The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Control Circuits
11
Control Circuits
This chapter presents a variety of ways to use an 8052-BASIC system for computer control.
The applications include switching power to a load, controlling a matrix of switches,
selecting the gain of an op amp, and controlling speed and direction of stepping and dc
motors.
Switching Power to a Load
You can use your 8052-BASIC system’s port bits to control power to all kinds of devices,
including those powered by alternating current (AC), or direct current (DC) at voltages other
than 5 volts. Figure 11-1 shows two port bits that control solid-state relays that switch power
to AC and DC loads.
A solid-state relay is a simple, safe way to switch power to devices that require high voltages
or currents. A logic voltage at the relay’s control inputs determines whether or not power is
applied to the load.
In a typical solid-state relay, the control voltage is electrically isolated from the switching
circuits, which contain an optoisolated triac or a similar device. Many AC solid-state relays
include zero-switching circuits, which reduce noise by switching power only when the AC
signal is near zero volts.
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