Nuts and Volts - January 2008

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.