The lightning detector connects to a 9-pin serial port via a 10 foot fiber optic cable; the fiber optic cable serves as electrical isolation in case the detector is directly hit by lightning. When the detector sees the electromagnetic pulse from a lightning strike, it connects an output (pin 7) on the serial port to one of the inputs (pin 1). According to the manual, this pulse is only 2 ms long. To catch this, we set pin 7 high and poll the serial port every 1 ms looking for a signal on pin 1.
The serial port interface can be tested by pressing the test button on the light transmitter at the opposite end of the fiber optic cable. The 9-volt battery inside the transmitter is only used when lightning is detected; thus it should last as long as the normal shelf-life of the battery (about two years).
Information on the lightning detector (model LSU-2002) is available online from www.stormwise.com; there is also a manual that comes with the detector.
The sound card is a four-channel Midiman DMAN 2044. Since Windows limits audio devices to having two channels, the Midiman card is treated as two separate devices. Audio buffers for the two devices are created side by side so that the data can easily be sent to the analysis module as a single buffer.
The microphones are $2.00 electret capsules bought from Digikey (Panasonic WM-61A). This particular microphone was selected because of its low price and high sensitivity compared to similar microphones (sensitivity = 18mV/Pa). The signal to noise ratio is >62dB. Each microphone is connected via a simple resistor/capacitor network to a $150 Rane preamp which supplies 48V phantom power. (Phantom power refers to power supplied over the same wires that carry the signal).
The wire between the microphone and the preamp is very short (<2 feet) to reduce noise. Each unit is surrounded by a plastic bag for weatherproofing; this plastic bag is surrounded by an outer layer of foam padding, Lycra, and other fabric to reduce the effect of the sounds created when rain drops hit the recording unit.
Cables connect each preamp output to the sound card inside the acquisition server PC. Ideally, we would like to have optical isolation somewhere between the microphones and the PC.
The locations of the microphones are needed for the analysis module to compute the direction of a lightning bolt; hence, this information is measured and recorded in a configuration file on the acquisition server. (See the System Setup section of the user manual for further details.)
author(s): Adam Seeger,
Eric D. Baker
last update: 05 May 1999 4:41p by Eric D. Baker