Eddy Sensor Boards

In 1993, Williams introduced eddy sensors with Twilight Zone. Eddy sensors can detect the ball through the playfield, and are useful where an opto is awkward. Twilight Zone used eddy sensors to identify the ceramic powerball.

The eddy sensor depends on an IC specifically designed for the purpose, the TDA0161. This chip makes a ooil oscillate, and can detect the losses in the coil. When a a ball comes near, the losses increase and the circuit stops oscillating. This design is more stable than designs that use frequency shift like a beachcomber's metal detector.

Eddy sensors can detect fast-moving balls. This is important for the Scared Stiff crate; the Theater of Magic trunk; the Cirqus Voltaire ringmaster; and the heads on Roadshow.

Tales of the Arabian Nights did not use eddys for its exit lanes. Instead, it placed an opto pair looking through the playfield. Safecracker and Cactus Canyon used special reed switches to detect the ball.

Earlier eddy boards used optocouplers as outputs. By Roadshow, Williams switched to a discrete circuit which increased the complexity of the board but presumably reduced the cost.

What Goes Wrong?

Eddys are reliable, but suffer from various long-term issues.

Twilight Zone The trough opto is one of the worst problems in the game, second only to the clock. It has a number of problems.

Connector Spring Failure The trough eddy board and its sensor are joined by a small cable. With the vibration from the trough, the pins lose spring and the connection becomes unreliable.

Damaged Sensor The sensor is adjustable, and needs to be within 1/16 of teh ball to work. If it is too close, the ball hits it and destroys the plastic. If it gets to the wire, the sensor will permanently fail.