About: a MAD man who fancies himself a genius

Hello friends

Today I am back with another project called DIY ULTRA CHEAP COMBINATION LOCK CIRCUIT

A combination lock is a type of locking device in which a sequence of numbers or symbols is used to open the lock. There are two types of combination lock available, Analog and digital. In analog type the sequence may be entered using a single rotating dial which interacts with several discs or cams, by using a set of several rotating discs with inscribed numerals which directly interact with the locking mechanism. On other hand digital locks are using keypads to enter the code

Now we are going to make a digital combination lock circuit using simple components. yes I know I can use an AUDINO. But my goal is different

Step 1: MY GOLE

To make a ULTRA CHEAP COMBINATION LOCK CIRCUIT using cheep and simple parts


This (Fig 1) is the circuit diagram of ULTRA CHEAP COMBINATION LOCK CIRCUIT

To make this circuit you need

1. CD4017 IC

2. Two 100k resistors

3. One 10k resistors

4. Two 1k resistors

5. One 2N3904 Or similar NPN transistor

6. One BC 556 Or similar PNP transistor

7. Tow capacitors of 1uF

8. 2 PCB

9. 12v relay

10. Push Button Switch (minimum 9 ,max any)

11. Wires (use Pitch Crimp Connectors, its looks clean)

12. 5 1N4148 Zener diode


This video will show you how the ULTRA CHEAP COMBINATION LOCKS CIRCUIT work

Step 4: HOW IT'S WORK?

This circuit uses a decade

counter IC CD4017. CD4017 is a 16 pin CMOS decade counter/ Divider. It takes clock signal from the clock input and turns on the 10 output in sequence, each time when it receives clock input pulses. It can count from zero to ten and its outputs are decoded. The IC starts counting from Q0, Q1 to Q9 also this IC allows user to count the carry using the Carry out CO 12th pin. High Signal in the pin-15 will reset the count and starts the counting again from Q0 It has 16 pins and the functionality of each pin is explained as follows:

· Pin-1: It is the output 5. It goes high when the counter reads 5 counts.

· Pin-2: It is the output 1. It goes high when the counter reads 1 counts.

· Pin-3: It is the output 0. It goes high when the counter reads 0 counts.

· Pin-4: It is the output 2. It goes high when the counter reads 2 counts.

· Pin-5: It is the output 6. It goes high when the counter reads 6 counts.

· Pin-6: It is the output 7. It goes high when the counter reads 7 counts.

· Pin-7: It is the output 3. It goes high when the counter reads 3 counts.

· Pin-8: It is the Ground pin

· Pin-9: It is the output 8. It goes high when the counter reads 8 counts.

· Pin-10: It is the output 4. It goes high when the counter reads 4 counts.

· Pin-11: It is the output 9. It goes high when the counter reads 9 counts.

· Pin-12: Carry out

(This is divided by 10 outputs which are used to cascade the IC with another counter so as to enable counting greater than the range supported by a single IC 4017. By cascading with another 4017 IC, we can count up to 20 numbers. We can increase and increase the range of counting by cascading it with more and more IC 4017s. Each additional cascaded IC will increase the counting range by 10)

· Pin-13: This pin is the disable pin.

(In normal mode of operation, this is connected to ground or logic LOW voltage. If this pin is connected to logic HIGH voltage, then the circuit will stop receiving pulses and so it will not advance the count irrespective of number of pulses received from the clock.)

· Pin-14: This pin is the clock input.

(This is the pin from where we need to give the input clock pulses to the IC in order to advance the count. The count advances on the rising edge of the clock.)

· Pin-15: This is the reset pin

(The reset pin should be kept LOW voltage (ground) for normal operation. If you need to reset the IC, then you can connect this pin to HIGH voltage.)

· Pin-16: This is the power supply (Vcc) pin.

(This should be given a voltage of 3V to 15V for the IC to function.)

These circuits need 4 digit codes to unlock. This means that there are 210 ways in which this combination can be set, which means that the probability of an intruder to break the code is 1 out of 210 ways. The fig show how I calculate it

In my case pin 3,2,4,7 is the order which are switches A to d in Fig. 5 respectively

In normal conditions, when we provide a power to this circuit via pin 16, a positive voltage appears at pin 3 of IC. When you press the switch A, the positive voltage from pin 3 pass to the pin 14 of IC, which is a clock input pin.

Then the positive voltage will moves to the pin 2. So the next switch is B .the same happens there, a positive voltage from pin 2 go’s to pin 14.As you can see, if we do not press the switch B, it will stop the loop, and if you push a wrong switch a negative voltage go’s to the base of the BC556 transistor via 3.2k resistor and it will become conductive. So a positive voltage go’s to pin 15, it will reset the count. If you press the code in correct order (pin 3,2,4,7)a positive voltage go’s through the base of the 2N3904 transistor via 10K resistor, its makes the 2N3904 is conductive and relay become on. If the relay is connected to a lock it will opens the locker

If you need more complex lock you can use pin 1 to 10 as input and pin 11 as output. To open this circuit you will need a 9 digit password. This is the order pin 3,2,4,7,10,1,5,6,9 .if you use a 9 digit password you must use more switches like 20 or more

Step 5: PROS & CONS


1. Simple and cost-effective

2. Easy to make

3. Minimal components required

4. No need for programming

5. You can add many switches if you want; it will increase the complexity


1. Not user-friendly to assign the password

2. Not recommended for high level security


This circuit has so many possibilities.The only limit is your can use it as a door locker or a safe can us it as a improvised car key or a complex switch for a light.even as a complex alarm clock ;-)

you can add a timer switch to the main so no one get the time to hack the system




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