The Microcontroller Idea Book - Jan Axelson's Lakeview Research

• What power supplies are available to power the sensor? (+12V, +5V)
The answers to these questions will help you narrow your choices as you research what’s
available.
On/off Sensors
Using Sensors to Detect and Measure
Sometimes, all you need to detect is the presence or absence of the sensed property. Some
simple sensors act like switches, with a low resistance in the presence of the sensed property,
and a high resistance in its absence.
There are many types of sensors that you can use in this way. A magnetic proximity sensor
responds to the physical separation of the items connected to each of the switch elements.
A vibration sensor responds to rapid motion. Both of these are often marketed as home-security
devices for use on doors or windows, but you might come up with other uses for them.
Another example is a mercury tilt switch, which uses a ball of liquid mercury as a conductor.
The switch contacts open or close when the switch tilts and the mercury rolls to the opposite
end of the switch. Figure 9-1 illustrates.
Figure 9-2 shows two ways to detect the state of on/off sensors like these.
Figure 9-2A is an unlatched input. When the resistance across the sensor is high, the pull-up
resistor brings the input voltage high. When the sensor’s resistance is low, the input goes
low.
You can connect this circuit to any unused pin on an input port. If you use the 8052-BASIC’s
INT1 input, you can use an ONEX1 statement to trigger a subroutine whenever the sensor
detects the property in question. If you use an ordinary port input, reading the port bit will
tell you the current state of the sensor.
In Figure 9-2B, when the sensor switches from high to low resistance, a 74LS73 JK flip-flop
stores the information as a high Q output, which your program can read at its leisure. After
reading the input, strobing the CLR input low brings Q low again, until the next sensing event.
The flip-flop “remembers” past events, so you don’t have to detect or respond to events as
they happen.
Figure 9-1. The tilt, or physical angle, of the mercury switch determines
which of its three terminals connect.
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