MYP Design Sensors, Inputs and Outputs Notes

Input, Process, Output: The Shape of Every Electronic Product

1. Nearly every electronic product runs on the same three stage chain: input, process and output.
2. The input notices something, the process decides what to do about it, and the output is whatever the user sees, hears or feels.
3. A security light is a clean example: a movement sensor is the input, the controller is the process, and the lamp is the output.
4. Drawing those three boxes before you choose a single component stops you buying parts the product never needed.
5. Some products have several inputs feeding one process, such as a greenhouse controller reading both temperature and soil moisture.
6. Programming that middle box is covered in the microcontroller articles, so this one stops at the components on either side of it.

Sensors Turn the World Into a Voltage

1. A light dependent resistor drops its resistance as light rises, from roughly a megohm in the dark to a few hundred ohms in bright sunlight.
2. A thermistor does the same job with heat, and the common NTC type falls in resistance as it warms up.
3. Neither one produces a voltage by itself, so you pair it with a fixed resistor to make a voltage divider and read the voltage at the join between them.
4. Swap the two parts of that divider over and the voltage now rises with light instead of falling, which is the fastest way to flip a sensor's behaviour with no code at all.
5. A potentiometer is an input the user sets by hand, sliding smoothly between 0 V and the supply voltage as the spindle turns.
6. Pick the fixed resistor to roughly match the sensor's resistance at the level you care about, so the voltage swings most where your threshold sits.

Switch Inputs: Push, Reed, Tilt and Ultrasonic

1. A push switch is the simplest input there is, and it needs a 10k pull down resistor to hold the pin at 0 V while the switch is open.
2. A reed switch closes when a magnet comes near it, with nothing touching, which is how a window alarm and a bike speedometer sense position.
   1. The glass tube means it works through a plastic case and survives damp, but it cracks if you bend the legs close to the body.
3. A tilt switch holds a rolling ball that bridges two contacts past a certain angle, giving you a cheap orientation input for about 50 pence.
4. An ultrasonic sensor such as the HC-SR04 fires a pulse of sound and times the echo, measuring from about 20 mm out to 4 m.
5. Ultrasonic readings go wrong on soft or angled surfaces, because a cushion absorbs the pulse and a slanted wall bounces it away from the receiver.

Outputs: Light, Sound and Movement

1. An LED is the cheapest output going, draws about 20 mA, and always needs the series resistor worked out in the circuits article.
2. A buzzer comes in two flavours: an active buzzer makes its own tone from a steady voltage, while a passive one stays silent unless you feed it a changing signal.
3. A display runs from a single seven segment digit up to a 16 by 2 character LCD or a small OLED, and the more it shows the more pins and code it costs you.

Outputs That Move

1. A DC motor spins continuously and runs faster on a higher voltage, but it pulls far more current than any microcontroller pin can give it.
2. A servo turns to a commanded angle, usually anywhere between 0 and 180 degrees, and then holds that position, which suits a latch, a gate or a robot arm joint.
3. Servos have three wires for power, 0 V and signal, and two or more of them need their own battery pack rather than the board's supply.

Transistors and Relays Switch What a Chip Cannot

1. A microcontroller pin supplies roughly 20 mA, while even a small motor wants 300 mA or more, so the pin cannot run the motor on its own.
2. A transistor solves that as an electronic switch, where a tiny current into one leg controls a much larger current through the other two.
3. A typical school arrangement uses a BC337 transistor or a logic level MOSFET, with a 1k resistor between the pin and the control leg.
4. Fit a flyback diode across any motor, relay coil or solenoid, because a coil switching off throws out a voltage spike that kills transistors.
5. A relay is a mechanical switch thrown by an electromagnet, so the low voltage control side never touches the circuit being switched.
6. Reach for a relay when the load is mains powered or needs full isolation, and a transistor when you want silent, fast switching or speed control.

Analogue or Digital Decides How You Wire It

1. An analogue signal can take any value across a range, like the voltage from an LDR sliding smoothly as a cloud crosses the sun.
2. A digital signal has only two states, high or low, which is all a push switch ever gives you.
3. A board reads digital inputs straight off the pin, but an analogue one has to pass through an analogue to digital converter first.
4. An Arduino Uno splits 0 V to 5 V into 1024 steps on its analogue pins, so one step is about 4.9 mV.
5. Some parts sit between the two, and an ultrasonic module sends back a digital pulse whose length codes the distance.
6. Decide which kind each input is early, because it changes the pin you wire to and the resistors you need around it.
7. You can always cut an analogue reading down to digital by setting a threshold, so the choice is part of your design rather than something the sensor forces on you.