SIGNAL CONDITIONING, DATA ACQUISITION, AND SPECTRAL ANALYSIS - Design and Development of Medical Electronic Instrumentation - page 221

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100uA EXCITATION 16

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R1
RS-271-110A

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BATTERY - (POWER
AND DIGITAL GROUND)

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Figure 5.8 Resistive sensors such as the thermistor shown can be read by injecting a known cur-
rent into the sensor and measuring the voltage developed across the sensor. (From Prutchi [1999].
Reprinted with permission from Popular Electronics , June 1999 © Gernsback Publications Inc.)

Warning! Be extremely careful when using the universal sensor interface to control
line-powered devices. Life-threatening voltages and currents are present in these circuits.
Isolate and fuse all circuitry properly on the power line path.

from an inexpensive digital kitchen scale can be excited from an op-amp (IC1A) driven by
D/A converter 2. When the scale is not loaded, the output from the cell is a voltage that

oats
somewhere in the range of 0 to the drive voltage generated by A/D converter 2. This output
voltage is slightly ampli

fl

fi ed by IC1B (gain 1.78). IC1C is used to cancel the load cell's base-

line level by of setting the output of IC1B by the amount established by D/A converter 1. The

signal developed by a real load is further ampli

ff

fed by IC1D before being fed to analog input
1. Resistor R4 and zener diode D1 protect the A/D input from output voltages produced by
IC1D, which may be beyond the acceptable input range. SCALE.BAS is the QuickBasic pro-
gram that implements an auto-zeroing digital scale by controlling the sensor's output o

fi

set.
The Universal Sensor Interface is also ideal for implementing low-cost process or envi-
ronmental control systems. For this type of application, the device's digital outputs are often
required to switch ac loads such as heaters, lamps, or motors. As shown in Figure 5.10, the
digital outputs of the universal sensor interface can be used to control an ac load connected
to the power line through an opto-isolated triac. The digital output signal is used to drive a
2N2222 transistor, which in turn controls the LED inside the optocoupler (IC1). The opto
device selected for this application is a zero-voltage switching triac which is used to drive
a power triac (Q1) capable of handling the necessary switching power. A device that can be
used to switch light loads is the Q4004.
Digital inputs can also be isolated. The circuit of Figure 5.11 shows how a PS2506 can be
used to sense an ac or dc voltage safely to signal a digital input line. R1 should be selected
to produce no more than

ff

20 mA of current through the LED of the PS2506.