Chain Reaction Robot:
Our robot is driven by a single motor and triggered by a single touch sensor. An extra motor was attached for programming purposes, but was not used.
The touch sensor was attached to the other robot but connected to our robot. Once the button was pressed, out robot was triggered to drive forward, dragging the other robot along with it.
Our robot then drove in front of an ultrasonic sensor of the next robot to trigger the next step of the EV3 chain.
Above: Overhead view of the robot (touch sensor not shown).
Below: Side view of the robot (touch sensor not shown).
Below: The touch sensor attached to the preceding robot.
Result: Our 3-brick-sub-system consistently worked.