The Skipped Resistor
Tinkercad helpThe Skipped Resistor
Project 1 · A switch and a light

The Skipped Resistor

Your LED lit up, but Tinkercad put a star on it. The star means too much electricity is going through the LED. A real LED would burn out. Here's why it happened and how to fix it.

What went wrong

Both legs of the resistor are in the same column (column 28).

Why it matters

Holes in one column are joined, so the electricity skips the resistor. Nothing slows it down.

The fix

Lay the resistor flat so its legs are in two different columns.

1

See it happen

This is your circuit, simplified. Start the simulation and hold the button, like in Tinkercad. Then switch to the fixed version and compare.

▶Try it
?Why does it still light?

The LED does get electricity, so it lights. The problem is how much. Without the resistor, there's far more current than an LED can take. Tinkercad still shows the light, but it adds the star to warn you a real one would be ruined.

2

Inside a breadboard

Under the plastic, metal strips join some holes together. Click any hole to see which other holes it's joined to.

●Click a hole
Or try:

!On a real breadboard

Some real breadboards split each rail in the middle. If the red or blue line has a break, the two halves aren't joined. Tinkercad's Breadboard Small doesn't do this.

3

Why the resistor gets skipped

Electricity always takes the easiest path. Think of two roads between the same two places: a narrow road with a toll gate (the resistor), and a wide free road (the metal strip). Almost everyone takes the free road.

⇄Two roads

★New word

When electricity takes a shortcut around a part, it's called a short circuit. Your resistor was shorted out by the metal strip in column 28.

How much current flows? (Ohm's law)

1 · What each letter means
  • Vsthe supply voltage: the Arduino's 5V pin gives 5 volts
  • Vfthe voltage the LED uses up: about 2 volts for a red LED
  • Rthe resistance in the way, in ohms (Ω)
  • Ithe current: how much electricity flows, in amps (A). An LED is happy at about 0.02 A (20 milliamps) or less
2 · In words

Take the LED's share away from the supply voltage. Divide what's left by the resistance. The answer is the current.

3 · The equation
I=Vs−VfR

The bigger R is, the smaller the current. A resistor is there to make R big enough.

4 · Worked example: change the numbers
4

Spot the mistake

For each breadboard, decide: does the part do its job, or is it skipped? Look at which strip each leg is in.

5

Fix it, step by step

Do these in Tinkercad. The picture changes with each step: the faded part shows where things were, and the dashed box shows what moved.

★Remember

Two legs, two different strips. If both legs of a part share a strip (same column and same half, or the same rail), the electricity skips the part.

Sources and more