PROJECT 02 – DIODES

OVERVIEW

A diode is a very simple electronics device that allows current to flow through it in one direction, but not in the other.

SIMPLE CIRCUITS

A few circuits for you to try. They will take only a few minutes and you do not need to show Mr. Campeau.

After you've built these circuits on a breadboard, you'll notice that the LED from circuit 1 lights up but the LED's from the other circuits dosen't. Do you know why?

Recall that a diode only lets current flow in 1 direction. In circuit 2, the diode is placed in the opposite direction from circuit 1. It is not allowing current to flow through. Remember that LED's are diodes too. In circuit 3, the LED is placed so that it is opposing current flow so it is not lighting up.

DIODE LOGIC

Diode logic (DL) allows us to create an AND gate and an OR gate (but no others.)

Diode logic suffers from voltage degradation (drop) from one stage to the next because diodes and resistors do consume voltage. This is a major issue and the reason DL isn’t used inside ICs.

DIODE OR GATE - DISCUSSION

At first glance, the diode OR gate may seem overly complicated for no reason. One might think that the diodes could actually be removed and simply replaced my normal wires. However, in the states when one input is 1 and the other is 0, the current would enter via one input and exit via the other (remember that inputs that are zero are grounded). So the diodes are necessary!

Furthermore, one may question why the resistor to ground is necessary. After all, if you are simply implementing this OR gate to light up an LED, then the resistor to ground won’t be necessary. However, when doing logic, we have to consider the possibility that there will be other gates beyond Q. And, for these gates to get an ouput of 0 from the OR gate, it needs to be able to connect whatever is beyond Q to ground – it is the resistor to ground that does this.

You might also wonder about that resistor to ground and how current flows through it when Q=1. First off, it does depend on the resistance value of the resistor. But yes, current will flow through two branches (one branch with the resistor to ground, and one with whatever is beyond Q).

DIODE AND GATE – DISCUSSION

The implementation of the diode AND gate is much easier to understand. If either input provides access to ground (zero), then the current entering from Vcc will all exit via that input.

Interestingly, when both inputs provide a 1, that is simply forcing the current from Vcc to go to Q. We usually imagine that the current entering the inputs is the same as the current exiting the output but that is not the case here.

To explain why we need a resistor, let’s consider the situation where either input (or both) has a zero. So current will flow from Vcc directly to ground. Without a load (resistor), this would be shorting out our circuit.

PROJECT EXPECTATIONS

You should create all five circuits on this page. Be sure you understand how to orient diodes.

You only need to show one circuit to Mr. Campeau. You decide between the Diode OR gate or the Diode AND gate.