Legacy electronics: timers, logic and voltage comparison
Explain 555 monostable/astable timing, programmable control, logic gates, truth tables and operational-amplifier comparison.
Section Paper 4 · Systems & Control (2026–2027) · Systems & Control · legacy option
Exam scope and assessment notes
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Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which logic gate produces a true output only when both of two inputs are true?
What you will learn
- Distinguish a monostable one-shot from an astable repeating circuit and use a supplied time-delay relationship.
- Interpret simple logic gates and truth tables and explain how a comparator can control an output.
A timer can produce one pulse or repeated pulses
A 555 timer circuit can be arranged as a monostable, which produces one timed output after a trigger, or an astable, which repeats between states. The time depends on the supplied formula and component values; substitute units carefully and check whether the requested quantity is a pulse duration or cycle.
A programmable integrated circuit can implement time delays or other logic through programmed behaviour. The older syllabus names PIC controllers and 4000-series ICs. The circuit diagram, component values, supply and output load must all match the task; no online example authorizes constructing a live circuit.
Make logic behaviour explicit
AND is true when all required inputs are true; OR is true when at least one is true; NOT reverses a state; NAND and NOR are the corresponding inverted outputs. A truth table lists every input combination so the design can be checked systematically.
An operational amplifier may compare two voltages and switch an output when one crosses the reference. Combine this with an LDR or thermistor to create a control decision, but consider a threshold range, noise, component tolerance and a safe default state.
Sources for this lesson
Pause and explain
What is the main output behaviour of an astable timer?
Worked example
A greenhouse fan should start when temperature is above a set level and stop below it. Describe an input, comparison, output and one design improvement.
Show the worked solution
- Use a thermistor in a potential divider as the temperature-dependent input.
- Compare the divider voltage with a reference using a suitable comparator or control IC.
- Switch the fan output through a correctly rated driver or relay.
- Add hysteresis or a suitable control band to reduce rapid switching near the threshold.
Answer A thermistor-divider voltage can be compared with a reference, and a transistor/relay stage can control the fan. Add a gap between on and off thresholds if rapid switching near the set point is a problem. The real circuit requires component ratings, protection and teacher-approved testing.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A security light should turn on if either of two doors opens. Which logic gate matches the requirement?
Give me a hint
Translate 'either' into a truth table and check the both-open case.
Compare my reasoning
- Name the two door-switch inputs.
- Write the desired output for 00, 01, 10 and 11.
- Select the gate whose truth table matches.
OR matches: output is off only when both doors are closed; it is on for 01, 10 and 11. Check whether a fail-safe default is needed if a switch or wire fails.
Look for these in your work
- Lists all input states
- Chooses a matching gate
- Considers faults
A timed dispenser should release one short pulse after a button press, not keep repeating. Which timer mode is appropriate and why?
Give me a hint
Compare a single pulse with a repeating waveform.
Compare my reasoning
- Identify the required output sequence.
- Select monostable or astable behaviour.
- State what timing parameter must be set and tested.
Monostable mode is appropriate because a trigger produces one timed pulse. Set the pulse length from the supplied design formula and verify it in a simulated or teacher-approved low-voltage model; an astable would repeat continuously.
Look for these in your work
- Matches one-shot behaviour
- Explains why astable is unsuitable
- Names a verification step
A voltage comparator chatters as a sensor reading fluctuates around the threshold. What design idea could reduce rapid output changes?
Give me a hint
Use a defined hysteresis band and choose thresholds from application evidence.
Compare my reasoning
- Identify the noisy threshold crossing.
- Separate turn-on and turn-off thresholds slightly.
- Test both transitions across expected sensor noise and conditions.
Hysteresis gives different thresholds for switching on and off, so small fluctuations near one value do not repeatedly toggle the output. Choose the band from measured sensor variation and user needs, then test normal and fault conditions under supervision.
Look for these in your work
- Identifies threshold chatter
- Explains hysteresis
- Requires evidence-based calibration
Common mistakes
- Confusing one-shot timing with repeated oscillation
- Writing a truth table with missing input combinations
- Connecting a motor directly to a logic IC output without checking ratings
What can you explain now?
What is the main output behaviour of an astable timer?
Compare with the explanation
It repeatedly switches between output states without requiring a new trigger for every cycle.
An astable circuit generates a repeating sequence; a monostable produces one timed pulse.
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Make it stick
Recall the main decision in “Legacy electronics: timers, logic and voltage comparison”, name the evidence that supports it, and identify one limitation that still needs checking.
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