Difference between revisions of "Activities/Turtle Art/Using Turtle Art Sensors"

Revision as of 22:36, 7 November 2010

CAUTION. APPLYING EXTERNAL POWER SOURCES TO THE XO LAPTOP'S MICROPHONE INPUT CAN CAUSE PERMANENT DAMAGE. BE SURE YOU READ AND UNDERSTAND THE FOLLOWING BEFORE CONNECTING POWER SOURCES TO YOUR LAPTOP. DO NOT CONNECT TO HAZARDOUS VOLTAGES

Specifications

The OLPC XO can measure external inputs with its microphone jack.

Standard 3.5mm 2-pin switched mono microphone jack; selectable 2V DC bias; selectable sensor-input mode (DC or AC coupled); selectable +20dB boost.

If using a stereo plug it is the tip (usually the red wire) plus the earth.

It is protected at the input by a 5V zener diode and has a measurement range of 0.4V to 1.85V. The allowable input is -0.5V to 5V. Inputs outside this range will cause excessive current and damage. Even a single 1.5V battery can cause damage if connected reverse polarity.

Addition of a 150k ohm series resistor would (I expect, no guarantee) give a reduced sensitivity in voltage mode (0-4V) but allow inputs to +- 100V without damage. Input impedance in resistance, volume and pitch modes is much lower but a 1k ohm series resistor should still allow inputs to +-12V.

If applying an external voltage, it is highly recommended that test leads be made up with a microphone plug and an inbuilt series resistor.

Voltage Mode

Measurement range is DC 0.4V to 1.85V. Voltages less than 0.4V report as 0.4V, voltages greater than 1.85V report as 1.85V. Accuracy is around 3% of full scale.

Resistance Mode

Measurement range is 750 ohms to 14k ohms, resistances less than 700 ohms report as 700 ohms, greater than 14k ohms report as 14k ohms. Accuracy is around 5% with respect to the full scale voltage measured accross the resistor, this translates to round 50 ohms at bottom scale and 2k ohms at top scale. (A series protection resistor of around 700 ohms would give a measurement range of 0 - 13k ohms and protection against +-8V inputs.)

Pitch Mode

Pitch Mode is set up for the the internal microphone but can also be accessed through the microphone jack, 2V DC bias is on, AC coupled, +20dB boost is on. The frequency of the strongest component is reported in Hz. The resolution is +-8Hz

Sound Mode

Raw data ranging -32000 to 32000, set up is for microphone, AC coupled, Bias on, Boost on. Sensitivity is 2uV per unit or 16mV full scale, so sine wave clipping sets in at round 10mV RMS.

Volume Mode

Ranging 0 to 32000, set up is for microphone, AC coupled, Bias on, Boost on. Volume is the rectified average of Sound, ie. volume=average(abs(sound))

Graphing the output

Source file as doc File:Oscillo.doc

Measuring Temperature

Connect the TDC05C247 thermistor, Specifications:

NTC (Negative Temperature Coefficient) Thermistor
Operating temperature range: -20 Celsius ~ +125 Celsius
Maximum power rating: 500mW
Nominal resistance at 25 Celsius 4.7k ohms

Teaching ideas:

measure resistance and temperature with a thermometer
construct your own calibration function
exponential decay
diurnal temperature
heat of reaction, weak acid and base

Measuring Soil Moisture

Two probes into the soil measured in resistance mode. Only a few cm of wire are required to bring soil within the resistance range of 700 ohms to 14k ohms

@@ Line 54: / Line 54: @@
 * diurnal temperature
 * heat of reaction, weak acid and base
+==Measuring Soil Moisture==
+Two probes into the soil measured in resistance mode. Only a few cm of wire are required to bring soil within the resistance range of 700 ohms to 14k ohms
 ==Links==