Yard lamp post photocell

[Becmar]

I have a client who leaves the low voltage system on until midnight or so. At that point, they want their front yard lamp post to kick in to retain some lighting until morning.

The client installed a photocell not more than 18" from a wall wash shell light and at the same altitude as the fixture. The fixture has the 20-watt maximum T3 lamp. The candle power is insufficient to kill the yard lamp. The photocell works. Come daytime, the lamp goes off. Placing the fixture any closer will yield an unacceptable hot spot of light in that location.

Are there photocells that require less candle power? Is there any other control capability for this application besides a timer on the yard lamp? I could not find any historical information on this website quite like what I am seeking.

Thank you for your input.

Bruce
BEC-MAR

[sitelights]

First look at the timer. Certain types, such as the Grasslin FM/1 Series which is mechanical or the FM/1 digi 20/42 Series digital (they are all interchangeable) are SPDT i.e. Single Pole Double Throw switches. In effect there are two throughputs (throws) one to the transformer allowing NO/NC (Normally Open/Normally Closed) which reverse their state at the moment the timer switch trips. If there is a photocontrol in series with the transformer the hot (120v) output shifts from terminal 5 to terminal 4 in the timer; this turns OFF the p/c which turns OFF the transformer. The throughput from number 4 then turns ON the post light which then stays ON all night until the p/c for the post lamp turns the light OFF.

We have done this a few times to achieve exactly what your client is asking you to do. Any timer with a SPDT switch will work; the NO/NC terminals must be identified. If you don't have the install/spec sheet trial and error will also work.

The SPDT timer will have five numbered terminals: 1 hot, 2 neutral, 3 switched, 4 NO and 5 NC. When number 3 opens or closes, 4 closes and opens. The timer output 5 turns the transformer p/c ON and OFF; the output 4 turns the post light ON (!) when the transformer turns OFF (!).

Using a landscape light beam aimed at a p/c* really does the control job while providing a more elegant (simple) and free solution. Since this won't work for you the alternative is to utilize a SPDT timer..and its attendant costs.

Of course, one must always do a cost/benefit analysis factored by determining the energy cost of running the post lamp dusk-to-dawn and having the low voltage lights operate dusk to midnight (or some other setting). This analysis offers a "goods" vs. simply "good" comparison: you can pay for the goods (energy) or you can do some good.

Dusk to dawn is about 4000 hours a year (it differs by latitude) so if a 100w (for example) lamp in the post light operates an average of 11 hours per night the d/d cost will be the local price per kWhr. At $0.15/kWhr the annual cost of the 100w post light d/d would be about $60.00/year; the midnight to dawn cost about $30.00/year.

The $30.00/year in energy cost could be avoided but the cost of avoiding the cost might be $150.00 thus it would not "payout" until the 6th year.

All energy savings are valuable beyond their market price (here comes the sermon) but that "conservation" is an altruistic concept. Let your conscience be your guide. Convincing your client to make the best choice benefits you: your client not only thinks you are a wizard, they have also saved money.

Questions?


*Yes, photocells/controls are available that have widely differing thresholds. A standard p/c does not provide much exactitude; in a sense they each have different "personalities". One must enter the realm of scientific instruments to do it perfectly rather than to just believe a manufacturer's specs (caveat emptor). A very high quality p/c will not achieve the mathematically precise control afforded by an astronomical clock: this instrument operates independent of "real" (clock) time; its base rests on the rock of sidereal (sun angle) time.

[Becmar]

Thank you. But I must admit, I am trying to decipher some of your reply in layman's terms.

Also, the cost savings in electricity is somewhat moot in this case. They simply do not wish to have the yard lamp competing with their low voltage lighting or else they could just go dusk to dawn with it. The energy savings by limiting it to midnight to dawn would be icing on the cake. I will research further to enable me to implement your ideas. Thanks again for your quick response.

[sitelights]

The photocontrol on the post light may be in addition to an interior switch; this is often the case when the p/c has been fitted for owner convenience. The existing switch remains in the ON position. A switchbox timer (fits in place of a normal interior switch) in place of the existing switch can be programmed for night operation after the low voltage lights switch OFF.