2) Differential Amplifier Circuit: In general, the output of
such a load cell is rather small – the maximum voltage change
of the load cell is less than 6 mV under stress corresponding to
the gravity of 50 kg object/subject (its capacity). This makes
it difficult to get the accurate value from the A/D converter.
Thus, we use a Differential Amplifier circuit to enhance the
signal that we get from the strain gauge. The amplifier model
that we use in our project is INA126 [2].
Vout = G ∗ Vin = (5+
80kΩ
Rg
) (2)
3) Power Switch Circuit: We also use a power switch circuit
to turn on/off the power supply, which is a simple p-MOS
FET and n-MOS FET circuit. The switch is controlled by the
transmitter tag, which uses a pin to pull down to the ground
to turn off the circuit or pull up to 3V to turn on the circuit.
In this way, we can conserve a significant amount of energy
when the measurement is not needed.
The whole circuit is shown in Figure 4.
4) Wireless Communication Component: We use a wireless
communication system developed in our group (details can be
found in [12], [11]) to convert the analog signal to digital
values and then transmit them through wireless links. The
system consists a transmitter that we refer to as PIP-Tags
which contains a 10-bit A/D converter with the range of 0 to
1.5V. It is low-cost, low-power, and easy to program. The PIP
tag has its own processor and radio transceiver. The basestation
has the same hardware as the PIP-Tags, with a tuned 900 MHz
monopole antenna attached. The basestation is also equipped
with a standard USB connection for data transfer to the laptop
which runs the signal processing algorithms. In fact, we could