We have a customer whose parking lot lights are controlled by the building automation system using input from a light sensor. With the arrival of spring and daylight-saving time, he has noticed that his parking lot lights are already on while it is still daylight. Knowing he is wasting money, he asked us to change the threshold so that the lights come on at dusk. Clearly, the light sensor signal threshold that brings on the parking lot lights in the control program needs to be tweaked. What is the best way to go about doing this? Although we live in a high-tech world, some things are best adjusted based on human perception. Whether the customer’s light sensor is a precision unit calibrated in foot-candles or an inexpensive uncalibrated unit, we suggest the following approach:
- Schedule a meeting with the building owner at the building shortly before dusk to select the light level when he prefers the parking lot lights turn on. If he defers to you, use your “engineering judgment.”
- Prior to the meeting, study the BAS control program to locate the place where the light sensor signal is checked against a threshold value to determine when to turn on the lights.
- Determine which controller AI input is receiving the light sensor signal and be sure you know how to read the light sensor AI input value on the BAS system screen.
- Meet again with the building owner shortly before dusk. Ask him to watch the parking lot and tell you when he wants the lights to come on. When he gives the OK, read the light sensor value on the controller AI input. Modify the control program to use this new value for the light turn-on threshold.
Using this approach, the actual precision of the light sensor is unimportant. The only important thing is that the signal value is repeatable for a given light level. You might find that a different threshold is needed in the morning for light turn-off. If that is the case, repeat the above procedure at dawn and, if necessary, modify your control program to use the two different thresholds depending on whether it is dawn turn-off or dusk turn-on. If the light level sensor ever needs to be replaced and it is a precision unit (calibrated in foot-candles from the factory), you should not need to change the thresholds in the control program. However, if you are using an inexpensive uncalibrated sensor, such as the Kele PSR-1, swapping the sensor will require a recalibration using the above procedure.
