low cost CMOS lock

I~ low colt CMOS lock A decimal keyboard, a CMOS IC, three transistors and an opto-coup- Ier ... that is about all that is needed to make this electronic lock with a three digit combination. Locking is achieved by means of a cascade of analogue switches, each of which is connected, through a programming matrix, to one of the keys on the keypad. Suppose line A is connected to key 2, line B to key 9, and line C to key 5. If key 2 is now pressed ES1 closes and stays closed because of the current delivered to it through R7. If key 9 is then pressed ES2 closes and remains closed (because ES1 is already closed). Now all that is needed is to press key 5, where- upon ES3 closes thus activating the opto-coupler, whose transistor then conducts. The keys not used in the ABC code must all be connected to the D line. When one of these keys is pressed, in error or in ignorance, line D sets ES4 to an active high logic level (which it keeps because of R6) and T1 conducts and thus disables the cir...