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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 coil 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.
The eddy sensor is a reliable part. However, they require an accurate set point that varies between boards, because of variations in the components.
Small Eddy Sensor
- Road Show, head jaw hit sensors (2)
- Theater of Magic, trunk hit sensor
- Scared Stiff, crate hit sensor
Twilight Zone Trough Eddy Sensor
What Goes Wrong?
Eddys are reliable, but suffer from various long-term issues.