When the visitor places a puck in a particular slot, the circuitry for the display is told two things: 1) there is a puck in this slot, and 2) whether or not is the right puck. This is accomplished by magnets imbedded in the pucks themselves. Refer to the image below left. Each slot has two reed switches. The one in the center (green circles) simply informs the circuit if there is a puck present. The one closer to the perimeter (blue circles) allows the circuit to determine whether or not it is the correct puck. (In the image, you can see the sensors for the slots corresponding to Uranus and Neptune.) When all the pucks are in, the circuit can give a thumbs up or down via the lights on the front of the panel (see image below right).

But here’s the problem: once a visitor has completed the activity, it is ruined for the next visitor because all the pucks are still in place. So, I built a little gadget to eject the pucks once the activity is complete. Refer again to the image above left. The motor on the left is connected to a shaft on which there are magnets (red circles). Once the activity is completed, a timer energizes the motor, which spins the shaft. The magnets on the shaft are situated to oppose magnets in the pucks. When the two magnets come face to face when the shaft is rotating, the pucks are ejected.

In this display, the visitor simply matches the planet to the moon by positioning the large wheels. This gadget is pretty simple. The circuit senses the position of the wheels with – you guessed it – magnets! See below. These images show the two sides of the ring that holds the reed switches (left), and the magnets (right). When the wheels line up properly, you get the Correct! light on the front of the panel.

Below is a picture of the tiny little circuit that analyses the inputs, and below that is the schematic.