|MadSci Network: Physics|
Motion sensors come from a group of devices developed to sense the presence or lack of something. This something could be solid, gas, light, heat or many other types of things. There are actually several means by which to get the desired results from a sensor - depending on what you are trying to "see". The three most common are optic (light), ultrasonic (sound) and resistive (electric). A motion sensor uses the optic and ultrasonic most often. We will look at ultrasonic (optic uses a different type of energy (light) but the *basic* principle is the same.
Ultrasonic sensors are devices that have a sort of speaker in them. The sound this "speaker" puts out is of a very high range (frequency) so you and I can't hear it. The sound creates waves (vibrations) through the air. The waves travel out until they hit something solid enough to bounch off. Some of these bouncing waves return to the sensor. The sensor picks up these waves with a transducer,(it is like your ear drums). The transducer creates a signal that the sensor can understand. Now the brains of the sensor (circuits) time how long it took for the waves to go out and back. If everything is normal, say nothing moves in front of the sensor, the sensor decides everything is the same as the last time it looked. If the waves are returned quickly (something moves in front of the sensor) the sensor "sees" the difference in the timing and will react by switching something on or off.
So what you have is a device that takes energy such as electric, converts it to some more usable energy like sound, sends it out and see's what kind of response it gets. Then converts this response back to the original energy and acts on the results. Some key words you can look up are
transducer photoelectric cell thermocouple optical-electrical devices ultrasonic devicesThese all have a use in sensors and will help you understand the way sensors work. As for your project. Both of your suggested methods would work if set up carefully. I would use a fluid level sensor for the switching device. This type of sensor is made for getting wet and is easy to set for the depth you want the tub to fill.
Some suggested reading would be: The Build It Book of Electronic Projects by Ruddf Graf, or How Things Work by Thomas Primmel
Always be very careful when using electrical devices. It makes life very comfortable when used properly. BUT it can be VERY DANGEROUS if used in the wrong way or the wrong place, like around water.. Make sure the device you use is designed for wet areas.
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