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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This circuit was specifically designed to recharge alkaline cells. The unusual connection of the transistor in each charging unit will cause it to oscillate, on and off, thus transferring the charge accumulated in the capacitor to the cell. The orange LED will blink for around 5 times a second for a 1.37V cell. For a totally discharged cell the blinking is faster but it will decrease until it will come to a stop when the cell is charged. You may leave the cell in the charger as it will trickle charge and keep it at around 1.6V. To set the correct voltage you have to connect a fresh, unused cell and adjust the trimmer until oscillations set in, then go back a little until no oscillation is present and the circuit is ready to operate. You should use only the specified transistors, LED colors, zener voltage and power rating because they will set the final voltage across the cell. A simple 9V charging circuit was also included: it will charge up to around 9.3V and then keep it on a trickle charge: the green LED will be off while charging and will be fully on when the battery is close to its final voltage.

A 2.5VA transformer will easily charge up to 4 cells at the same time although 2 only are shown in the schematic. In order to minimize interference from one circuit to the other they have nothing in common except the transformer and, in order to show a balanced load to the transformer, half of the charging units will use the positive sinewave and the other half the negative sinewave. Make sure to use high beta transistors such as BC337-25 or better BC337-40. Given the dispersion of the transistor parameters it might happen that oscillations do not take place. Use a slightly higher zener voltage: 7.5V instead of 6.8 or a green led in place of the orange ones.

All types of alkaline cells can be recharged: it will take 1 day for a discharged AA cell or 9V battery and up to several days for a large D type cell. The best practice is not to discharge completely the cell or battery but rather to give a short charge every so often although admittedly this is not easy to achieve. Do not attempt to recharge a totally discharged cell or a cell showing even the slightest sign of damage.

I tried successfully to recharge NiMH cells as well. Although the charging profile for these cells is quite different from alkaline cells, the circuit seems to work fine provided you do not leave them in the charger forever, because of the possibility of overcharging especially for the smaller batteries.

The mains transformer must be suited for the voltage available in each country: usually 230Vac or 115Vac.

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