The circuit presented here can be-used as a lock for important elec¬tronic/electrical appliances. When card is inserted inside its mechanism, depending upon the position of punched hole on the card, a particular appliance would be switched on. The card is inserted just like a floppy disk inside the disk drive. This card should be rectangular in shape with only one punched hole on it. T7 o The circuit uses eight photo-transistors (T1 through T8). When there is no card in the lock, light from incan¬descent lamp L1 (40-watt, 230V) falls on all the photo- transistor de¬tectors. Transistor T8 is used as enable detector for IC1 (74LS244). When light is incident on it, it conducts and its collector voltage goes low. This makes transis¬tor T16 to cut-off, and its collector voltage goes high. This logic high on its collector terminal will inhibit IC1 as long as light is present on photo-transistor, IC1 will get enabled only when the card is completely inserted inside the lock mechanism. This arrangement en-sures that only the selected appliance is switched on and prevents false op¬eration of the system. You can make these cards using a black, opaque plastic sheet. A small rect¬angular notch is made on this card to indicate proper direction for insertion of the card. If an attempt is made to insert the card wrongly, it will not go completely inside the mechanism and the system will not be enabled. When card for any appliance (say appliance 1) is completely inserted in the mechanism, the light will fall only on photo-transistor T1. So only T1 will be on and other photo-transistors will be in off state. When transistor T1 is on, its collector voltage falls, making transistor T9 to cut-off. As a result, collector voltage of transistor T9 as also pin 2 of IC1 go logic high. This causes pin 18 (output Q1) also to go high, switching LED1 on. Simulta¬neously, output Q1 is connected to pin 1 of IC2 (ULN2003) for driving the relay corresponding to appliance 1. Similarly, if card for appliance 2 is inserted, only output pin 16 (Q2) of IC1 will go high”making LED2 on while at the same time energising
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http://electronicsforuefylinux/circuit/feb2003/apr99_cardlock.pdf
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http://electronicsforuefylinux/circuit/feb2003/apr99_cardlock.pdf