The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Using Sensors to Detect and Measure
Listing 9-1. Reads and clears a flip-flop output connected to an input port pin.
10 REM clear flip-flop
20 XBY(0E400H)=0
30 XBY(0E400H)=1
40 DO
50 REM read port
60 A=XBY(0E000H)
70 REM see if bit 0 is set
80 IF A.AND.1=1 THEN GOSUB 200
90 WHILE 1=1
100 END
200 PRINT “vibration alarm”
210 REM clear flip-flop
220 XBY(0E400H)=0
230 XBY(0E400H)=1
240 RETI
Many sensors have analog outputs, however. They vary continuously in response to changes
in the properties they sense. For example, the resistance of a Cadmium-sulfide (CdS)
photocell varies with the intensity of light hitting it. If you want to use an analog sensor like
this in an 8052-BASIC system, you need to add some components to convert the analog
signal to digital.
A comparator provides a way to detect a specific analog voltage. Figure 9-3 shows how to
use a comparator to detect a specific light level on a photocell.
A comparator is a special form of op amp with analog inputs, but a digital output. In Figure
9-3, pin 4 is a reference voltage, and pin 5 is the input being sensed. When the sensed input
is lower than the reference, the comparator’s output is low. When the sensed input is higher
than the reference, the comparator’s output is high.
R1 and the photocell form a voltage divider. As the light intensity hitting the photocell
increases, its resistance decreases and pin 5’s voltage rises. To detect a specific light level,
adjust R2 so that VOUT switches from low to high when the light reaches the desired intensity.
You can read the logic state of VOUT at any input port pin.
R4 is a pull-up resistor for the LM339’s open-collector output. R3 adds a small amount of
hysteresis, which keeps the output from oscillating when the input is near the switching
voltage.
The Microcontroller Idea Book 157