Become a subscriber (Free)

Join 29,000 other subscribers to receive subscriber sale discounts and other free resources.
Don't worry -- your e-mail address is totally secure. I promise to use it only to send you MicroZine.

Transistor Electronic Code Lock

This project, a transistor electronic code lock from 1968,  shows how successive transistor chains can be used to implement a key coded door lock.

The interesting thing is that it is all done without any kind of programming. Only transistors are used yet it achieves the same effect as a programmable system.

If keys are pressed in the wrong sequence the whole thing resets.

The way it works is to charge up a chargeable element in a transistor block which enables the next block so control is passed from key to key (each key is associated with a chargeable transistor block).

If you don't hit the keys within the discharge time then you have to start again and you can't just start at any old key, you have to press keys in the right order to get to the last one which enables the door solenoid.

Executive Summary of the Transistor Electronic Code Lock

This specification describes an electric code lock including successive code steps, which are to be energized in a predetermined order for the release of the lock. Improper actuation of one code step results in resetting of all previously set steps and alarm.

Background of the Transistor Electronic Code Lock

The security or lock mechanisms available today range from the simple to the complex. The simple devices tend to be unreliable and easily tampered with or bypassed, whereas the more complex devices may be extremely bulky and expensive. Most coded access devices have a limited number of combinations, and the combinations may be difficult to change.

An example of a coded actuating device is a mechanical combination lock, which requires the rotation of a dial to a plurality of correct positions in a predetermined sequence in order to actuate a latch or bolt mechanism. The mechanical arrangement employed is highly complex, especially in locks having a large number of possible combinations. An additional drawback is the known ability to determine the proper combination by surreptitious means.

Summary of the Transistor Electronic Code Lock

The present design eliminates the disadvantages of such previously known devices and is primarily directed on an electronic code lock mainly characterized by a number of serially arranged code step circuits, each of which includes an eletcronic relay device and a chargeable element and a circuit part common for all steps, each step' being so connected to succeeding step that the charging of the chargeable element of a previous step prepares the charging of the chargeable element of the following step at the application of activation pulses to said step in a predetermined order.

Figure 1 : : schematic for the transistor electronic code lock

View larger image here.

Description of the Transistor Electronic Code Lock

The design will now be more closely described in connection with the accompanying drawing, FIG. 1, which illustrates a practical embodiment of the design. The figure shows a cross wire field with one side connected to plus potential. A pushbutton switch 0-9 and a resistor 10-19 are arranged in each horizontal wire.

Between the switches and the resistors are crossing vertical wires A-F and each such vertical wire is connected to a certain horizontal wire. The junctions are marked a-f, The vertical wires are connected to minus potential via resistors 26, 29, 32, 35, 38 rectifiers 27, 30, 33, 36, 39, capacitors 20-25, transistors 41-45 and a common resistor 60.

These vertical wires together with their respective minus connection circuits are nominated as code steps of the established chain.

The base electrodes of the transistors 41-45 are via resistors 47, 49, 51, 53 and 55 connected to the next previous step at a point between the rectifier, which can be a diode, and the capacitor of said step and via suitable coupling resistors 48, 50, 52, 54 and 56 to minus. A further transistor step 46 completes the chain and has its base connected to minus via resistor 58 and also directly to the emitter of transistor 45, which latter electrode in turn is connected to common resistor 60 via a blocking diode 57.

The emitter electrode of transistor 46 is connected to minus via a resistor 59 and the collector electrode is connected to plus via un-locking means, for example a winding which is conducting state releases the lock mechanism.

According to the design the pushbuttons are to be pressed down in a predetermined order for releasing the lock. A special safety circuit arrangement is therefore arranged which reacts on wrong pushbutton actuation sequence. This circuit arrangement is shown as individual discharge circuits for the capacitors 20-24, and represented by the diodes 28, 31, 34, 37 and 40, which are 15 commonly connected to a transistor 64. Transistor 64 is part of a control circuit, which also includes transistors 62 and 63 and associated coupling elements 61, 66 and 67.

Transistors 62 and 63 are interconnected base-to-collector and a resistor 66 and 67, respectively, is connected at one end to the base-collector junctions and at the other end to the emitters. The junction between the emitter of transistor 62 and the resistor 67 is via a resistor 68 connected both directly to the interconnected horizontal wires and via resistors 69 and 70 to minus, while the emitter of transistor 63 and resistor 66 are directly connected to minus.

The junction between resistors 69 and 70 is directly connected to the base of transistor 64, the emitter of which is directly connected to minus, while the collector is connected to diodes 28, 31, 34, 37 and 40. A coupling capacitor 71 is arranged in parallel to the resistor 70.

A special alarm circuit includes transistor 65, resistors 73, 74, 75 and the alarm relay 76, 77 and is connected 35 between plus and minus. This circuit is also connected to the collector of transistor 64 via a diode 72 of opposite orientation to diodes 28, 31, 34, 37 and 40 in the discharge circuits of the code steps.

