Free Details Of How To Build A Simple Keypad-Operated Switch - Using Cheap Off-the-ShelfComponents.

All the information you need to build a simple keypad code-operated switch - using cheap off-the-shelf components.

COMMENTS SUGGESTIONS	Keypad Switch No.2 - Support Material	MORE KEYPAD CIRCUITS

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Circuit Description

Click Here For A Photograph Of The Prototype.

Circuit Diagram For A
Keypad-Operated Switch

The circuit uses a Cmos 4081. It contains four separate two-input AND gates. 

When power is applied - pin 4 of the IC is low. 

Current flows through the emitter-base junction of Q2 and R8 - into pin 4. 

This switches the transistor on - and it in turn energizes the relay. 

The four keys you've connected to A, B, C & D - are used to de-energize it. 

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Cmos 4081 gates only produce a high output when both inputs are high. 

R3 holds pin 1 high. This enables gate 1.

When "A" is pressed - pin 2 is taken high by R1. 

Since both inputs of gate 1 are now high - its output at pin 3 goes high. 

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Pin 3 does two jobs. 

Firstly, it latches itself on by holding pin 2 high through R4. 

In this way - pin 2 will stay high after "A" is released. 

Secondly, Pin 3 "enables" gate 2 by taking pin 12 high. 

In other words - pin 3 is doing for gate 2 - what R3 does for gate 1. 

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Now - in exactly the same way - when "B" is pressed - pin 13 is taken high. 

Since both inputs of gate 2 are now high - its output at pin 11 goes high. 

This output latches itself on through R5 - and "enables" gate 3 by taking pin 8 high. 

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Again - in exactly the same way - when "C" is pressed - pin 9 is taken high. 

Since both inputs of gate 3 are now high - its output at pin 10 goes high. 

This output latches itself on through R6 - and "enables" gate 4 by taking pin 5 high.

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Now - when "D" is pressed - pin 6 is taken high. 

Since both inputs of gate 4 are now high - its output at pin 4 goes high - and latches itself on through R7. 

With pin 4 high - the Q2 base-current is cut off. 

So the transistor switches off - and the relay drops out. 

While pin 4 remains high - the relay will remain de-energized. 

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However, if one of the keys connected to "E" is pressed - Q1 will switch on and take pin 1 low. 

Because one of its inputs is low - the output of gate 1 will go low. 

This takes one of the gate 2 inputs low - so the gate 2 output will go low. 

It in turn takes one of the gate 3 inputs low - so the gate 3 output will go low. 

Gate 3 takes one of the gate 4 inputs low - so the gate 4 output will go low. 

This restores the Q2 base-current. 

The transistor switches on - and the relay energizes.

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If - at any time during the code entry sequence - one of the "Wrong" keys connected to "E" is pressed - base current through R1 will switch Q1 on. 

Q1 connects pin 1 to ground and this removes the initial gate 1 enable provided by R3. 

So the attempted code entry fails. 

In other words, the sequence of support that each gate provides - by enabling its successor - is removed the moment Q1 takes pin 1 low. 

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The capacitors are there to slow down the inputs of the Cmos 4081 - and make them less sensitive to stray signals. 

However, the capacitors may continue to store energy - even after the power is switched off. 

They will - in effect - remember that the code has been entered. 

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If the circuit is to power-up with the relay energized - the code entry sequence must be reset.

That's the purpose of R9. It does for pin 2 - what Q1 does for pin 1. 

If there is any charge in C1 - it will be removed by R9 - the moment the power is switched off.

So - when power is re-applied - pin 3 is low. 

And - because the other three gates are no longer enabled - their outputs are low also.

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With a 12-key pad there are 12 possible choices for "A", 11 choices for "B", 10 choices for "C" and 9 choices for "D". 

This means that there are (12 x 11 x 10 x 9 = 11 880) different codes available. 

Using a bigger keypad with more "Wrong" keys - will increase security - by making an even larger number of codes available. 

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I specified a relay with a minimum coil resistance of 270 ohms. Use a higher value if you wish.

With a 12-volt supply - the maximum current through Q2 will be about 45mA.

This is well within the limits of a BC557 - which has an IC(max) of 100mA.

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There is nothing special about the transistors.

Provided the NPN/PNP requirement is satisfied - any small transistors with a gain (hfe) greater than 100 and an IC(max) of at least 100mA should do.

But remember that the pin configuration of your transistors may be different from that of the BC547 and BC557.

I have used a SPCO/SPDT relay. But you can use a multi-pole relay - if it suits your application.

Parts List

Suppliers

Worldwide RS Components

UK & Ireland Maplin

Do not use the "on-board" relay to switch mains voltage. The board's layout does not offer sufficient isolation between the relay contacts and the low-voltage components. If you want to switch mains voltage - mount a suitably rated relay somewhere safe - Away From The Board.

Construction Guide

Click here if you're new to constructing stripboard projects.

The terminals are a good set of reference points. To fit them, you may need to enlarge the holes slightly. Then turn the board over and use a felt-tip pen to mark the 21 places where the tracks are to be cut. Before you cut the tracks, use the "actual size" drawing to Check That The Pattern is Correctly Marked .

When you're satisfied that the pattern is right - cut the tracks. Make sure that the copper is cut all the way through. Sometimes a small strand of copper remains at the side of the cut and this will cause malfunction. Use a magnifying glass - and backlight the board. It only takes the smallest strand of copper to cause a problem. If you don't have the proper track-cutting tool, then a 6 to 8mm drill-bit will do. Just use the drill-bit as a hand tool - there's no need for a drilling machine.

Pattern for Cutting
The Tracks on the
Underside of the Board

Pattern for Cutting
The Tracks on the
Underside of the Board

Actual Size Of Pattern

Pattern for Cutting
The Tracks on the
Underside of the Board
ACTUAL SIZE

Next fit the 9 resistors and the Ten Wire Links. For the links - I used bare copper wire on the component side of the board. Telephone cable is suitable; the single stranded variety used indoors to wire telephone sockets. Stretching the core slightly will straighten it; and also allow the insulation to slip off.

The resistors are all shown lying flat on the board. However, those connected between close or adjacent tracks are mounted standing upright.

Add The Nine Resistors -
The Ten Wire Links

Add The Nine Resistors -
The Ten Wire Links

Now fit the remaining components - the diode - the two transistors - the five capacitors - the relay - and the IC socket. Note that the BC557 PNP transistor is the one drawn with a red emitter - and that the emitter is in the bottom position. If you are building the alternative version of this circuit using a BC547 NPN transistor - it should face in the opposite direction - with its emitter in the top position - similar to Q1.

Add The Remaining Components

Turn the board over and examine the underside carefully - to make sure that there are no unwanted solder bridges or other connections between the tracks. If you backlight the board during the examination - it makes potential problem areas easier to spot. When you're satisfied that everything is in order - add the 7 solder bridges.

Add The Seven Solder Bridges

Finish off by inserting the Cmos 4081 into the socket. Pin 1 of the IC should be in the top left-hand corner. Check that all 14 pins have entered the socket. Sometimes - instead of entering the socket - a pin will curl up under the IC.

You're Now Ready To Test Your Circuit

Keypad Layout

General Information

Test Your Finished Circuit Board

Schematic Diagram And Layout	The Rest of Ron's Circuits	Write To Ron	More Free-to-Use Circuits	Circuit Exchange International

Keypad Switch No.2 - Support Material

Parts List

Construction Guide

Actual Size Of Pattern

Keypad Layout

General Information