The c-meter measures the resistance of the human body using 2 electrodes normally made of tin.
The signal feeds into a 24 bit ADC and then via an opto-isolated serial
interface to a desktop or laptop computer.
Using a Windows program, the PC (computer, not preclear) displays small changes in resistance
on a dial as shown below.
Power:
The c-meter is powered by 4 "AA" batteries.
Input:
2 electrodes are used. One is connected via a
22k resistor to a 1.25 volt reference and the other is grounded.
MPU:
The ADC and output is controlled by an
MSC430F123
microprocessor.
ADC (Analog Digital Converter):
The ADC used is an
LTC2440
due to its low noise and high data output rate.
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.
