Tab Electronics Guide to Understanding Electricity ... - Sciences Club

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338 Chapter Twelve
A comparator circuit “compares” two voltages. When one exceeds the
other, a signal is provided.
The op amp compares an adjustable reference voltage, provided by P1,
to an input voltage. When the input voltage exceeds the reference voltage,
the op amp output goes high, turning on Q1, which energizes the relay.
The relay, of course, can be used to turn on, or provide power to, just
about any kind of device. For precise, repeatable performance, the positive
rail should be regulated by an LM7812 (NTE966) positive voltage regulator.
Circuits of this nature are used primarily for control applications. For
example, different variables such as temperature, humidity, or light could
be utilized to control other circuits, appliances, or machinery. For a security
light controller, a photoresistor and fixed resistor, in series as a voltage-divider
network, could be connected between the positive rail and
ground. The input to the op amp could be connected to the junction
between the two. If this circuit were to be placed outdoors, the resistance
of the photoresistor would increase as the sun started to set. When
the voltage drop across the photoresistor became high enough to exceed
the reference voltage applied to the op amp, the relay would be energized
turning on an outside light.
When two op amps are configured as comparators, one referenced to a
low voltage and the other to a higher voltage (called high and low trip points),
the resulting circuit is called a window comparator. Window comparators are
used to control a variable within a set of operating parameters.
Watt an Amplifier
The simple circuit of Fig. 12-11 is a 1-watt IC audio amplifier. P1 is the
volume control. The rest of the circuit is self-explanatory.
Figure 12-11
A 1-watt audio
amplifier.
Input
0.22 F
P1
10
k
2
3
9 to 15 Vdc
6 5
4
10
Output
8
470 F
0.1 F
ICI NTE823