MYP Design Electronic Circuits and Components Notes

A Circuit Diagram Is a Map, Not a Picture

1. A circuit diagram shows what is connected to what using standard symbols, and it deliberately ignores where the parts physically sit on the board.
2. Lines are wires with no resistance, and a dot at a crossing means the two wires join while a plain crossing means they pass over each other.
3. Draw the positive supply as a rail along the top and 0 V as a rail along the bottom, so anyone reading it follows the circuit downwards.
4. Conventional current is always drawn flowing from positive to negative, even though electrons drift the other way, and every datasheet you will ever read uses that convention.
5. Six symbols cover most school circuits: cell, switch, resistor as a plain rectangle, LED as a triangle with two small arrows, motor as M in a circle, and buzzer.
6. Drawing a symbol wrongly is a real fault in your design folder, because a reader has no way of knowing what you meant to put there.

Series and Parallel Behave Completely Differently

1. In series the components sit one after another on a single loop, so exactly the same current passes through every one of them.
2. The supply voltage splits between series components, so two equal resistors across a 9 V battery take 4.5 V each.
3. Break a series circuit anywhere and the whole thing stops, which is why old fairy light strings went dark when a single bulb blew.
4. In parallel each component sits on its own branch straight across the supply, so every branch gets the full supply voltage.
5. Branch currents add up, so a battery driving four LEDs in parallel is supplying four times the current and flattens four times as fast.
6. Series resistances simply add, so 220 ohms plus 330 ohms is 550 ohms, while parallel resistances always come out lower than the smallest one on its own.

Voltage, Current and Resistance: One Sum Ties Them Together

1. Voltage, in volts, is the push that drives charge around a circuit, and it is measured across a component with a voltmeter.
2. Current, in amperes, is how fast that charge flows, and most school circuits work in milliamps where 1 mA is 0.001 A.
3. Resistance, in ohms, is how hard a component makes it for that current to get through.
4. Ohm's law links all three as V = I x R, and rearranging gives I = V divided by R, and R = V divided by I.
5. Worked example: put 9 V across a 470 ohm resistor and the current is 9 divided by 470, which is 0.019 A or 19 mA.
6. Power in watts is voltage times current, so that resistor is turning 9 x 0.019, about 0.17 W, into heat, which a standard 0.25 W resistor handles comfortably.

Resistors and Capacitors: Holding Back and Holding Charge

1. A resistor limits current and sets voltages, and its value is printed on the body as coloured bands.
2. Read the bands from the end with three close together: the first two are digits, the third is how many zeros follow, and the gold or silver band is the tolerance.
3. Red, red, brown, gold reads as 2, 2, one zero, so 220 ohms give or take 5 per cent.
4. A potentiometer is a resistor the user can change by turning a spindle, which is how a volume knob and a lamp dimmer work.
5. A capacitor stores charge on two plates and gives it back again, measured in farads, though real parts are microfarads or nanofarads.
6. A 100 microfarad capacitor across the supply smooths out the voltage dip when a motor starts, which stops a microcontroller resetting every time the wheels move.
7. An electrolytic capacitor has its negative leg marked with a stripe, and fitting one the wrong way round makes it swell and sometimes burst.

Diodes and LEDs: Why the Series Resistor Is Not Optional

1. A diode passes current one way only, and the bar on the symbol and the stripe on the body both mark the end the current leaves by.
2. That one way behaviour makes a diode a cheap guard against a user pushing the battery in backwards.
3. An LED is a diode that gives out light, with a long leg for positive and a short leg next to a flat on the rim for negative.
4. An LED does not follow Ohm's law, because above its forward voltage, roughly 2 V for red and 3.2 V for blue or white, its resistance collapses and the current runs away.
5. Connect one straight across a 9 V battery and it will pull far more than its 20 mA rating and burn out within seconds.
6. A series resistor fixes that by soaking up the leftover voltage, so on 9 V with a 2 V red LED the resistor drops 7 V and needs to be 7 divided by 0.02, which is 350 ohms.
7. No 350 ohm resistor exists in the standard range, so fit 390 ohms, since a little too much resistance only dims the LED while too little destroys it.

Switches Do More Than On and Off

1. A push to make switch only conducts while your finger is on it, which is what a doorbell or a game button needs.
2. A push to break switch does the opposite, staying closed until pressed, which is how a fridge light and many safety cut outs work.
3. Toggle and slide switches latch, staying wherever the user left them, so they suit a main power switch.
4. Switches are described by poles and throws: SPST turns one circuit on or off, SPDT sends one input to either of two outputs, and DPDT switches two separate circuits together.
5. Every switch carries a current rating on its body, and a 0.5 A switch will weld its own contacts together if you run a 3 A motor through it.

Breadboard, Stripboard, PCB: Three Stages of One Circuit

1. A breadboard grips component legs in sprung clips with no soldering, so you can pull a circuit apart and rebuild it twenty times in one lesson.
2. Its holes are joined in rows of five either side of the centre channel, and the two long rails down the edges are for power and 0 V.
3. Breadboard is for testing only, because the clips loosen, jumper wires drop out and a knock can change your circuit without you seeing it happen.
4. Stripboard is a board of holes with copper strips running along the back, where you solder the legs and then cut a strip with a spot face cutter wherever the track must be broken.
5. It is permanent, cheap and quick, and a missed track cut is the single commonest reason a stripboard circuit does nothing when you switch on.
6. A PCB is drawn in software such as KiCad or EasyEDA, then etched in school or ordered from a manufacturer, and it gives the neatest and most reliable result.
7. Work through all three in that order and take photos each stage, because that progression is exactly the making evidence criterion C asks for.