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When resistors are connected in series it is equivalent to having a single resistance whose value is the sum of the series resistors.
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But calculating parallel resistance is very different.
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Think about why series capacitors would combine like parallel resistors, and vice versa.
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A diode is like a one-way valve. Current can flow when the anode is more positive than the cathode. Otherwise flow is largely blocked.
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Transistors have only a mediocre analogy in the water pipe system. Here we will only consider MOSFET transistors, which are most common these days.
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MOSFET Transistors (or MOS for short) are four terminal devices. However, two of the terminals are usually tied together. So we usually only concern ourselves with three: the Gate, the Source, and the Drain. (The fourth terminal is the Bulk.) The transistor samples the voltage difference between the Gate and the Source. That voltage then determines the effective resistance between the Drain and Source. (Please note that this is a significant simplification of transistor behaviour, but it should be good enough for almost everything we do.)
Transistors are very high gain. This means the input voltage on the Gate only has to change by a tiny amount to cause the output resistance to change dramatically. In digital systems we generally think of transistors as simply on (low resistance) or off (almost infinite resistance).
One gotcha with transistors is that they care about the polarity of the voltage applied to them. In other words if the voltage on them is reversed they will not function correctly.
As the name implies, and LED is a diode. Current only flows in one direction. But when it does and LED emits light.
LEDs generally should not be connected directly to a battery or power supply. They need the current to them limited. Usually a resistor is the most convenient choice. Typically the resistor should be chosen to put about 10mA thru the LED. Most LEDs drop about 2V across them when running. But blue or white LEDs drop closer to 3V.
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There are many sensors used in robotics. But typically they act mostly like the devices listed above. A light sensor will have an output which switches just like a transistor. Similarly for a hall effect sensor. Nevertheless, there are some differences.
Some digital sensors such as hall effect sensors and optical encoders will look like a transistor with a resistor pullup. The schematic will look something like this:
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This output can be connected directly to a digital input. The transistor will pull the signal to ground (when appropriate). And at other times the pullup resistor will pull the signal to the positive supply, usually +5V.
If the digital sensor has an open drain output, i.e. no pullup resistor, then you will have to supply one. Otherwise the signal will never give any output other than 0.
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