Measuring Nocturnal Light

An optional feature for the BatLogger II

I developed a MoonLight sensor for the HOBO logger back in 2002, and find that it is interesting to correlate bat activity and light levels. So, when I designed the logger mounting plate for the BatLogger II, I provided for an optional nocturnal light sensor.

The sensor requires only two parts, and is powered by the HOBO logger, so doesn't add to the overall power requirements of the bat activity logging system.

Below is a plot sequence made with a MoonLight sensor. Data from 5 consecutive nights have been merged to display the relative shifts in time and light intensity as the moon phase passes from waxing, to full, and then waning. The night of the 28th shows a particularly variable response as the moon was repeatedly obscured by passing clouds.

The long descender at the end of the plot for 2/25 was created by wrapping the logger and sensor in a thick black blanket, and putting it in a dark closet for 10 minutes. This essentially provided a reference point that represented "absolute" darkness.

The schematic diagram of the MoonLight sensor is shown to the left. Note that it is a very simple device to build, as long as you use the correct components. The sensor circuit was developed specifically to work with an Onset Hobo logger.

The circuit derives power from the logger via the red wire from the logger voltage cable. The white wire is the sensor voltage tracked by the logger, and the black wire is the logger's reference ground.

The photo cell I was originally using was a Clairex part number CL705HL, which was discontinued in 2003. The photocell I am currently using is the Silonex NSL5540. Only these two CdS cells are recommended for use as a MoonLight sensor element.

The value of Rs in the circuit determines the sensitivity and scale of the sensor. A value of 10 Megohms provides characteristics comparable to the plots shown above, and generally tends to be the best value.

NOTE: The value of Rs should always be greater than 5K to prevent damage to the HOBO logger.

You should understand that MoonLight sensors may vary somewhat in sensitivity from one to the next, due to the manufacturing tolerances of the photocells. So you will need to establish the sensitivity range for your particular sensor based on a little experimentation. If you are monitoring nocturnal light over a long period of time, you will be able to establish a full moon "calibration" reference value for the sensor. From that point, you can relate the illumination data to the full moon reference level.

Tony Messina - Las Vegas, Nevada - email:

Information on the BatLogger II was first published on this page in May, 2010 ... last updated November, 2010