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Comprehensive Review of Mica Sensor Board Features and Functionalities

The Mica Sensor Board, reviewed by Alec Woo at the NEST Meeting on November 21, 2001, integrates diverse sensors, including light, temperature, microphone, sounder, accelerometer, and magnetometer. Its compact design (2.25 x 1.25 inches) houses a 2-layer PCB layout and features I2C and 1-Wire communication protocols. The review discusses critical components, including voltage divider designs, amplification, and filtering. It also highlights the board's applications in distance estimation through sound detection and the operational specifications of each sensor for optimal performance.

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Comprehensive Review of Mica Sensor Board Features and Functionalities

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  1. Mica Sensor Board Review Alec Woo November 21, 2001 NEST Meeting

  2. What can we do with it? • Light sensor • Temperature sensor • Microphone sensor • 2.6kHz Sounder • 2 Axis Accelerometer • 2 Axis Magnetometer

  3. Board Layout • Size • 2.25 x 1.25 square inches • Same size as the Mica Board • 2 Layer Board (Top and Bottom) • No ground plane • One sided 51 pin connector • top in a stack of daughter cards

  4. Sensor Physical Placements Magnetometer (bottom) Accelerometer (top) Light (top) Temp (top) 1.25 in Microphone (top) 2.25 in Sounder (top)

  5. Signal and Power Interface

  6. Potentiomenter Control Interface • Same Interface as adjusting the Radio Signal Strength.

  7. Alternative Control Interfaces (ie. Bus Interface) • I2C • 2 pins instead of 4 pins • I2C_BUS_1_CLK, I2C_BUS_1_DATA vs. LED1,LED2,PW6, PW7 • Higher overhead • I2C commands vs. flipping pins • Chip addressing is hardwired • 1 Wire • Only need 1 pin instead of 4 pins • Higher overhead • Basically implement a 1 wire protocol Bus Master • Every 1 wire chip has unique 64 bit addressing • Use a 1 wire Bus Master Chip with serial interface • In any case, at least one extra pin can be freed to avoid using INT3 pin

  8. Light Sensor • Clairex CL9P4L 10kohm • Voltage Divider Design • Same as before

  9. Temperature Sensor • 2 options • The usual one • YY14406 • 10kohm, 0.2C accuracy, 0-75C • $6/unit • Alternative • ERT-J1VR103J • Negative temperature thermistor • 10kohm, 1C accuracy, -40 to 125C • $0.43/unit @1000 or $0.96/unit • Voltage Divider Design • Populate the one you want

  10. Sounder • Piezoelectric • Resonant at 2.6kHz +/- 500Hz • 85dB sound pressure • Weight 4grams • Diameter 29mm • $ pending from Taiwan • Circuit + sounder draws 1.5mA from measurement • Sine Wave as Output waveform

  11. Microphone • Panasonic WM-62A • 500uA max • <5kHz is good as observed • 20Hz to 16kHz from spec • omni directional • 6mm in diameter • $2.12/unit @1k

  12. Amplification and Filtering • Pre Amp • Amplification = -100 • Passive RC • 159Hz – 6.4kHz • Amp • Adjustable Amplification = -1 to -101 • Active Bandpass filter • 2.6kHz +/- 500Hz Pre Amp Passive RC High pass and Low pass filters Amp Active Bandpass Filter mic_out mic_bandpass_out ~4.7mA for circuit prototype + microphone

  13. MIC_BANDPASS_OUT Signal • Biquad Active Filter • P. 278, The Art of Electronics • Tunable • center frequency(fo) and bandpass bandwidth (BW) • Bandpass bandwidth determines quality of the filter • Center frequency can shift while bandpass bandwidth remains the same • fo = 1/2RFC • BW = 1/2RBC

  14. Example of Time of Flight Estimation • Peak Detection wrt periodicity identifies sounder signal • Period of 2.7kHz = 370us first peak (1/4 period) = 92.5us • t1 = t2 – 92.5us +  • Sampling rate determines granularity of t2 and affects  • Uncertainties in frequency also affects  • Distance = (speed of sound) * t1 Sender e.g. t1 = 865us for 1 foot t2 Receiver End of RF Signal t1

  15. Accelerometer • 2 Axis • ADXL202E • 2mg resolution at 60Hz • Filter capacitors set to be 50Hz bandwidth • 600uA current consumption • Uses the raw analog output channel for both X and Y • Duty cycle output are not used but fanned out • $13.38/unit @1k

  16. Magnetometer • 2 Axis • HMC1002, $20 @100 • +/-6 gauss (earth’s field +/-0.5 gauss) • 27ug at 10Hz • ~5mA • 2 stage amplification • 29 * 41 = 1189 • Digital Pot to adjust 2nd stage amplification to avoid railing on both axis • Amplifiers are not Rail-to-Rail (0.66V to 2.33V) • Added a virtual ground chip to give better voltage reference/(roll back to voltage divider) • Fan out reset pins for demagnetizing the chip

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