Brigadoon - An Occasional Corner on the Internet
Arduino Based Weather Station
Designed by Mark Little
"ATMega328/P Datasheet Complete" by Atmel
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Interfacing with the Tipping Bucket Sensor
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It is possible to use a wide range of sensors to detect the tipping of the bucket, but the many Tipping Bucket Sensors use a reed switch activated by a magnet as the bucket tips from one side to the other. This is used to create a pulse that can be used to trigger the interrupt to count the number of bucket tips.

The simplest solution is to force the trigger pin to use an internal pull-up and use the reed switch of the tipping bucket sensor to pull the pin to ground. The risk with this scheme is that there may be a long run from the sensor to the Arduino and that electrical noise induced into the long cable run can result in false triggers.

Some early commercial home weather stations which used a long lead from the rain gauge sensor to the control unit were susceptible to this kind on interference and uses tried all sorts of filters to reduce the problems. Some of these included wrapping the lead around a toroid, or including a low pass filter on the lead. The modern alternative is to have a microprocessor in the sensor head and/or use a transmitter to send the rainfall information back to the main unit by radio.

In this case of this weather station design, the Arduino will be located adjacent to the rain gauge, so the external resistor is not actually required as the Arduino input pin will be set to use the internal pull-up.
When the magnet is close to the reed switch (as it passes through the centre point of its tipping cycle), the switch will close, taking the level on Pin 2 (INT0) low, causing an interrupt. This interrupt will cause the rainfall counter to increment.