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.

1 What Goes Wrong?

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

2 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. There are two solutions. You can solder the wires directly to the boards. Or you can buy or make a new cable.

Damaged Sensor

The sensor is adjustable, and needs to be within 1/16 of the 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. Although the sensor may look beaten up, the problem is still usually the cable.

Out of Tune Circuit

The design doesn't accommodate allvariations of parts and conditions, and may end up being too robust and never respond to a close ball - or it may never start up. One fix to try for a stuck red light is to add some twists to the cable,

Failed Opto Output

The output is an opto across the switch matrix. A short to the switch matrix can destroy the opto. Replace the opto if the LED works correctly but there is no switch signal.

2.1 How does Twilight Zone keep track of where the powerball is?

The powerball is a ceramic ball used in 'Twilight Zone' and the machine discovers it's location by using a combination of four different methods:

• 1. By using the eddy sensor in the under playfield tunnel between the piano, camera, powerfield exit and dead end that ends up in the slot. If a ball triggers the piano, camera or dead end switches and then the slot switch triggers *but* the eddy wasn't triggered in between, then we know the powerball must have gone through.

• 2. By using the eddy sensor in the ball trough

• 3. By failing to grab a ball successfully on the loop magnets.

• 4. By counting the powerball as it goes in and out of containers, such as the lock and trough. So if the machine knows there's two balls in the trough and you drained the powerball during single ball play, it'll be out the trough again in 2 balls time.