What it is?
This project can be considered a remarkable concept as it is based on a single active component. It can also be considered a logic circuit that requires few components.
How does the project work?
To start the relay it is necessary to press 4 buttons (S1, S2, S3 and S4). If any of the S5-S8 buttons is pressed, then the relay does not receive voltage. The keyboard can be a membrane with sensors or a set of 8 distinct buttons.
The supply voltage used must be equal to the working voltage of the relay used.
A transistor of the type BD135 will be able to switch up to 0.5A, while a 2N2222 works up to 0.2A.
List of required components:
8 x Switches
1 x D1 = 1N4007
1 x Relay (K1) = 12V
1 x T1 = BD135 / 2N2222
2 x Resistant (R1, R2) = 1k
1 x Terminal contact
1 x Breadboard test board + connecting wires
For a better understanding of the operation of the circuit and to make this assembly on the breadboard we will need electronic scheme presented below: