Build Your Secure 4-Digit Door Release Circuit Diagram

Description

This circuit is designed for operating an electrical door-release mechanism – yet it possesses potential applications beyond that. Input the desired four-digit code, and the relay will activate for the duration dictated by components C4 and R4. Utilize the relay contacts to power the release mechanism. The standby current is practically negligible, rendering battery power a viable option.

The circuit is initially drawn using a 12-volt supply – however, it functions effectively across a range of 5 to 15 volts. Simply select a relay appropriate for the supply voltage intended. Substitute the SPCO/SPDT relay with a multi-pole relay – if it aligns with your specific application requirements.

Schematic Diagram

IMPORTANT!!!

The “on-board” relay should not be utilized for switching mains voltage. The board’s layout does not provide sufficient insulation between the relay contacts and the low-voltage components. If mains voltage switching is desired – install a relay rated for safe operation in a secure location – Away From The Board.

Notes

Select the four keys you wish to utilize as your code – and connect them to “A B C & D”. Wire the common terminal to R1 and all the remaining keys to “E”. When you press these four keys – in the correct order – the relay will energize.

With the established values for C4 & R4, and R4 set to its maximum value – the relay will de-energize approximately one minute after “D” is released. However, if C4 is replaced with a 100nF capacitor – and R4 is replaced with a 4k7 fixed resistor – the relay will de-energize instantly upon the release of “D”.

Any keys not wired to “A B C & D” are connected to the base of Q2. Whenever one of these “Incorrect” keys is pressed – Q2 takes pin 1 low, and the code entry fails. Similarly, if “C” or “D” is pressed out of sequence – Q4 or Q3 will take pin 1 low, and the code entry will fail. Should an error occur while entering the code – simply begin again.

The Keypad must be of the type featuring a common terminal and a separate connection for each key. For a 12-key pad, look for 13 terminals. A matrix type with 7 or 8 terminals will not be suitable. A 12-key pad possesses eight “Incorrect” keys connected to “E”. To enhance security – utilize a larger keypad with more “Incorrect” keys.

Veroboard Layout