Here is an electronic code lock, which can be used as a door latch or key for ignition, etc. Operation is fairly tricky and there lies the beauty of the circuit. There are a number of switches, actually nine, which should be operated in a particular and owner – configured -sequence, or else the circuit will not drive the final SCR. The switches can of course be reconfigured as desired. SCR Output should be used with a suitable driver such as a solenoid for the door latch or a relay for the key.
The circuit is shown in Schematic 29. CD 4066 quad bilateral switch is made use of here. 12 V DC powers the circuit through SI. External switches S2—S9 are not connected in the same order as their number and that is part of the trick.
52 is a dummy switch, when pressed, LED D2 lights up only to fool the intruder. It is not connected to the rest of the circuit.
53 is the next switch. This operates internal switch 1 of CD 4066. When this switch is pushed, it pulls up trigger terminal (Pinl), and switch across 13 and 2 (SW1) is closed. It stays closed because of the feedback action of 3.3M resistance (Rl). Dl lights up indicating the closure of one switch in the sequence.
This powers the second internal switch (SW2) consisting of 5, 4, 3 pins. Power reaches Pin 5 and Pin 4 is the trigger terminal. When S5 switch is pushed on internal switch across 5 and 3 (SW2) closes. It charges CI capacitor 47uf through 100K resistance (R3). It can now feed the next switch as long as the capacitor can hold charge. CI is discharged through D3 and R5, which mean that next switch should be operated before this charge finishes.
To add to the confusion, the next switch is actually two switches in series comprising of S4 and S7 with trigger terminal at Pin 6. If they are pressed simultaneously, only if they are pressed simultaneously, internal switch across pins 8 and 9 (SW 3) closes. This charges 47uF capacitor (C2) through 100 k resistor (R6) which discharges through D4 and R7. Hence one has to press the next switch S8 before this charge is completed.
When S8 with trigger terminal at Pin 12 is operated in time, internal switch across pins 11 and 12 (SW4) closes.
SCR is fired now through R9. SCR drives a solenoid or a coil or any other drive mechanism of the lock. Final LED (D6) also lights up.
S9 is a blind switch only to fool the inadvertent user. S6 is another clever switch. This lights up LED D5 but also starts a piezo buzzer warning that somebody is fiddling with the lock. A 2200 uF capacitor charges and keeps the buzzer for some time. Use of capacitor is deliberate. It also makes the rogue user take a quick run.
Construction with CMOS IC is simple and straight. The trick here is to lay out the switches in a haphazard sequence, known only to the authorized user. Provision must also be made for easy change of code. With nine switches available, permutations are really many. Wiring must be carefully done to avoid false triggering.
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