AND Truth Table
A B X
No No No
No Yes No
Yes No No
Yes Yes Yes
A
No
No
Yes
Yes
OR/NOR Truth Table
B OR (X) NOR (X)
No No Yes
Yes Yes No
No Yes No
Yes Yes No
circuit described next.
The LED conducts
when the output voltage of the op-amp
rises above the LED’s
forward voltage. This
allows enough base
current to turn on
transistor “A” in the
inverter (logical NOT gate) circuit.
NOTed NOR
gate = AND gate
A B X
Yes Yes Yes
Yes No No
No Yes No
No No No
■ FIGURE 6
leg of the voltage divider needs to be
determined experimentally because it
will depend on the light and dark
resistance values of the CdS photoresistor you select for this application. I
found that aiming the photoresistor at
a white ceiling obtains an accurate
indication of the ambient light level.
As the light on the photoresistor
brightens, the resistance falls and the
voltage across the photoresistor (and
at the non-inverting input of the op-amp) drops. On the other hand, as the
light on the photoresistor dims, the
resistance rises and the voltage across
the photoresistor increases.
The 741 functions as a voltage
comparator in this application. When
the voltage across the photoresistor
falls below the voltage set at the
inverting input of the op-amp by the
10K ohm potentiometer, the output
voltage of the op-amp switches from
six volts to two volts. The output voltage of the op-amp switches from two
volts to six volts when the voltage
across the photoresistor rises above
the voltage at the inverting input of
the op-amp.
Consequently, the output voltage
of the op-amp is + 2 volts when the
room is too bright to need the emergency lights, such as when sunlight is
filling the room or the AC lighting is
on. The output voltage is + 6 volts
when the room is dark enough to justify using the light, such as nighttime
or the AC lighting is off.
A green LED serves two purposes.
It provides an obvious indication of
the light triggering level, and it interfaces the op-amp to the digital logic
■ FIGURE 7
Digital Logic Circuits
Digital logic circuits receive
signals that represent the two
binary digits 0 (NO/OFF) and 1
(YES/ON). A voltage of six volts
represents a YES signal, and a
voltage less than two volts represents a NO signal. Here’s how
the individual components work.
Think of the emitter and collector
terminals of a transistor as the
terminals of a pushbutton switch
and the base terminal as the switch’s
button. A YES signal presses the
button by allowing current to flow
through the transistor’s base-emitter
junction. This causes the transistor to
conduct electricity through the
emitter and collector terminals. A NO
signal releases the button and the
transistor returns to a non-conducting
state. This is a highly simplistic version
of how a transistor operates, but a
more accurate explanation that delves
into the physics of semiconductors is
beyond the scope of this article.
Input transistor A receives a signal
from the op-amp to answer the question, “Is it dark?” Input transistor B
receives a signal from the Lithonia IC
to answer the question, “Has the AC
power failed?” If the answers to question A and question B are YES, then
the logic circuit’s output X responds
with a YES and turns on the light
switch transistor (Q1) on the emergency light’s main circuit board. A
logical AND gate circuit fulfills the
requirements of this application.
■ FIGURE 8
Diode-Transistor-Logic
The diode-transistor-logic (DTL)
circuit in Figure 6 functions as an
AND gate, but the actual logic
circuit I used is defined as a NOTed
40
January 2008