Onboarding

Circuit Challenge: Design and Simulate a 12 V Switched Circuit

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Purpose

Confirm that a new Tech Team member understands basic circuit reasoning and can design a simple circuit and verify it in simulation before working on the car’s electronics. This is one design-and-simulate assignment, done individually. It complements the lecture and tutorial quizzes and the build documentation quiz. No physical build and no LiPo batteries are involved.

Tool: Falstad circuit simulator

The club has no established simulator licence, so this challenge uses the Falstad Circuit Simulator. It is free, runs in the browser, needs no account, and a finished circuit can be shared as a link (File > Export as Link), which makes it easy to review. If you strongly prefer it, Tinkercad Circuits (free Autodesk account required) is an acceptable alternative.

The assignment: 12 V switched indicator circuit

Design and simulate one circuit in Falstad:

  1. A 12 V DC voltage source, like the car’s 12 V power board terminals.
  2. Two LED branches in parallel, each with its own current-limiting resistor sized for 10-15 mA, assuming a 2.0 V LED forward voltage drop.
  3. One switch that turns both LED branches on and off together.
  4. A fuse (100 mA rating) in series with the supply, before the branches.

What to submit

  1. Circuit understanding answers, a few sentences each (this is Part 1 of the assessment; the Falstad circuit is Part 2):
    • U1. The resistor calculation: how Ohm’s law gave your resistor value for one branch, and the nearest standard resistor value you used.
    • U2. Predicted current versus the simulated current shown in Falstad for each branch, and why any difference.
    • U3. What would change if the two LEDs were wired in series instead of parallel: the voltage each LED sees, the single resistor value you would use for 10-15 mA, and what happens to total current.
    • U4. Why the 100 mA fuse sits in series with the supply before the branches: what it protects, and why that position and not after the split.
    • U5. Why each LED branch needs its own resistor rather than one shared resistor before the split.
  2. The Falstad share link (File > Export as Link) for the working circuit.

Send both to the Tech Team Lead in Discord.

Rules

  • Individual work, about 45-60 minutes.
  • The Tech Team Lead marks it and reports the result to Internal Affairs.
  • You may use Ohm’s law notes, resistor charts, and the Falstad help and examples. Nobody designs it for you.
  • Consistent with club onboarding standards: pass line above 70%, unlimited retries, attempts logged by Internal Affairs.

Grading (10 points, pass at 8: more than 70%)

  • Circuit understanding: the five answers U1 to U5 (5 points, one each).
  • Correct topology: 12 V source, fuse, one switch, two parallel LED branches each with their own resistor (3 points).
  • Simulation runs and simulated currents match predictions within reasonable tolerance (2 points).

Why this circuit

The same reasoning shows up on the car: 12 V distribution on the power board, correct current limiting for loads, fusing, and switching. Getting this right in simulation is the safe first step before working on real car wiring under SOP-TECH-05 Soldering and Electronics Assembly. SOP-TECH-01 Lithium Battery Safety applies once real batteries are involved - not in this challenge.