The device measures the resistance of the human body via 2 tin electrodes.
The signal feeds into a 24 bit ADC and then via an opto-isolated serial
interface to a desktop or laptop computer.
The PC, using a Windows program, displays small changes in resistance
on a dial as shown below.
Input:
2 electrodes are used. One is grounded and the other connected via a 22k
resistor to a 1.25 volt reference.
ADC (Analog Digital Converter):
The ADC used is an LTC2440 due to its low noise
and high data output rate.
MPU:
The ADC and output is controlled by an MSC430F123
microprocessor.
Output:
The ADC sends data to a serial port at 300Hz. The data lines are optically
isolated from the computer.
Data is sent at 19,200 baud, data bits 8, parity none, stop
bits 1, flow control none.
The 24 bit value is transferred in 4 bytes. The msb is used to synchronize
the
data by being set on the first of the 4 bytes only.
The numbers above correspond to those in the LTC2440
datasheet (page 11).
2 buttons on the unit can be used to control the sensitivity.
The indicated bits are set in byte 4 of the data when the buttons are pressed.
The lowbattery bit is set when the battery voltage goes low.
