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

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

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

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

Signal and Power Interface Sensors or Actuators ADC Channel Power Control MUX Setting LightADC1PW0 TemperatureADC2PW1 Sounder-PW2 MicrophoneADC5,PW3INT3 = 1 Mic. BandpassADC5,PW3INT3 = 0 2 axis Accelerometer ADC3,4PW4 2 axis Magnetometer ADC6,7PW5

Potentiomenter Control Interface Gain Adjustment ResistorINCDECSelect Microphone Gain 0 – 100kLED1LED2PW6, PW7 Magnetometer 1 st Axis 0-50kLED1LED2PW6, PW7 Magnetometer 2 nd Axis 0-50kLED1LED2PW6, PW7 Same Interface as adjusting the Radio Signal Strength.

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

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

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

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

Microphone Panasonic WM-62A 500uA max <5kHz is good as observed 20Hz to 16kHz from spec omni directional 6mm in diameter

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 AmpPassive RC High pass and Low pass filters AmpActive Bandpass Filter mic_out mic_bandpass_out ~4.7mA for circuit prototype + microphone

MIC_BANDPASS_OUT Signal Biquad Active Filter P. 278, The Art of Electronics Tunable center frequency(f o ) and bandpass bandwidth (BW) Bandpass bandwidth determines quality of the filter Center frequency can shift while bandpass bandwidth remains the same f o = 1/2  R F C BW = 1/2  R B C

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 End of RF Signal Sender Receiver t1 t2 e.g. t1 = 865us for 1 foot

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

Magnetometer 2 Axis HMC1002, +/-6 gauss (earth’s field +/-0.5 gauss) 27ug at 10Hz ~5mA 2 stage amplification 29 * 41 = 1189 Digital Pot to adjust 2 nd 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