Download Open Source Electronics for Laboratory Physics Chapter 3: Sensors

Saint Cloud State University, MN, USA
[email protected]
AAPT workshop W03 July 26, 2014
CH3: Sensors
 Resistive sensors (thermistors, photo resistors etc.)
 Analog sensors (PASCO or Vernier analog sensors etc.





output voltage represents measurement)
Digital on-off sensors (photo-gates, Hall-effect
switches etc.)
Digital pulse width sensors (sonic rangers)
Digital sensors with serial output (some sonic rangers)
Digital sensors with I2C bus (accelerometers, magnetic
sensors, gyroscopes etc.)
Digital sensors with SPI, One-Wire, SDI-12 interfaces
Resistive sensors (10-bit)
 Thermistor: doped semiconductor that decreases
resistance with increasing temperature.
1
1 1
𝑅
= + ln
𝑇 𝑇0 𝐵
𝑅0
𝐵
𝑇=
𝑅
ln
𝐵
−𝑇
𝑅0 𝑒 0
 Light dependent resistor (LDR): doped semiconductor
that decreases resistance with increasing light
intensity. Also called photo resistors.
Wikipedia.org
Sensing resistance
𝑉𝑠𝑒𝑛
Vsen
𝑅 = 𝑅𝑓𝑖𝑥
𝑉𝑠𝑒𝑛
5𝑉 − 𝑉𝑠𝑒𝑛
Rfix
The easiest way to sense resistance is to form a
voltage divider using a fixed resistor and the
variable resistor.
R
For best accuracy, the value of the fixed resistor
should be equal to that of the variable resistor
when it is in the middle of its range.
Analog input
GND
𝑅
=
∗ 5𝑉
𝑅 + 𝑅𝑓𝑖𝑥
OSPL has on-board 10Kohm pull-up resistors on
channels A1, A3, and A5 so there is no need to
add a resistor to the circuit. This makes
prototyping and cabling a lot easier.
Breadboard
 All 5 holes in a row are connected and they act as
junctions you see on a schematic.
 All holes along a red bus are connected but different
red buses are separate. Same goes with blue buses.
Breadboard vs. schematic
Photo resistor
𝑅 = 𝑅𝑓𝑖𝑥
𝑉𝑠𝑒𝑛
5𝑉 − 𝑉𝑠𝑒𝑛
Integers calculation will truncate accuracy of the
result. Make sure to use floating point numbers
when necessary to maintain accuracy.
Use channel 0 (leftmost)
Wiring:
One end – White
Other end – Black
Thermistor:
𝑅 = 𝑅𝑓𝑖𝑥
𝑉𝑠𝑒𝑛
5𝑉 − 𝑉𝑠𝑒𝑛
𝑇=
ln
𝐵
𝑅
𝑅0
𝐵
−
𝑒 𝑇0
Integers calculation will truncate accuracy of the
result. Make sure to use floating point numbers
when necessary to maintain accuracy.
Use channel 0 (leftmost)
Wiring:
One end – White
Other end – Black
Analog sensors (10-bit)
 Vernier direct temperature probe (DCT-DIN)
 Slope 55.55DegC/V or 100DegF/V
 Intercept -17.7DegC or 0DegF
1. Analog voltage
2. Slope
3. Intercept
1
2
3
 Vernier dual range force gauge (DFS-DIN)
 Slope -4.9N/V (±10N) or -24.5N/V(±50N)
 Intercept 12.25N (±10N) or 61.25N (±50N)
Reading Vernier analog sensors
Read sensor
voltage
Convert
reading x with
y=ax+b
Format result
send to LCD
or PC
Delay (for
mere
humans)
Send measurement to PC, 4 lines of code
reading=analogRead(channel);
result=a*reading+b;
Serial.println(result);
delay(200);
Line 1: acquire data:
Line 2: scale data for output:
Line 3: output result:
Line 4: pause momentarily for user:
Display measurement on the LCD, 5 lines of code
reading=analogRead(channel);
result=a*reading+b;
lcd.clear();
lcd.print(result);
delay(200);
Line 1: acquire data:
Line 2: scale data for output:
Line 3: clear LCD:
Line 4: output result:
Line 5: pause momentarily for user:
Optional: Reflective sensor
Integers calculation will truncate accuracy of the
result. Make sure to use floating point numbers
when necessary to maintain accuracy.
Use channel 0 (leftmost)
Diode side Wiring:
Left pin – 150ohm resistor
150ohm resistor – Red
Transistor side wiring:
Left pin – White
Right pin – Black
ADXL335 Accelerometer
1
2
3
Digital on-off sensors
 PASCO photogates
 No obstruction-digital HIGH
 Obstruction-digital LOW
 Predefined blocker distance (1cm) for speed calculation
 10 spokes on pulley for angular speed calculation
 To calculate speed:
1.
2.
3.
4.
5.
6.
While signal is HIGH, wait for LOW.
When it becomes LOW, store time stamp and wait for HIGH again.
When it becomes HIGH again, wait for LOW.
When it becomes LOW, subtract stored time stamp from current time stamp.
Divide predefined blocker distance by the time elapse to find speed.
Best way to sense a photogate is to use an object implementing state machine
Pulse-width sensor
 Sonic ranger
 Start ranging by pulling the trigger pin to HIGH for




10us or longer then return it to LOW
Ranger will emit ultrasonic pulses and detect its echo
Ranger pulls the echo pin HIGH for the amount of
time it takes sound to travel the distance round trip
Detect this pulse width with pulseIn() and calculate
distance with speed of sound.
Accuracy depends on timing accuracy of the receiver.
Sonic ranger code and wiring
Use channel 0 (leftmost)
Wiring:
VCC – RED
Trig – Brown
Echo – White
GND – Black
1
1. Initialize pins A0 as output and A1 as
input.
2. Initialize the serial port to 9600
baud rate.
3. Define a variable to store result.
4. Toggle the INIT pin to start ranging.
5. Get result with a function pulseIn().
6. Convert the result into distance in
mm and print it to serial monitor.
2
3
4
5
6
Serial interface sensor
 Maxbotix sonic rangers (car reverse parking sensors)
 Free running at around 10Hz
 Prints “R1234\n” where 1234 is the range in mm.
 Requires some C-string to integer conversion if range
value is to be used programmatically
 Can be directly connected to a PC
 Measurements are done on board and suffers NO loss
of accuracy during transmission.
2
IC
sensors
 Inter-Integrated Circuit bus connects to many sensors.
 Two wires for a large number (127) of different sensors
 Each sensor module has a different address
 Measurements are done on board and suffers NO loss
of accuracy during transmission.
 Requires the Wire library to communicate with and
some understanding of the commands on data sheets.
ADXL345 Accelerometer
ADXL345 3-axis accelerometer
 ADXL345 has maximal range of 18g in all 3 dimensions
 First set up the device (range, power up)
 Then read the device data register for X, Y, and Z
acceleration.
Stores persistent values of offset for each axis
Controls data range and accuracy
Stores x, y, and z acceleration measurements, low and high bytes
How to set offset registers
How to set data format registers
10DOF sensor board wiring
1.
2.
3.
4.
VCC_IN – Red
GND – Black
SCL – Write
SDA – Brown
Note the diagram is a mirror image of
the actual sensor board.
The code is so long I can only fit a small portion on this slide.