Theory — Logic Gates and ICs
Back to Electronics hub GlossaryLogic gates and truth tables, the control circuits you must be able to describe (op-amp comparator and the 555), and the IC pin-outs on the data sheet.
Logic gates & truth tables
Key words
- Logic gate
- a part that gives a 1 or 0 output from 1/0 inputs.
- Truth table
- a table of every input and the output it gives.
- Combinational
- gates joined so one gate's output feeds the next.
A logic gate takes digital inputs (1/0) and gives a digital output set by its truth table. You must identify a gate from its symbol or truth table and complete a truth table. The symbols below are the distinctive shapes (the IEC operator is in brackets).
Combinational logic — work out the intermediate level first. Gates can be chained (up to 4 inputs, 2 outputs). Here an AND gate gives the intermediate signal G = A AND B, then an OR gate gives the output F = G OR C. Fill the G column before the output — exactly what the exam's "complete the truth table" question expects.
| A | B | C | G = A·B | F = G + C |
|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 0 | 1 |
| 0 | 1 | 0 | 0 | 0 |
| 0 | 1 | 1 | 0 | 1 |
| 1 | 0 | 0 | 0 | 0 |
| 1 | 0 | 1 | 0 | 1 |
| 1 | 1 | 0 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 |
An AND gate gives an output of 1 only when inputs are 1. An OR gate gives 1 when input is 1. A NOT gate its input.
A system has two outputs from inputs A and B: X = A AND B and Y = A NOR B. Complete both output columns for all four input rows.
Answer
You can now…
- identify a gate from its symbol or truth table.
- complete a truth table, working out any in-between level first.
Control circuits you must describe
Key words
- Comparator
- an op-amp that compares a signal with a set reference voltage.
- Astable
- a circuit that switches by itself — a continuous square wave.
- Monostable
- one trigger gives one output pulse of a fixed length.
You should be able to give a short "how it works" description for each of these circuits — and recognise the circuit diagram. All resistors are drawn the UK (BS) way, as a rectangle. The transistor switch is the fourth circuit you must describe.
Op-amp comparator. The op-amp compares two voltages: a sensor divider on one input and a set reference level on the other. When the sensor voltage rises above the reference, the output swings high (LED on); below it, the output is low. This switches an output at a chosen light or temperature level.
Learn it in Lesson 2 · The comparator
555 astable. With no trigger needed, the 555 runs continuously: C charges through R1 + R2 and discharges through R2, so pin 3 gives a steady square wave that switches between high and low. Bigger R or C values make it slower (lower frequency) — a flashing-LED or tone circuit.
Learn it in Lesson 4 · The 555 astable
555 monostable. It has one stable state. A short trigger pulse on pin 2 makes pin 3 go high for one fixed time set by R and C (a longer time for bigger R or C), then it returns low on its own — a one-shot timer (e.g. a timed light).
Learn it in Lesson 3 · The 555 monostable
A 555 is wired so that one push of a button makes an LED stay on for exactly 5 seconds, then it goes off on its own. Is this an astable or a monostable, and what two components set the 5-second time?
Answer
You can now…
- describe an op-amp comparator.
- describe a 555 astable and a 555 monostable, and say what sets their timing.
IC pin-out reference
Key words
- IC
- an integrated circuit — a "chip".
- Pin-out
- which job each pin (leg) of the chip does.
- Notch
- the mark that shows the pin 1 end of the chip.
These are the pin-outs given on the SQA data sheet, so you can recognise them in the exam and complete IC pin-out diagrams. The notch (or dot) marks the pin 1 end.
741 op-amp (8-pin)
555 timer (8-pin)
7408 — Quad 2-input AND
7432 — Quad 2-input OR
7400 — Quad 2-input NAND
7402 — Quad 2-input NOR
7486 — Quad 2-input XOR
7404 — Hex NOT (inverters)
On a 555 timer, which pin is the output, and on a 7408 AND-gate chip which pins are the power supply (Vcc) and ground (0 V)?
Answer
You can now…
- read the 741, 555 and 7400-series pin-outs.
- find the power (Vcc) and ground (0 V) pins on a chip.
Logic Lord — build the truth tables
Fill in the output (Q) column for every gate. Tap a cell to cycle it through 0 and 1. Get every cell right to earn the Logic Lord badge — completing the six Theory badges also unlocks Theory Triumph.
Check your understanding
Pick an answer to see feedback on it. You can then try the others. Your score counts first tries only.
RAG self-check
Tap Red / Amber / Green for how confident you feel.