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Schmitt Trigger Inputs, Decoders
TTL Switching In this lab we study the switching of TTL devices. To do that we begin with a source that is unusual for logic circuits, an offset sine wave from an oscillator. Experiment 1: Setup the function generator as in Figure 1 to make a sine signal that is offset positively, with a maximum output not greater than 5 volts. The goal is to obtain a voltage that swings from a value near zero to a positive value that will appear as a high
state to TTL input (about 2 volts) as shown in Figure 2. The diode is for protection of the TTL devices that will be connected to this source. It ensures that the input of the source does not go more negative than – 0.7 volts. An ideal diode would not permit any negative voltage. If the offset source would otherwise have some negative swing then the ideal diode would lead to an output that looks like Figure 3. For a real diode the output, seen on the scope, looks like Figure 4.
Function Generator
V Sine^150 Ω Offset
Figure 1 : Sine Signal Generator
Figure 2 : Sine Wave with offset.
Experiment: Vary the level of the sine signal and the offset so that you see an output like Figure 4. Then set the level and the offset so that the output resembles Fig. 2.
Figure 3 : Sine Wave with Offset
Figure 4 : Sine Wave with Diode
LED Arrays The next experiments in this lab use the light-emitting diode arrays. There are 10 LEDs in a single DIP connected as shown in Figure 6.
The only problem is that you do not know which side of the package is which. Experiment: Discover the answer by a test circuit like that shown in Figure 7.
Remember the answer. The quad two-input NOR gate: The 7402 (or 74LS02) is a quad two-input NOR gate. The pin-out is shown in Figure 8:
Figure 6 : DIP with LEDs.
5V 150 Ω
Figure 7 : Test Circuit.
Experiment: Verify the NOR truth table for one or two of the logic gates of a 74LS02: [This chip also needs a +5 volt supply at pin 14 and a ground at pin 7.]
Six NOR gates make a 2-to-4 Decoder Suppose that we want to turn on any one of four circuits from some remote location. We could send control signals along four individual control lines. Alternatively, we could encode the data in a digital word and decode it downstream with a scheme like this: Table 1 : Two to Four Line Decoder Code Action Downstream A B 0 0 Turn on circuit 0 0 1 Turn on circuit 1 1 0 Turn on circuit 2 1 1 Turn on circuit 3 The decoder at the downstream location is a 2-line to 4-line decoder. We can make it from six NOR gates as shown below in Figure 9.
ground
+5V
Figure 8 : Pinout of 7402.
decoder to light the LEDs of the array we let the decoder sink current, as shown in Figure 10 below. resistor to keep from burning out the LEDs with too much current.^ A TTL chip^ can sink quite a lot of current. Therefore, we need a current limiting Question: Why don’t we need a resistor on each LED of the array? Experiment : Build the decoder shown in the figure below and verify its operation.
Table 2 : Three-to-eight decoder. C B A Output Value 0 0 0 0 0 1 0 1 0 0 1 1 1 0 0 1 0 1 1 1 0 1 1 1
+5 V
3K A
B
C
Figure 10 : Three to eight decoder test circuit.
