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Grove One-Wire Wind Speed and Rainfall Sensor

2014-11-10 00:43  
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The DS2423 1-Wire® RAM with Counters is a fully static, read/write memory for battery operation in a low-cost, six-lead TSOC, surface-mount package. The memory is organized as 16 pages of 256 bits each. In addition, the device has four counters, two of them with external trigger inputs called A and B. Each of the counters is associated with a memory page. A counter without external trigger input increments each time data is written to the page it is associated with (write cycle counter). The counters triggered by inputs A and B, respectively, increment with every low-going pulse on their input. All counters are readonly. They are automatically cleared to 0 when the battery is connected. The battery-backed memory offers a simple solution to storing and retrieving information pertaining to the equipment where the DS2423 is installed and its frequency of use. The scratchpad is an additional page that acts as a buffer when writing to memory. Data is first written to the scratchpad where it may be read back for verification. A copy scratchpad command will then transfer the data to memory. This process ensures data integrity when modifying the memory. A 64-bit registration number is factory lasered into each DS2423 to provide a guaranteed unique identity which allows for absolute traceability and acts as node address if multiple DS2423 are connected in parallel to form a local network. Data is transferred serially via the 1-Wire protocol, which requires only a single data lead and a ground return.

DS2423 Application Circuits

DS2423

Many events call for measurement of either a total, or a count per unit time (rate). Examples include wind speed and rainfall or the number of times a wheel has rotated, from which rpm and distance can be computed. A magnetically actuated reed switch used as an input to a DS2423 counter allows such events to be easily measured. A basic reed switch circuit suitable for a rain gauge or wind speed sensor is shown in Figure 1. The dual diode BAT54S serves to protect the circuit from signals that go below ground, and, with C1, provides a local source of power. While the DS2423 has an internal pull-up resistor to keep the input from floating, its high value (~22 Mohm) can make it susceptible to noise. To avoid generating spurious counts during turn-on and minimize noise pickup, an external 1 Mohm pull-down resistor is substituted. Except for lithium backup (not shown), this is the counter circuit used in the 1-Wire rain gauge. Here, a small permanent magnet moves past the reed switch each time a tipping bucket fills and empties. This momentarily closes the reed switch that increments the counter, indicating that 0.01 in. of rain has fallen. A similar circuit is used in the 1-Wire weather station to measure wind speed. The same circuit (also with lithium backup) has also been used as a hub-mounted wheel odometer. Conveniently, the DS2423 also contains 4096 bits of user-accessible SRAM that can be used for temporary storage, or, where lithium backup is provided, for calibration, location, and routine inspection information.