How To Build A Simple Decimal To BCD Decimal Converter

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Circuit Description

This circuit uses a Cmos 4081.

In the diagram on the left there are two small arrows - top and bottom.

These indicate that the positive line is connected to pin 14 - and the negative line is connected to pin 7. 

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The Cmos 4081 contains four distinct circuits known as "gates". 

All four gates share the same two connections to the power supply. 

However - apart from that - they operate entirely independently of one another. 

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We will look closely at the operation of gate 1.

But - since all four gates operate in the same way - anything said about gate 1 will apply to the other three gates as well.

Pins 1 & 2 are the inputs of gate 1 - and its output is at pin 3. 

By altering the voltages on the input pins - you can turn the output pin on and off. 

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When the output is on - pin 3 is connected internally to the positive rail.

When it's off - pin 3 is connected internally to the negative rail.

When the output is on - pin 3 is said to be "high".

And when the output is off - pin 3 is said to be "low". 

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The two inputs of a 4081 gate are identical to one another. 

They operate in exactly the same way. 

So from a circuit design point of view - they are interchangeable. 

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Each input has a very high internal resistance (impedance). 

So almost no current flows into the input pins. 

Instead, the mere presence of the voltage at the inputs is all that's required to make the gate operate. 

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In the circuit on the left - the push switch will connect pin 1 directly to the positive rail.

There's no need for an external current limiting resistor. 

The high impedance of pin 1 allows you to connect it directly to the positive line. 

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The box filled with zeros and ones is known as a "Truth Table". 

In the table - a "one" represents a high - and a "zero" represents a low. 

The first two columns represent the gate's two inputs. 

The third column represents the output.

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The table lists the four possible combinations of input and output - available with a Cmos 4081 gate. 

When the two inputs are low - the output will be low.

This is represented by the top row of the table - where the entry is 
                "0 + 0 = 0". 

When the two inputs are high - the output will be high.

This is represented by the bottom row of the table - where the entry is 
               "1 + 1 = 1". 

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There are two more possible situations. 

These arise when one of the inputs is high and the other is low. 

These two configurations are covered by rows two and three of the table. 

In both cases the output is low. 


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The gates in a Cmos 4081 are known as "AND" gates. 

With an "AND" gate - you'll only get a high output - when both inputs are high. 

That - in a nutshell - is all you need to remember. 

It summarizes neatly - the contents of the Truth table. 

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In standby mode R9 holds pin 2 high.

Since only one of its input pins is high - the output pin is low.

When a Cmos output pin is low - it's connected internally to the negative line.

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This means that pin 3 cannot supply current to the LED.

It also means that pin 1 is connected to ground - by pin 3 - through R1.

In other words - pin 1 is low.

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While at least one of the inputs is low - the output will be low.

See the Truth table.

So - in standby mode - the output is "off" - and so is the LED.

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When the button is pushed - it takes pin 1 high.

R9 is already holding pin 2 high.

And - because both the inputs are now high - the output will go high also.

While pin 3 is high - it's connected internally to the positive line - and it does two different jobs.

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The first job is to latch itself on.

It does this by taking pin 1 high - through R1.

So when the button is released - pin 1 will remain high.

And since both pins 1 & 2 are high - the output will remain high also.

Pin 3's second job is to provide current to light the LED.

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To sum up - each gate has one input pin held high by R9.

If a keypad switch takes the second input pin high - the output pin will go high.

The output pin will remain high after the key-switch is released.

And the LED indicator will remain lit.

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The four LEDs represent a four bit binary display.

In decimal terms - they can display any number from 0 to 15.

In hexadecimal terms - from 0 to F.

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The diode network - between the keypad switches and the four input pins of the 4081 - ensures that each keypad switch turns on the correct combination of LEDs.

That is - switch 1 turns on "A"

Switch 2 turns on "B"

Switch 3 turns on "A" and "B" etc.
 
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Diode Network

The diode network - between the keypad switches and the four input pins of the 4081 - ensures that each keypad switch turns on the correct combination of LEDs.

In the diagram on the left - the diodes are colour coded - according to the keypad switch they serve.

When a switch is pressed - it takes the input pin of one or more of the gates high.

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The diodes only conduct in one direction.

They create a network of one-way paths.

They allow several switches to take the same input pin high - and still keep those switches separate from one another.

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For example - switches 1, 3, 5, 7 & 9 can all take input "a" high.

Switches 3, 5, 7 & 9 ARE all connected to switch 1 - and do the same job as switch 1.

But switch 1 is NOT connected to switches 3, 5, 7 & 9.

So switch 1 will not take "b" or "c" or "d" high.

If the diodes were replaced by wire links - all the switches would be connected to all the input pins.

And every switch would turn on all four LEDs.

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The diodes allow you to select the LEDs that each individual switch will light. 

Switch 1 turns on "A" - but so do switches 3, 5, 7 & 9

Switch 2 turns on "B" - but so do switches 3, 6 & 7

Switch 4 turns on "C" - but so do switches 5, 6 & 7

Switch 8 turns on "D" - but so do switches 9 & 10

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You can have up to sixteen switches (0 to 15).

Switch 0 will turn off all four LEDs.

And switch 15 will turn all four LEDs on.

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The Reset

Before a fresh number can be displayed - the old number has to be deleted.

In other words - any LEDs that are lighting - have to be switched off.

We do this by taking one input pin - from each of the four gates - low.

See the Truth table.

If at least one input pin is low - the output pin will be low also.

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The purpose of Q1 is to take one input pin - from each gate - low.

Every keypad switch is connected to the base of Q1 - through the diode network, R10 & C2.

At the beginning of every keystroke - there's a brief moment when current flows from the keypad switch - into the base of the transistor.

This current switches the transistor on - and the transistor connects one input pin from each gate to ground.

In other words - Q1 takes pins 2, 6, 9 & 13 briefly low.

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This resets all four outputs - to low.

Once C2 charges through R10 - Q1 will switch off - and R9 will take the four input pins high again.

When R9 takes the inputs high - the new number will be displayed.

It will be displayed - until the next time a keypad switch is pressed.

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The keypad switch representing zero - behaves a little differently.

It's not required to take any input pins high.

It simply operates Q1 - and resets all four outputs to low.

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The values of R10 & C2 determine the length of the reset pulse.

It has to be short.

It needs to end before the keystroke ends.

Otherwise the new number will not be displayed.

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R11 discharges C2 after the keypad switch is released.

That way - current will flow into the base of Q1 - when the next keypad switch is pressed.

In effect - R11 resets the reset switch.

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R12 connects the base of Q1 to ground.

This keeps the transistor switched firmly off - until it's needed.

C1 & C3 filter out any stray high frequency signals that might interfere with the operation of the gates.

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Decimal to BCD - Support Material

Circuit Description

Diode Network

The Reset