Digital Code Lock – DIY Project

A Digital Code Lock project is introduced in this article which is a really simple and low cost DIY project. There are many other digital code lock circuits available some use a certain set of switches to be pressed in a certain time and some use microprocessor or different ICs.

This circuit is a really simple circuit that uses NE555 timer IC and contains 10 switches to control the lock. Only 6 out of 10 switches are used in a particular sequence to open the lock.

Note: The circuit is tested by myself and in 100% working condition.


Components Required:

  1. Center taped step down transformer 6V-0-6V
  2. 4x 1N4001 Diode
  3. 10x Push Button
  4. NE555 IC
  5. 6V Relay (100 ohm)
  6. 1 uF Capacitor (16V)
  7. 0.01 uF Capacitor
  8. 5 uF Capacitor (16V)
  9. 1000 uF Capacitor (16V)
  10. LED red
  11. 1 Mohm Resistor
  12. 2x 10 Kohm Resistor
  13. 1 Kohm Resistor
  14. 820 ohm Resistor
  15. Solenoid

Circuit Diagram:

Digital Code Lock DIY Project(Click to enlarge image)


Here the keying-in code is rather unique. Six switches are to be pressed to open the lock, but only two switches at a time. Thus a total of three sets of switches have to be pressed in a particular sequence. (Of these three sets, one set is repeated.)

An essential property of this electronic code lock is that it works in monostable mode, i.e. once triggered, the output becomes high and remains so for a period of time, governed by the timing components, before returing to the quiescent low state. In this circuit, timer IC 555 with 8 pins is used. The IC is inexpensive and easily available. Its pin 2 is the triggering input pin which, when held below 1/3 of the sup-ply voltage, drives the output to high state. The threshold pin 6, when held higher than 2/3 of the supply voltage, drives the output to low state. By applying a low-going pulse to the reset pin 4, the output at pin 3 can be brought to the quiescent low level. Thus the reset pin 4 should be held high for normal operation of the IC.

Three sets of switches SA-SC, S1-S8 and S3-S4 are pressed, in that order, to open the lock. On pressing the switches SA and SC simultaneously, capacitor C3 charges through the potential divider comprising resistors R3 and R4, and on releasing these two switches, capacitor C3 starts discharging through resistor R4. Capacitor C3 and resistor R4 are so selected that it takes about five seconds to fully discharge C3.

Depressing switches S1 and S8 in unison, within five seconds of releasing the switches SA and SC, pulls pin 2 to ground and IC 555 is triggered. The capacitor C1 starts charging through resistor R1. As a result, the output (pin 3) goes high for five seconds (i.e. the charging time T of the capacitor C1 to the threshold voltage, which is calculated by the relation T=1.1 R1 x C1 seconds).

Within these five seconds, switches SA and SC are to be pressed momentarily once again, followed by the depression of last code-switch pair S3-S4.

These switches connect the relay to out-put pin 3 and the relay is energised. The contacts of the relay close and the solenoid pulls in the latch (forming part of a lock) and the lock opens. The remaining switches are connected between reset pin 4 and ground. If any one of these switches is pressed, the IC is re-set and the output goes to its quiescent low state. Possibilities of pressing these reset switches are more when a code breaker tries to open the lock.
LED D5 indicates the presence of power supply while resistor R5 is a cur-rent limiting resistor.

Summary: Press switches SA SC in unison at first ,then press switches S1 S8 in unison just after releasing SA SC within 5 seconds,after releasing S1 S8 again press SA SC in unison within 5 seconds and just after it press S3 S4 in unison and your relay will be energised.

– You can enlose this circuit in a suitable enclosure and put digit numbers on all the buttons from 0-9. And remember the combination so only you can open the lock through the code. You may also add a green LED at the end just before the relay in series with a suitable value resistor so you can check the status of the relay if its on or off just as i have added in the breadboard circuit.

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