organizations like the IDA (International Dark Sky Association) that want sky glow reduced or eliminated during the last half of the night. Two basic "Part Night" concepts exist. One is simple and not very desirable; the other is more complicated but is preferred.

1) Over the years, almost all photocontrol suppliers have made or prototyped a fixed delay control. For example, ON at dusk, OFF four (or maybe 6) hours later. On average, this is OK. Except in early winter, when sunset is 4:00 PM and the lights go out at 8:00 PM when people are still Christmas shopping. Unless you are south of the equator, in the early summer, when sundown is 10:00 PM, the lights on a fixed timer control would not go out until 2:00 AM. Like most compromises, a fixed delay OFF control doesn't work too well. But it's cheap and simple.

2) With modern electronic technology, true half night controls are now possible at a low cost. In simplest terms, an electronic timer measures the duration of the first night, the value is divided in half and becomes the length of time that the light stays ON after dusk. Every night thereafter, the circuit again measures the night and resets the ON duration. It continuously updates itself. If there is a power failure, the controls become a standard "all night" control for the first night and then resumes its operation as a normal half night control. Special circuitry in the control's "brain" allow a quick field test without disrupting the timing calculations. The obvious advantage to this electronic control over a conventional timer is that it resets automatically, never needs adjustment and is the same size and shape as a standard control. The DTL preferred model is DPN124-2.6-TMGN.

What a good Part Night control should do:
Ø Turn-ON at dusk.
Ø Turn-OFF part way through the night.
Ø Be interchangeable with an ANSI locking type control.
Ø Be affordable.
Ø Last over 10 years

What is affordable? It depends on your savings. Assume a typical 175 watt mercury lamp with a 25 watt ballast loss. Also assume that energy cost is $0.08/kwhr. Finally, assuming a standard photocontrol provides 4200 annual burning hours, 2100 hours would be half of the full night.

(175W+25W/1000w/kW) X $0.08/kwhr X 2100 hours = $33.60 /year saved

Energy savings are much greater as luminaire size goes up. $33.60 is for a 175 watt fixture.

Metal halide users take note: When used for car lots, large wattage metal halide fixtures are a natural for Part Night controls. With rated 12,000 hour lamp life, metal halide life can be increased by using part night controls. Leave some HPS on all night for security, put the metal halide "business hour" fixtures on Part Night controls. Energy and re-lamp cost are saved.