Free Details Of How To Build A Simple Keypad Tamper-Alarm - Using Cheap Off-the-Shelf Components.

COMMENTS SUGGESTIONS	Keypad Tamper-Alarm - Support Material	MORE KEYPAD CIRCUITS

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Circuit Description

Click Here For A Photograph Of The Prototype.

Circuit Diagram For
A Keypad Tamper-Alarm

The Keypad Tamper-Alarm circuit uses a Cmos 4017 decade counter.

It counts the number of "Wrong" keystrokes.

When it reaches eight - it sounds the alarm and locks the keypad.

The alarm will continue to sound - and the keypad will remain locked - for a period of time set by the values of C3 and R3.

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"Y" is connected to the negative side of the keypad circuit's relay coil. 

While the keypad switch is off - the negative side of its relay coil is high.

So "Y" holds the reset pin of the 4017 high - through D2.

This disables the 4017 decade counter - and it cannot respond to keystrokes.

In other words - while The Keypad Operated Switch is off - pushing the keypad buttons will have no effect on the Tamper-Alarm circuit.

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When The Keypad Operated Switch is turned on - the negative side of the relay coil is connected to ground.

So "Y" is no longer able to hold the reset pin high.

Instead - R2 takes pin 15 low.

This resets the 4017 counter to zero - and pin 3 goes high.

Now - every time the 4017 clock input is taken high - the counter will advance by one.

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"X" connects the 4017 clock input to pin 1 of the 4081 - through R1.

Whenever a "Wrong" key is pressed - and pin 1 of the 4081 goes low - "X" takes the 4017 clock input low.

When the key is released - and pin 1 of the 4081 goes high again - "X" takes the 4017 clock input high.

When the 4017 clock input goes high - its counter advances by one.

In other words - every time a "Wrong" key is pressed and then released - the 4017 counter advances by one.

After eight "Wrong" key have been pressed and released - the count will reach pin 9.

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When Pin 9 goes high - it does two jobs.

Firstly, it supplies base current to Q1 - through R5.

This switches the transistor on.

The transistor connects the negative side of the buzzer to ground - and the buzzer sounds.

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The Q1 collector is also connected to the "Com" terminal on the keypad.

When Q1 switches on - it connects the "Com" terminal to ground - through D1.

While the "Com" terminal is held low by Q1 - none of the 4081 inputs can be taken high by the keypad switches.

So the code cannot be entered.

In other words - the keypad is disabled.

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The second job done by pin 9 of the 4017 is to take pin 13 high - through C3.

Because C3 is discharged - it acts like a wire link between pins 9 and 13.

While pin 9 holds pin 13 high - the counter is inhibited.

In other words - while pin 13 is high - inputs at the clock terminal will not advance the counter.

So the count will stay at eight - the buzzer will continue to sound - and the keypad will remain disabled.

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How long this lasts depends on the values of C3 and R3.

With the values shown - it takes about five minutes for C3 to charge through R3 - and for pin 13 to go low.

When pin 13 goes low - the count advances to nine.

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Pin 11 only goes high very briefly.

It takes pin 15 high - through D3.

The instant pin 15 goes high the counter is reset to zero - so pin 11 will go low.

And when pin 11 goes low - R2 takes pin 15 low again.

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Since pin 9 is once again low - Q1 is switched off.

So the buzzer is silent - and the keypad is no longer disabled.

Also - because C3 has discharged into pin 9 - the capacitor is ready to inhibit the count the next time eight is reached.

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You don't have to use the pin 9 output.

You can choose to activate the Keypad-Alarm after fewer "Wrong" keystrokes.

All you need do is connect R5 and C3 to the output of your choice - and D3 to the next output in the count.

For example - if you connect C3 and R5 to output four (pin 10) - you must connect D3 to output five (pin 1). 

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R1 and C2 are there to slow down the speed at which the clock input operates.

The time it takes C2 to charge and discharge increases the length of the input pulse required to advance the count.

This reduces the effect of keypad contact-bounce. It ensures that any given keystroke will only advance the count by one.

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D1 creates a one-way path. 

It allows Q1 to take the "Com" terminal low.

But - when the transistor is switched off - it prevents the negative lead of the buzzer (or relay) from taking the "Com" terminal high.

If this were allowed to happen - current through some of the keypad keys might be excessive.

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D2 also creates a one-way path.

When the keypad switch is off - D2 allows the negative lead of the relay coil to take the reset pin high.

But when the keypad relay is energized - it prevents Q6 from holding the reset pin low.

If Q6 were to hold the reset pin low - then D3 couldn't take it high when the count reaches nine.

D3 also creates a one-way path.

It allows a high pin 11 to take the reset pin high - but prevents a low pin 11 from holding the reset pin low.

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Relay coils and some sounders produce large reverse voltage spikes that will destroy Cmos ICs.

D4 short circuits these spikes at source - before they can do any damage.

Its presence means that you can connect the coil of a small relay to the buzzer terminals - and use its contacts to switch a much louder siren.

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The relay should have a minimum coil resistance of about 270 ohms. Use a higher value if you wish.

With a 12-volt supply - the maximum current through Q1 will be about 45mA.

This is well within the limits of the BC547 - which has an IC(max) of 100mA.

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Sw1 simply disconnects the buzzer (relay coil) from Q1.

It means that you can choose to turn-off the noise - without disabling the keypad lock-out feature.

Sw 2 is the reset switch.

It provides a rapid charge path for C3 - through R4.

It resets the 4017 counter - stops the noise - and unlocks the keypad.

R4 is there to limit the maximum surge current through the switch.

This means that Sw2 can be any type of normally-open switch.

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Construction

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 18 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 .

Actual Size Of Pattern

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

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.

Keypad Tamper-Alarm
Construction Guide

Next make and fit the Four Wire 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.

Then fit the 5 resistors. The resistors are all shown lying flat on the board. However, those connected between close or adjacent tracks are mounted standing upright. See The Photograph Of The Prototype.

Next - fit the transistor and the four diodes. Pay particular attention to the orientation of the diodes. Note that D1 is mounted with its kathode (the side with the bar) pointing downwards. See The Photograph Of The Prototype.

Keypad Tamper-Alarm
Construction Guide

Fit the three capacitors and the IC Socket. Pay particular attention to the orientation of the electrolytic capacitor. Note that electrolytic capacitors generally have a light coloured stripe down the side next to the negative terminal. C3 is mounted with its positive terminal facing downwards. See The Photograph Of The Prototype.

Next - 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 4 solder bridges.

Finish off by inserting the Cmos 4017 into the socket. Pin 1 of the IC should be in the top left-hand corner. Check that all 16 pins have entered the socket. Sometimes - instead of entering the socket - a pin will curl up under the IC.

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 Tamper-Alarm - Support Material

Circuit Description

Construction

Actual Size Of Pattern