Logic Gates SOLVED

Here is an example of an AND gate with attached to switches. While one switch is open and another closed, it generates the logical output (but should not). Can anyone lend any assistance?

by kevbot89
May 30, 2017

2 Answers

Answer by Garrett

Try adding a pull down resistor after each button. Or you could replace the buttons with a SPDT switch and use 5V and Gnd as the inputs and feed that into the and gate. Its because when the button isn't pressed there is no input to the AND gate so its undefined.

ACCEPTED +4 votes
by Garrett
June 02, 2017

Answer by mrobbins

@Garrett's answer is exactly right! Pull-down resistors would help here. Here's your circuit with pull-down resistors added:

(Click the circuit, click "Simulate", then "Run DC Solver." The output is now 0 as you expect!)

FYI, this is not just a simulation issue. It's potentially a real-world issue too, with different types of digital logic making different assumptions if an input is left "floating" (unconnected). It's always good practice to add an explicit pull-down or pull-up resistor. Sometimes, for example in many microcontrollers or other embedded systems like the Arduino and Raspberry Pi, these internal pull-up resistors and pull-down resistors can be turned on by software.

Another way to test out the AND gate is just to use digital clocks as your inputs, which always supply a solid logical 0 or 1:

(Click the circuit, click "Simulate", then "Run Time-Domain Simulation." This shows all four combinations of inputs for the AND gate.)

+2 votes
by mrobbins
June 02, 2017

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