Mica Sensor Board Review

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
Accelerometer (top)
Magnetometer (bottom)
Light (top)
Temp (top)
1.25 in
Microphone (top)
2.25 in
Sounder (top)
5
Signal and Power Interface
Sensors or Actuators ADC Channel Power Control MUX Setting
Light ADC1 PW0
Temperature ADC2 PW1
Sounder - PW2
Microphone ADC5, PW3 INT3 1
Mic. Bandpass ADC5, PW3 INT3 0
2 axis Accelerometer ADC3,4 PW4
2 axis Magnetometer ADC6,7 PW5
6
Potentiomenter Control Interface

• Same Interface as adjusting the Radio Signal
  Strength.

Gain Adjustment Resistor INC DEC Select
Microphone Gain 0 100k LED1 LED2 PW6, PW7
Magnetometer 1st Axis 0-50k LED1 LED2 PW6, PW7
Magnetometer 2nd Axis 0-50k LED1 LED2 PW6, PW7
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 _at_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
• lt5kHz is good as observed
• 20Hz to 16kHz from spec
• omni directional
• 6mm in diameter
• 2.12/unit _at_1k

12
Amplification and Filtering
Pre Amp
Passive RC High pass and Low pass filters
Amp
Active Bandpass Filter
mic_out
mic_bandpass_out

• Pre Amp
• Amplification -100
• Passive RC
• 159Hz 6.4kHz
• Amp
• Adjustable Amplification -1 to -101
• Active Bandpass filter
• 2.6kHz /- 500Hz

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

• 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

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 _at_1k

16
Magnetometer

• 2 Axis
• HMC1002, 20 _at_100
• /-6 gauss (earths 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