How To Build A Simple Keypad Switch With A Tamper Alarm

This circuit is designed to be used with the Universal Keypad Operated Switch. A few genuine mistakes - made during code entry - will not activate the device. But repeated attempts to find the correct code by trial and error - will cause the keypad to lock-up - and the Buzzer to sound. Connect the coil of a suitable relay to the Buzzer output - a

nd you can sound a Siren instead. The circuit uses a Cmos 4017 decade counter. It counts the number of times a "Wrong" key is pressed. When the count 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. With the values shown - this takes about five minutes. Then the keypad becomes active once more - and the alarm is silenced. Two different control switches are provided. The first - Sw1 - simply stops the noise. Removing the lock from the keypad is left to the timer circuit. Sw1 gives you the option to silence the alarm - and still retain the keypad protection. Sw2 shortens the time it takes for C3 to charge. It both removes the lock from the keypad - and stops the noise. Any small normally-open switch will do. If you use a magnetic reed-switch - you can hide the switch within the keypad housing - and use a magnet to reset the alarm. There are three connections to be made between the Tamper-Alarm and the Keypad Circuit. Connect "Com" on the Tamper-Alarm Circuit to "Com" on the Keypad Circuit. Connect "X" on the Tamper-Alarm Circuit to Pin 1 of the 4081. And connect "Y" on the Tamper-Alarm Circuit to the junction of the relay coil and the collector of Q6. The count advances by one every time a "Wrong" key is pressed. A wrong key is any key that takes pin 1 of the 4081 low. Since "E" takes pin 1 low...

Leave Comment

characters left:

New Circuits