Question 9.PE.6: To investigate the decoder For this exercise you will need t...

Practical Digital Electronics for Technicians

To investigate the decoder
For this exercise you will need the following components and equipment:

1 – 74LS42 ic (BCD to decimal decoder)
1 – +5 V dc power supply
10 – LED (5 mm) and series resistor (270 Ω)

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1 Connect up the circuit of Figure 9.26. The pin connection diagram of the 7442 ic is shown in Figure 9.27.

2 Work through and complete the truth table of Figure 9.28. You should notice that, for a particular binary input, the corresponding decimal output is logic 0 with the LED being unlit. Thus the outputs are active low, which accounts for the style of the truth table.

Conclusion

How should the LED and resistor be connected in order for the LED to be lit when the output is low?
Hint. Look ahead to Chapter 10!

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