Wednesday, March 07, 2012

Diodes and Diode Logic

A standard diode with its cathode (negative pin) and anode (positive pin) labeled.

Introduction To Diodes

When you are first introduced to diodes they seem pointless because all you are told is that they only allow electrical current to travel through the diode in one direction, but that happens to be a pretty big deal in circuits . Diodes are the simplest semiconductor device used in electronics: not the smallest but the simplest.  And if you know more about LEDs than you do diodes, it might be nice to know that  LEDs (light emitting diodes) are a special type of diode that emits visible light.

Schematic diagram of a diode connected to an LED (light emitting diode). DC power is the same type that AA, AAA, D, ect. batteries use. Notice the similarities and differences between the diode schematic symbol versus the LED schematic symbol. Their symbols are very similar because LEDs are diodes! The side of the diode with line is the cathode (negative pin) and the other side (where the triangle is tallest) is the anode pin (positive pin).

Schematic diagram of a diode connected to an LED in the opposite direction. Diodes only allow electrical current to flow in one direction, from anode to cathode, but this diode is placed in reverse which stops any current from flowing to the negative side (Ground) of the DC power supply. Therefore the LED is not lit because it has no electrical current running through it. Similarly if you plug an LED backwards the circuit will not run correctly (might even blow up the LED), so be careful of the way you set up your diodes. Anode (positive) to Cathode (negative).

Diode Logic Implementation

Diode logic implementation is the process of creating logical functions out of combinations of diodes in series or parallel. The logic functions are made possible because diodes ability to only allow current to flow in one direction can be utilized as a switch. In this post, I will teach you the set up for diode implementation of the AND, and OR gates.

This is a diode logic circuit for an AND gate. An AND gate demands that both inputs, we can call them A and B,  are connected to positive voltage indicating a logic high. The two arrows pointing away from the LEDs (Light emitting diodes) are filled in with a color if the light is on indicating a logic high.
An AND gate can be created by placing two diodes in parallel.  Parallel diodes means that the diodes act independently of each other and meet at a common point of the greater circuit. An AND has a logic low output when either of the two inputs or both inputs are at a logic low (off/false).

*Double click for a better view! Notice that the top input is now hooked up to ground (the three line triangle symbol) which functions as a logic low (off/false) therefore the output of the AND gate is logic low (off/false).
An OR gate can be created by flipping around one of the two diodes (in parallel like the AND gate example above). The two inputs oppose each other resulting in an OR function. It looks like this!
1 Input is HIGH or true and the other is LOW (false) which is processed through the OR gate of diodes to result in a positive current (logic HIGH/true) through the logic indicating LED.

If you enjoyed this post and want more posts on diode logic or diode function, then email me at and simply type "diode logic" or something and I will make it happen. Thanks.

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