# Use Diodes and Resistors to Perform Logic

Date: 7/29/2015

Using NI Multisim I created a simply design to show how to use basic components to create an AND & OR gate.

Diode logic gates use diodes to perform OR and AND logic functions as shown in the circuit diagram. Connection of the LED at the output is optional which simply displays the logical state of the output, i.e. the logic state of output is 0 or 1, if LED is off or on, respectively.

Diodes have the property of easily passing an electrical current in one direction, but not the other. Thus, diodes can act as a logical switch. Diode logic gates are very simple and inexpensive, and can be used effectively in limited space.

However, they cannot be used extensively due to the obvious logic level shift when gates are connected
in series. In addition, they cannot perform a NOT function, so their usefulness is quite limited. This type of logic circuit is rarely found in integrated form.

### OR Gate (74ls32)

If one or both inputs are at logic “1” (5 volts), the current will flow through one or both diodes. This current passes through the resistor and causes the appearance of a voltage across its terminals, thereby obtaining logic “1” on the output.

Here only a logic “0” (0 volts) on the output when both inputs are in logic “0”. In this case, the diodes do not conduct, there is no current through the resistor R and there is no voltage across its terminals. As a result the voltage at Vout is the same as ground (0 volts)

### AND Gate (74ls08)

When both inputs are at logic “1″, the two diodes are reverse biased and there is no current flowing to ground. Therefore the output is logic “1” because there is no voltage drop across the resistor R.

If one of the inputs is logic “0”, the current will flow through the corresponding diode and through the resistor. Thus the diode anode (the output) will be logic “0”.

This method works fine when the circuits are simple, but there are problems when you have to make interconnections with such gates.

Reference material:

## Saturday, July 25, 2015

### 74ls163 Clock Counter Project|Atanua

A while back I posted a blog about the 74ls163 Synchronous Counter. In this blog I will revisit the same TTL using Atanua software. After playing with the 73ls163 I gained a little more insight about it.

I found that (referring to the diagram) by changing the input values you can program the count up starting number. This TTL can only count up! To do this you will use input switches 1-4
Switch A is used to clear the counter and pin 2 is for clock connection. The "Ripple carry output" (RCO) used to enable successive cascaded stages. *floating pin in the diagram.

After setting your desired starting number, you can enable P, T & Load to start the count. You will see your project initiate from the programmed value. However, to keep the clock in a loop additional TTL's must be added; the 4 input Nand & Invertors. A nice project write up can be found here:

Check out my YouTube video to see the simulation of 74ls163

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