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

COMMENTS SUGGESTIONS	Universal Keypad Switch - Support Material	MORE KEYPAD CIRCUITS

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Circuit Description
Click Here For A Photograph Of The Prototype.
	The key you've connected to "E" is used to energize the relay. And the four keys you've connected to A, B, C & D - are used to de-energize it. All the other keys on the keypad are connected to "F". ======== When "E" is pressed - current from the "Com" terminal flows through D2 and R9 - into the base of Q6. This switches Q6 on - and the transistor connects the negative side of the relay coil to ground. This causes the relay energize. Q6 also connects the green LED to ground. This provides a visual indication that the relay is energized. ======== Q5 & Q6 are wired to form a complementary latch. When Q6 switches on - it also connects the base of Q5 to ground. In so doing - it switches Q5 on - and current through R10 & Q5 flows into the base of Q6. Thus, Q6 uses Q5 to supply itself with base current. In this way - the relay remains energized after "E" is released. For a detailed account of how a complementary latch works - click on the link below. ======== To de-energize the relay - you have to press A, B, C & D in the right order. The IC is a Cmos 4081. It has four two-input AND gates. These gates only produce a high output when both inputs are high. ======== To begin with - pin 1 is held high by R5. This enables gate 1. When "A" is pressed and pin 2 is taken high - the output of gate 1 - at pin 3 - goes high. This output does two jobs. Firstly, it latches itself on by holding pin 2 high through R2. In this way - pin 2 will stay high after "A" is released. Secondly, Pin 3 enables gate 2 by taking pin 5 high. In other words - pin 3 is doing for gate 2 - what R5 does for gate 1. ======== Now - in exactly the same way - when "B" is pressed and pin 6 is taken high, the output from gate 2 latches itself on through R3. And it enables gate 3 by taking pin 12 high. ======== Again - in exactly the same way - when "C" is pressed and pin 13 is taken high, the output from gate 3 latches itself on through R4. And it enables gate 4 by taking pin 8 high. ======== Now - when "D" is pressed and pin 9 is taken high - the output from gate 4 latches itself on through R6. But this time it also switches Q4 on - through R8. ======== Q4 shorts the Q6 base current to ground. Without its base current - Q6 switches off. So the relay drops out - and the green LED is extinguished. ======== When Q6 switches off - the relay coil takes its collector high. It also takes the base of Q5 high. So Q5 switches off. And the base current Q5 was supplying to Q6 - is cut off. In other words - the complementary latch is unlatched. ======== The "right" keys are connected to A,B,C & D. The keys connected to "F" are the "wrong" keys. If one of the "wrong" keys is pressed - current through R7 will flow into the base of Q3. This switches the transistor on - and it connects pin 1 to ground. The initial gate 1 "enable" provided by R5 is removed - and pin 3 goes low. Pin 3 takes pin 5 low - so the gate 2 "enable" is removed. Pin 4 takes pin 12 low - so the gate 3 "enable" is removed. Pin 11 takes pin 8 low - so the gate 4 "enable" is removed. The sequence of support that each gate provides - by enabling its successor - is removed the moment pin 1 is taken low. In other words - every time you press one of the "F" keys - the code entry sequence fails. ======== D1 allows the key you've connected to "E" - to act as though it's connected to "F". But it prevents the keys you've connected to "F" - from behaving as though they are connected to "E". In other words - "E" is able to switch Q3 on - but "F" cannot energize the relay. ======== This is important for two reasons. Firstly, after the correct code has been entered - pin 10 is high and Q4 is switched on. Because Q4 diverts the Q6 base current to ground - "E" is prevented from turning Q6 on. However - D1 allows "E" to behave as though it's connected to "F". "E" can switch Q3 on - take pin 1 low - reset the code entry sequence - and so switch Q4 off. When "E" is pressed - it begins by resetting the code entry sequence - and then it energizes the relay. ======== The second reason is that - when it comes to entering the code - "E" is a "wrong" key. D1 allows it to act as such - by permitting it to switch Q3 on. ======== D2 prevents current - through R10, Q5 & R9 - from reaching the base of Q3 - through D1 and R7. Given the addition D1 makes to the Q3 threshold voltage - D2 is probably unnecessary. But I've included it to be absolutely sure. ======== There is one other situation that must also be covered. That's where the "right" four keys are pressed - but in the "wrong" order. This eventuality is dealt with by Q1 and Q2. ======== To begin with - if any key other than "A" is pressed - nothing will happen. Until "A" is pressed - the code entry sequence cannot start. And - once "A" is pressed - it must be followed by "B". ======== If it's followed by "C" - then pin 4 will still be low. While pin 4 is low - it's connected internally to the negative line. So the emitter of Q1 is connected to the negative line. And current from "C" will flow through its base-emitter junction. This will turn Q1 on - and take pin 1 low. ======== In exactly the same way - if "D" is pressed before "C" - pin 11 will be low. While pin 11 is low - it's connected internally to the negative line. So the emitter of Q2 is connected to the negative line. And current from "D" will flow through its base-emitter junction. This will turn Q2 on - and take pin 1 low. ======== It's only when the four keys are pressed in the right order - and the Q1 and Q2 emitter paths to ground are closed - that the code entry will be successful. ======== R1 limits the base current through Q1 and Q2. The capacitors are there to slow down the inputs of the Cmos 4081 - and make them less sensitive to stray signals. ======== 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. ======== I've drawn a SPCO/SPDT relay. But you can use a multi-pole relay - if it suits your application. Relay coils produce large reverse voltage spikes that will destroy Cmos ICs. D3 short circuits these spikes at source - before they can do any damage. ======== 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 Q6 will be about 45mA. This is well within the limits of the BC547 - which has an IC(max) of 100mA. ======== 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. ========
Click Here To Learn More About The Complementary Latch.

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 the relay somewhere safe - Away From The Board.

Parts List

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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 30 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 make and fit the Fourteen 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.

Add The Fourteen Wire Links

The next stage is to fit the 11 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.

Add The Eleven Resistors

Now fit the transistors and diodes. Note that the PNP transistor is the one with its emitter arrow coloured red. And note that D2 is mounted standing upright. See The Photograph Of The Prototype.

Fit the Capacitors, the IC Socket and the Relay.

Add The Capacitors
The IC Socket And
The Relay

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 6 solder bridges.

Add The Solder Bridges To
The Underside Of The Board

Add The Solder Bridges To
The Underside Of The Board

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.

Test Your Finished Circuit Board	Three Ways To Modify The Circuit	Add A Beep To The Keystrokes	Add A Tamper Alarm To The Keypad

Keypad Layout

One of the advantages of making your own keypad is that you can choose the number of keys you use. For example, with a 16-key pad the number of choices are: 16 for "A" 15 for "B" 14 for "C" 13 for "D" This gives (16 x 15 x 14 x 13) = 43 680 possible codes.

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

Universal Keypad Switch - Support Material

Circuit Description

Parts List

Construction

Actual Size Of Pattern

Keypad Layout

General Information