Circuit Simulation

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This is the Circuit simulation area of the course — how to use simulation software to design a circuit, test it on screen and find faults before you build it for real. The electrical theory, including logic gates and IC pin-outs, is in the six theory topics on the Electronics hub.

Concept 1

What is simulation software?

  • Simulation software (such as Yenka and the on-site simulators) models a circuit on screen: you place components, wire them together and the software calculates how it behaves.
  • You can read voltages and currents with virtual meters, view waveforms on a virtual oscilloscope, and switch inputs — all without any real parts.
Try it: the 555 astable simulator and the stripboard builder are simple simulators you can run in the browser; the Yenka guide (PDF) covers the full software.
Concept 2

Why simulate before you build?

  • Cheaper & safer — no components are wasted and nothing is damaged while you experiment.
  • Faster — change a value or a connection in seconds and see the effect immediately.
  • Test the design — confirm it behaves as intended and measure voltages, currents and waveforms safely on screen.
  • Find faults early — a printout or layout from the simulator shows up mistakes before any soldering.
✍️ How to say it

I simulated the circuit first because _______.

It also means _______.

See example reasons
"…because it does not waste or damage any parts." · "…it is quicker to change a value and test again." · "…it finds faults before I build it." · "…I can measure voltages safely on screen."
Practice 1

Give two reasons an engineer simulates a circuit before constructing it.

Answer
Any two of: it avoids wasting/damaging components; it is quicker to change and re-test; faults and design errors are found before building; voltages and waveforms can be measured safely on screen.
Concept 3

When to simulate

  • At the design stage, and always before construction.
  • Whenever you change a component value and want to see the effect (e.g. a different resistor or capacitor).
  • To investigate how a circuit behaves — resistor networks, logic, and timing such as a 555 astable.
Practice 2

You have a working design but want to make an LED flash faster. State how simulation helps before you rebuild the circuit.

Answer
Change the timing resistor/capacitor in the simulator and re-run it to check the new flash rate (and that nothing else breaks) before changing the real circuit — saving parts and time.
Concept 4

How to use it

A simple loop you can follow with any simulator:

  1. Build the design — place the components and wire them up.
  2. Run it — power the circuit and switch the inputs.
  3. Take measurements — virtual voltmeter, ammeter and oscilloscope.
  4. Compare with the expected result from your calculations.
  5. Refine — change values or connections and run again until it works.
You can also convert between a circuit diagram and a layout diagram, and check the IC pin-outs (741, 555, 7400-series) on the Theory page before wiring an IC.
Practice 3

After building a circuit in the simulator, you measure 0 V where you expected 9 V. State the next two things you would do.

Answer
Compare the measurement with the expected value, then check the build for the fault (e.g. a missing wire, wrong component value or a part the wrong way round), correct it and re-run.
Concept 5

Designing a system: input → process → output

Before simulating, sketch the design as three stages and choose a sub-system for each block. Identifying the stages also makes fault-finding easier.

Input
LDR voltage divider (senses light)
Process
741 comparator + transistor switch
Output
Relay drives a lamp

A system is built from three stages: an senses the surroundings, a decides what to do, and an does the job. Simulating first lets you find faults you build the real circuit.

Practice 4

A circuit sounds a buzzer when it gets too hot. Identify a suitable input, process and output sub-system.

Answer
Input: thermistor voltage divider. Process: comparator (switches when the temperature voltage passes a reference) driving a transistor. Output: a buzzer (or relay + buzzer).
Concept 6

Fault-finding from a simulation printout

Compare the voltages shown at the marked test points with what you expect, and check each part against the design. Common faults are:

  • Wrong supply voltage set on the source.
  • Wrong resistor value (or wrong colour code / BS1852 entered).
  • Wrong orientation of a polarised part — diode, LED, electrolytic capacitor or transistor put in the wrong way round.
  • Wrong IC pin connection — e.g. a 555 (or logic-gate) pin wired to the wrong place. Check it against the pin-out reference.
Once the simulation is correct, work through the pre-power-up checklist before building or powering the real circuit.
A power-rail circuit with a 470 ohm resistor and an LED drawn reversed (reverse-biased) between the rails +V0V R 470Ω LED
Fault: the LED (orange) is reversed — its cathode/arrow points to the + rail, so it is reverse-biased and will not light.
Practice 5 — identify three errors

A simulated lamp circuit does not work. Checking the test points against the design, the printout should show a 6 V supply, a 470 Ω resistor and a forward-biased LED, but instead shows: a 9 V supply, a 47 Ω resistor, and the LED arrow pointing at the + rail. Identify the three errors and the fix for each.

Answer
1) Supply set to 9 V not 6 V — reset the source to 6 V. 2) Resistor is 47 Ω not 470 Ω — fit the correct value (it limits the LED current). 3) The LED is reversed (reverse biased) so it cannot light — turn it round so its arrow/cathode faces the 0 V side.
Concept 7

Costing a build from component codes

Once a design works in the simulator you cost it from a parts list — each component's order code, pack quantity and pack price — to work out the total build cost. Real builds are rarely one-offs: here we cost the parts to make a class batch of 100 prototypes, which is why components are bought in packs.

Get prices and order codes from a supplier's online catalogue. The three used in Scottish schools are:

Resistors: always order 0.25 W carbon-film resistors — they suit every National 5 circuit and are sold in bags of 100 for about £1.10, so buying 100 prototypes' worth costs barely more than buying one.

ComponentSupplierOrder codeDescriptionPack qtyPack price
Resistor 1 kΩ 0.25 W carbon filmRapid62-03560.25 W ±5% CF, 250 V100£1.10
Resistor 10 kΩ 0.25 W carbon filmRapid62-03940.25 W ±5% CF, 250 V100£1.10
Red LED 5 mmKitronik35205 mm, 2.0 V fwd, 20 mA100£6.00
Electrolytic cap 100 µF 25 VRS711-1418radial, 25 V, ±20%100£8.50
BC548 NPN transistorRapid81-003045 V, 100 mA, TO-92100£6.00
555 timer IC (DIP-8)Kitronik2452NE555, 4.5–15 V10£2.50

Example codes and prices only — always check the supplier's live catalogue before ordering.

Practice 6

Each prototype uses 3 resistors, 1 red LED and one 555 timer. Using the packs above, work out the cost of the resistor, LED and 555 packs needed to build 100 prototypes.

Answer
Resistors: 100 boards × 3 = 300, so 3 bags of 100 = 3 × £1.10 = £3.30. LEDs: 100 needed = 1 bag of 100 = £6.00. 555 timers: 100 needed = 10 packs of 10 = 10 × £2.50 = £25.00. Total for these parts = £3.30 + £6.00 + £25.00 = £34.30 (about 34p per board).

🔧 Simulation tools

Open the on-site simulators to design and test circuits.

555 Astable simulator Stripboard builder Yenka simulation guide (PDF)
Interactive

The Operator — design the system

For each brief, choose an input, a process and an output sub-system that work together. Satisfy all three briefs to earn the 🎛️ The Operator badge.

Choose the sub-systems scored

Match the sensor to what is sensed, a switching process, and an output that does the job.

View my progress

Check

Check your understanding

Answer the multiple-choice questions, then mark yourself with the RAG self-check.

RAG self-check

Tap Red / Amber / Green for how confident you feel.