The lock release operation has to be made stepwise and in predetermined step order, from left to right in the figure. This means actuation of the push buttons 1, 3, 0, 5, 4, 7 in mentioned order for the iIIustrated example.

Actuation of button 1 results in charging of capacitor 20 from plus via point a, resistor 26, diode 27 and resistor 60 to minus. The transistors 62 and 63 are then biased via resistors 11 and 68 and an ignition pulse resulting from the voltage drop over resistor 60 at the charging of the capacitor 20. The voltage over resistors 69 and 70 will be so low that transistor 64 remains non conducting.

By charging the capacitor 20 the transistor 41 receives base potential and goes conducting, thus preparing the charging circuit for the capacitor 21, which when button 3 is depressed will be charged via resistor 29, diode 30, the collector-emitter path of transistor 41 and the common resistor 60. The same conditions as described above are then repeated for transistors 62, 63 and 64.

When capacitor 21 is charged the transistor 42 receives base voltage and goes conducting thus preparing the charging circuit for capacitor 22, etc., until the whole chain has been passed and the transistor 46 has been made conducting by depression of the last button 7. Transistor 46 then completes a circuit from the plus to minus via the lock relay which then releases the lock.

If the wrong button is actuated, for example button 0 instead of button 3 at the second step operation, no charging circuit is prepared for the capacitor 22. The ignition pulse then fails to appear, which pulse should be generated due to voltage drop over resistor 60 and this prevents transistors 62, 63 from being conducting.

Transistor 64, on the other hand, receives enough base voltage via the circuit including resistors 10, 69, 70, to be conducting and thus completes a discharge path for the capacitor 20 via diode 28 and this last mentioned transistor.

As shown in the figure all discharge circuits are so constructed that all already charged capacitors are discharged as soon as a following button actuation is wrong. If two 5 or more buttons are actuated simultaneously two or more of the resistors 10-19 are parallel connected and the transistor 64 will be conducting in spite of the voltage drop over the transistor combination 62, 63.

Further when transistor 64 is made conducting the alarm circuit 76 77 is activated.

The transistor electronic code lock arrangement according to the present design shows a relatively simple and cheap device which includes automatic blocking and alarm release when wrongly operated. The illustrated embodiment can of course be 15 modified to include a greater or smaller number of code steps, if desired, and other code combinations by selecting other coupling points in the cross coupling field are also possible.

Click here for more project ideas.

New! Comments

Have your say about what you just read! Leave me a comment in the box below.

Claim Your: Useful

"Arduino Software Guide"

   Right Now...

Jump from the transistor electronic code lock page to
Best Microcontroller Projects Home Page.

Privacy Policy | Contact | About Me

Site Map | Terms of Use


Visit our Facebook Page:

   Click Here

Recent Articles

  1. Using the MAX6675 or How to easily measure Extreme temperatures

    How to use the MAX6675 and an Arduino to measure temperatures from 0°C to 1024°C with two components: A chip - the MAX6675, and a Sensor: - a type-K thermocouple.

    Read more

  2. How to use the DHT22 (or DHT11) with an Arduino; full code and description. Also including a comparison of the DHT11 vs DHT22.

    The Essential Guide to the DHT22/11 humidity sensor including a library code guide for the Arduino. Learn how to determine Humidity, Dew point and Heat Index.

    Read more

  3. How to Use the MAX7219 to drive an 8x8 LED display Matrix on the Arduino.

    The Essential Guide to the MAX7219; What it is and how you can easily use one in any of your projects.

    Read more

Sign up for MicroZine
''The'' Microcontroller Newsletter

Enter your first Name and primary email address in the form below:

And receive absolutely FREE a full project for:

"Measuring Analogue Voltages
Without An ADC"

(Using only one pin).

Instant Download:
You Can
Get It Right Now

Warning: This project could be  Removed 
at any time.  

It will  NOT be 
available indefinitely SO
To avoid 
disappointment  get it:


Don't worry -- your e-mail address is totally secure. I promise to use it only to send you MicroZine
Remember this is a project with full description and fully debugged C Source code - and it's not available from the main website.

You can only get it through this newsletter.

To get exclusive access Enter your first name Name and primary email address Now in the form above.:

But wait !

There's more...

You'll receive more
free and exclusive reports as well as site information and site product updates

Scroll up to the form above and sign up NOW. Don't forget it's FREE and if you don't like it, you can unsubscribe at any time.

Click Here Now to use the form above to get your Valuable information absolutely free.

Readers Comments

"I wanted to thank
you so so so much
for all the information
you have provided in
your site it's


- Ranish Pottath

"This site really is
the best and my favorite.
I find here many useful
projects and tips."

- Milan


"Awesome site,
very, very easy and nice
to navigate!"

- Matt

Learn Microcontrollers

"Interested in

Sign up for The
Free 7 day guide:


"I am a newbie to PIC
and I wanted to say
 how great your
site has been for me."

- Dave


"Your site is a great
and perfect work.

- Suresh


"I couldn't find the correct
words to define
yourweb site.

Very useful, uncovered,
honest and clear.

Thanks so much for
your time and works.

- Anon