Digital Entry Lock Circuit

Posted on Jun 28, 2012

A block pinout diagram of the LS7220 keyless-lock IC is shown. The keypad must provide each key with a contact to a common connection. In this case, the common connection goes to the positive supply rail so that when a key is pressed, a positive voltage is passed through to the wire associated with that key. Each of the 12 keys are brought out to separate wires, and each wire is connected to a different pin of a 24-pin socket (SOI). To activate (unlock) the circuit, a preprogrammed four-digit access code must be entered in the proper sequence. The four-digit access code must be entered in the proper sequence.

Digital Entry Lock Circuit
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The four-digit access is programmed into the circuit by connecting jumpers between terminals of a 24-pin plug-in header. When the correct access code is entered (in the proper sequence), positive voltages appear at pins 3, 4, 5, and 6 of Ul. That causes IJ1 to output a positive voltage at pin 13, which is fed through resistor R2 to the base of Ql, causing it to conduct. With Ql conducting, its collector is pulled to ground potential, energizing relay Kl. The normally open relay contacts close, switching on any external device. Capacitor C2 controls the total time that the output of Ul at pin .13 is positive after the release of the first key. With a value of 3.3 for C2, active time after release of the first key is about two seconds, assuming a 6-V supply or four seconds with a 12-V supply. Therefore, if you push the subsequent keys too slowly, the relay might not close at all! To increase the time allotted for code entry, you will have to increase the capacitance of C2.

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