This section describes the circuit diagram of simple electronic combination lock. The central part of this circuit is the IC LS 7220. This circuit is used for the activation of a relay which controls a device on/off states while pressing a combination of 4 digits from keyboard. To set the combination, connect the appropriate switches to pin 3, 4, 5 and 6 of the IC through the header. Then connect the rest of the switches to pin 2 of IC. This is for correcting an invalid key press. The output relay will be activated if and only if the correct key combination is pressed for a preset time. In this circuit, the value of preset time is determined by the capacitor C1. Preset time can be increased by increasing the value of capacitor C1.
Pin Diagram of LS 7220
- Capacitors: C1 (1uF 25V), C2 (220uF)
- Resistors: R1 (2.2K)
- Transistors: Q1 (2N3904 NPN Transistor)
- Diodes: D1 (1N4148 Rectifier Diode)
- Other Components: K1 (12V SPDT Relay)
- U1 (LS7220 Digital Lock IC)
- S1-S12 (SPST Momentary Pushbutton)
- HD1 (12 Position Header)
- Use a good quality PCB to assemble this circuit.
- For the key pad, arrange switches in a 3X4 matrix on a PCB.
- The circuit can be operated from 5V to 12V.
- In this circuit, preset time for key press is set as 6s.