Running LED
Two button control
Relay clock
Toggle on-off
Electronic key
Bistable relay

Elevator control
Traffic light

Two hand control

Two hand control relays are often used for industrial machine safeguarding. Due to this system the operator must use both hands to control the machine. Just for fun I created a basic circuit which turns on a relay by pressing two buttons at the same time, with a small tolerance. By pressing one button C2 capacitor starts charging. In this condition Q1 turns on within 0,5 seconds or less and prevents Q2 from activating. If both buttons are pressed almost at the same time the relay turns on and activates C1 capacitor. In this condition Q1 turns on within 3-4 seconds. Then the operator has to release the buttons and press them again.
This is just an hobby circuit. Remember that two hand control systems must be approved by safety standards.

Watch on Youtube