3.5 Additional Sensor and Actor Devices

	The figure to the left shows a sketch of a standard 0.1 inch male header as it is used both for input and output plugs. This particular part is a sensor plug, as it has three pins in an asymmetrical layout. This encoding is very important in order not to confuse the polarity of the sensor plug, because some sensor devices may be critical to reverse voltage.
Please note that the next gereration Crickets, the so-called Red Dot Crickets, use 4-pin sensor plugs that are incompatible with the ones describes above. See the MIT Media Lab’s documentation for more information.
	Sometimes, there is the wish to connect ordinary switches to the Cricket input ports. They may be of the type shown on the picture to the left, but they may as well be mic-ro-switch-es, mercury switches, or of any other kind. This is fairly simple to realize, as the pair of wires that come from the switch just have to be soldered to a sensor plug’s ‘signal +‘ and ‘signal –‘ pins.
However, there is one thing that should be paid attention to: Although the sensor plugs are encoded by the unused pin as shown above, they still fit into the Crickets’ output ports. If this happens, both the attached sensor and the Cricket can be destroyed by excessive currents!     For this reason, it is suggested to insert a small 1kOhm resistor into one of the sensor lines, e.g. directly at the switch or at the sensor plug. This value will not harm the reading of the switch as it is still close enough to zero ohms.
	A touch-less sensor is also great for many purposes, as to kids, it sometimes appears like magic. But the Lego-compatible sensor shown to the left is not magic, just magnetic – one of the tiny reed contacts in the upper corner of the picture has been integrated into a flat type of Lego brick.     While the small glass tube just fits into the larger one of the two bricks at an angle of 45 degrees, the other Lego brick takes a tiny yet powerful bar magnet, diagonally as well.
When the two pieces approach each other closer than eight millimeters, the reed switch is triggered by the magnetic field and closes its contact. However, this happens only if the glass tube and the magnet are at a perpendicular position. Thus, the sensor can also be used as a revolution counter, as the contact will open and close twice when the magnetic piece is rotated by 360 degrees.     Just as commercial Lego switches do, this touch-less sensor already contains a 1kW resistor in its housing, so no external protector is needed here. A second one will not hurt however, so when in doubt, you better add a resistor at every sensor plug.     In the way that has been shown with the above examples, many useful sensors can be created for the Crickets. Most of the switches and variable resistors are also inexpensive parts which kids may use and modify in order to learn how natural parameters like temperature, pressure, and brightness are converted into their electrical equivalents, voltage or current.
	Inspired by the Crickets’ new ability to control four actors at the same time, some girls suggested to build up a Cricket "light show" using a couple of differently colored light bulbs. 12V, 4 watt bulbs have been chosen for this purpose, as up to 15 pieces of the same color could then be connected in parallel for great disco effects.

3.6 Mechanical Preparations

While Märklin Metall is very adequate to serve as a mechanical basis for pneumatics applications due to its high strength and durability, there is no standard piece in the construction kit that would fit to the cylinders. To provide a solid fixing of the Festo micro cylinders to a Märklin model, the adapter shown to the left has been created.