LogicRoo / Combinational Gates / Electronic Lock Circuit Using Digital Logic — Secure Door Lock | Logicroo

Electronic Lock Circuit Using Digital Logic — Secure Door Lock | Logicroo

A simple electronic lock circuit walkthrough — components, circuit connections, working principle, password input, logic operation, wiring steps, and an interactive electronic lock you can explore in the browser.

Parts bin

What you need

One counter, a handful of buttons and a transistor to do the heavy lifting. Test with an LED first and only fit the solenoid once the sequence behaves.

×1 CD4017 Decade Counter U1

×6 Push Buttons, momentary — the keypad S1–S6

×1 Push Button, momentary — re-lock S7

×1 BC547 NPN Transistor — output driver Q1

×1 1N4007 Diode — coil flyback D1

×1 12V Solenoid Lock or Relay LOCK

×2 100 kΩ Resistors — clock & reset pull-downs R1, R2

×1 1 kΩ Resistor — transistor base R5

×1 0.1 µF Capacitor — power-on reset C1

×1 LED + 220 Ω Resistor — unlocked indicator LED1, R6

×1 Breadboard BASE

×— Jumper Wires BASE

×1 9–12V Supply PWR

Operating principle

How a counter enforces an order

The clever part isn't the chip. It's that each key is wired to the one moment it's allowed to work.

A CD4017 keeps exactly one output high and steps to the next on every clock pulse. Normally the outputs are treated as results. Here they're treated as permissions: each correct key sits between the output for its position and the clock input.

So key 3 only does something while Q0 is high — that is, only as the first press. Push it later and the output feeding it is low, so closing that contact connects one dead wire to another and the counter doesn't move. The order is enforced by the wiring, not by any decision being made.

The wrong keys don't need to be clever either. They go straight from +V to the reset pin, which drops the counter back to Q0. Progress isn't just paused by a mistake, it's erased — someone guessing has to start over every time.

Changing the code

The code is the wiring. Move Q0's wire to a different key and that key becomes the new first digit. Nothing else changes.

Longer codes just use more outputs — Q0 through Q5 for a six-digit code, with the unlock taken from Q6 instead.

Repeating a digit is the one thing this design can't do. Two outputs would have to share a key, and pressing it once would clock the counter twice.

More keys make it stronger: with six keys a four-digit code is one of 1,296 — with ten keys it's one of 10,000.

Assembly

Building it on a breadboard

Five steps — get the counter resting reliably at Q0 before wiring a single key, or every later fault will look like a keypad problem.

Give the 4017 a clean resting state

Seat the counter: pin 16 to +V, pins 8 and 13 to ground. Run R1 from the clock input (pin 14) to ground and R2 from reset (pin 15) to ground so neither pin can float. Add C1 from +V to pin 15 — that brief charging pulse at power-up forces the counter to Q0 instead of a random output.

Gate each correct key with the previous output

This is the whole idea. Wire Q0 to one side of key 3, Q1 to key 1, Q2 to key 5, Q3 to key 2 — and tie all four of their other sides together into the clock input. A key can only pass a signal through if the output feeding it happens to be high, which means only the right key at the right moment advances the counter.

Wire every other key to reset

Take the remaining keys — 4 and 6 here — and run them from +V straight to the reset pin. Pressing one clears the counter to Q0 instantly, so a wrong key doesn't just fail to advance, it throws away whatever progress had been made.

Take the output off Q4

Q4 goes high only after all four correct keys have landed in order. Run it through R5 to the base of Q1, put the solenoid or relay coil between +V and the collector, and the emitter to ground. D1 sits across the coil, cathode to +V, to absorb the reverse spike when the coil switches off.

Add the re-lock button and test it

Wire S7 from +V to the reset pin as well — that's your lock button. Now test slowly: press 3, 1, 5, 2 and the lock should open. Press 3, 1, 4 and it should be back at square one. Try the sequence in the wrong order and confirm nothing happens.

If it misbehaves

Opens too easily, or never opens at all

Floating pins cause the first problem and missing gate wires cause the second. Check both before suspecting the chip.

It unlocks on almost any key — the clock input is floating. R1 has to hold pin 14 low between presses, otherwise stray charge clocks the counter on its own.
The first key never advances it — check that Q0 actually reaches that key's contact. Without the gating output, the key has nothing to pass through to the clock.
It resets itself halfway through a correct sequence — contact bounce. A single messy press can look like two events; keep the keypad wiring short and consider a small capacitor from the clock pin to ground.
The lock buzzes or the counter resets when it opens — the coil is dragging the supply down. Check D1 is fitted the right way round, and give the solenoid its own supply with a shared ground.
It locks itself again the instant the last key is released — Q4 is reaching the reset pin somewhere. Only Q4's path to the transistor should exist.
It comes up unlocked after a power cycle — C1 is missing or open. On power-up a 4017 can land on any output, and the power-on reset pulse is what prevents that.