Sunday, April 12, 2015

Make:Electronics, Experiment 19: Learning Logic

With experiment 19 of Make:Electronics I'm entering the world of pure digital electronics. This experiment explains the basics of the 7400 family chips. It is also a prelude to experiment 20. First an LM7805 voltage regulator is used to to provide precisely 5 volts DC. Then a simple circuit is built around the 74HC00 chip (the NAND chip of the 7400 family). This circuit (figure 4-46 of the book), shown on the image of the breadboard below, demonstrates the function of one NAND gate (out of four on the chip). Two tactile switches provide input on the NAND gate and the LED provides a visible output. As expected the the LED lights up if none or one tactile switch is pushed. If both tactile switches are pushed the light of the LED goes out.

Next I replaced the 74HC00 with the 74HC08 chip, which is the AND chip. This chip functions opposite to the NAND chip. So only when I push the two tactile switches simultaneously the LED lights up.

In the last part of this experiment a diode is introduced. It connects the output of the first AND gate with an input of the same gate (figure 4-79 of the book). There is only one tactile switch left since the first pin is connected directly to the positive voltage. Once the tactile switch is pushed the LED lights up and stays on even if the switch is let go (like a latch).

Overview of experiment 19 with the LM7805 on the right. 

With 74HC00 NAND chip.

Latch configuration with pin 1 connected to positive voltage and a diode connects pin 3 to pin 2.

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