DIY an electronic card lock security system

This is a DIY electronic card lock security system that can be used as a lock to open or close important electronics/appliances. When the card is inserted, a specific device will open based on where the holes are punched in the card. The card should be rectangular with only one punched hole in it. Unused/damaged cards (i.e. expired credit cards, student cards, etc.) can be used with this circuit.

How the Cardlock Security System Works

The electronic card lock security system circuit uses eight phototransistors (T1 to T8) to sense light. When there is no card in the lock, light from incandescent lamp L1 (40 watts, 230 volts) shines on all phototransistor detectors. Transistor T8 serves as the enable detector for IC1 (74LS244). When light is incident on it, it conducts electricity and its collector voltage becomes low. This causes transistor T16 to turn off and its collector voltage to become high. As long as there is light on phototransistor T8, this logic high on its collector terminal inhibits IC1.

IC1 is only enabled when the card is fully inserted into the locking mechanism. This arrangement ensures that only selected devices are turned on and prevents incorrect operation of the system.

You can make these cards using black opaque plastic sheets. A small rectangular notch is made on this card to indicate the correct orientation for inserting the card. If you try to insert the card by mistake, it will not fully enter the mechanism and the system will not boot.

When the card of any device (e.g. Device 1) is fully inserted into the mechanism, light will fall on the phototransistor T1 only. So only T1 will be on and the other phototransistors will be off. When transistor T1 is turned on, its collector voltage drops, causing transistor T9 to cut off. As a result, the collector voltage of transistor T9 and therefore pin 2 of IC1 becomes logic high. This causes pin 18 (output Q1) to also go high, turning on LED1. At the same time, output Q1 is connected to pin 1 of IC2 (ULN2003) and is used to drive the relay corresponding to device 1. Similarly, if the card of device 2 is inserted, only output pin 16 (Q2) of IC1 will become high level, and LED2 will light up at this time. Simultaneously energize the device 2 relay via ULN2003. The same goes for other situations/devices.

The system generates a musical tone while the card is in the mechanism. This is achieved with the help of diodes D1 to D7, which provide a wire-OR connection at their common cathode junction. When any output of IC1 is logic high, the common cathode junction of diodes D1 to D7 also becomes logic high, allowing IC3 (UM66) to produce a musical tone.

In this electronic card lock security system, circuit IC1 (74LS244) is used as a buffer for Schmitt trigger. All outputs of this IC (Q1 to Q7) are connected to IC2 (ULN2003) which is used as a relay driver. IC2 consists of seven high current relay drivers with integrated diodes. Therefore no external freewheeling diode is required.

When the input of this IC goes to logic high, the corresponding output goes to logic low and the relay connected to that pin becomes energized. This turns on the specific device and corresponding LED.

Once a specific card is inserted to open a specific relay, that relay is locked via its second pair of contacts. Therefore, even if the card is removed, the specific device remains on. The same goes for all other relays/appliances. After removing the corresponding card, the only way to de-energize the latching relay is to close the corresponding switch (S1 to S7), which will cut off power to the required relay.

The +5V and +12V supplies can be obtained by using the usual arrangement of step-down transformers, rectifiers, filters and regulators (using 7805 and 7812 etc.), you can find circuit designs in the Power Supply category.