Data Sheet

-34-

43. Triple Input ‘OR’ Gate

Build the circuit as shown on the left. If you turn on any one of the

switches, you will light up the lamp (76). If you want to turn off the

lamp (76), you must turn off all the switches. Electronic OR Gates

can have two or more inputs but the function is still the same. All

inputs must be False (OFF) for output to be False (OFF).

44. The Resettable Fuse

Build the circuit as shown on the left. If you turn on any one of the

switches, you will light up the two LEDs at the same time. If you

want to turn them off, you must turn off all the switches.

Positive-Temperature-Coefcient, or PTC, thermistors – also known

as resettable fuses are devices that have very low resistance until

a current is reached, then they get warm and the resistance

changes limit the current.

Pretend the reed switch (83) and LEDs are a PTC fuse and pretend

the magnet (7) is a piece of ice. Place the magnet (7) near the

reed switch (83) to cool down the PTC fuse, allowing current to

ow and the LEDs to light. Move the magnet (7) away from the

reed switch (83) and the PTC fuse gets warm, trips and prevents

current from owing through the LEDs and thus they do not light.

1st level