Fig. 02 · Robotics Lab · Ohm's law with a meter

LED + resistor

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Bench

No reading yet

Meter reads (mA)

No reading yet

Current draw

0 mA

Build and break

1BuildNext up
Unplug it
The trainer comes plugged in, its LED lit at 220 Ω. Unplug the USB-C cable before you touch a part: never work on a live board.
2
Strip it to the bare PCB
3
Fit the low parts
4
Fit the tall parts
5
Solder every joint
6
Jumper cap and meter
7
Predict, then plug in
8
Move the jumper to 1 kΩ
9
Move it to 10 kΩ
10
Back to 220 Ω
11
Break it: no resistorBreak it
12
Break it: the LED in backwardsBreak it
13
Break it: C1 in backwardsBreak it
14
Break it: leave out R4Break it
15
Break it: a cold joint on the LEDBreak it
16
Break it: swap the bench supply's leadsBreak it
17
Break it: 12 V on the 5 V inputBreak it
18
Break it: a 10 mA current limitBreak it
19
Back on USB

Break it on purpose

Each fault is one that catches real builders. Trigger it, read the symptom on the bench, then fix it.

No resistor (jumper on NONE)
overcurrent
Why it happens
Symptom: A bright flash and OL on the meter, then the LED is dark for good and the meter reads 0.
With 0 Ω in the row, nothing in I = (V − V_F) / R limits the current. Only the LED's own few ohms and the charger's limit hold it back: hundreds of milliamps against the LED's 30 mA maximum. The die and its bond wire overheat in a moment and the LED goes open.
Fix: Unplug, move the jumper back to 220 Ω and fit a new LED: a burnt LED never comes back.
LED in backwards
part in backwards
Why it happens
Symptom: The LED stays dark and the meter reads 0.00.
An LED conducts only from anode to cathode. Turned round it is reverse-biased, and at 5 V (its reverse rating) only a few microamps of leakage flow. It survives; it just stays dark.
Fix: Unplug and turn D1 round: the long leg (anode) in the + hole, the flat side toward −.
C1 in backwards
part in backwards
Why it happens
Symptom: C1 hisses and its vent opens. The LED keeps working.
An aluminium electrolytic blocks current one way only. Reverse-biased it leaks more and more, heats up, makes gas and the scored vent on top opens. The LED doesn't need C1, so it stays lit, but the capacitor is ruined.
Fix: Unplug and fit a new C1 with its stripe (−) toward GND.
R4 left out (a USB-C pull-down)
missing part
Why it happens
Symptom: Nothing at all: the LED is dark and the meter reads 0, as if the charger were dead.
A USB-C charger keeps its 5 V off until it sees a 5.1 kΩ pull-down on the CC pin, so a loose cable end is never live. R4 and R5 are those pull-downs, one for each way up of the plug, and without R4 this way up gets no power. A USB-A-to-C cable always gives 5 V, which hides the mistake until someone uses a C-to-C cable.
Fix: Unplug, fit R4 (5.1 kΩ) and solder it.
Cold joint on D1
cold solder joint
Why it happens
Symptom: The LED flickers and the meter jumps between 13.6 and 0.
A cold joint is solder that never wetted the pad properly: dull, grainy, sometimes cracked. It touches, then it doesn't, so the loop opens and closes. On a robot this is the fault that comes and goes when it moves.
Fix: Unplug and reheat the joint with a little fresh solder until it flows into a shiny cone.
Bench supply leads swapped
reverse polarity
Why it happens
Symptom: The LED stays dark, then C1 vents.
J2 has no reverse-polarity diode, on purpose: one would eat 0.7 V of the 3 V this lesson is about. With the leads swapped, the 5 V line sits at −5 V. The LED is reverse-biased and simply blocks, but C1, right across the supply, is reverse-biased too, and vents.
Fix: Unplug, swap the leads (red to +), then fit a new C1.
12 V on the 5 V input
wrong voltage
Why it happens
Symptom: The LED glares and the meter reads 45.5 mA. A few seconds later it flashes and goes dark.
(12 V − 2 V) / 220 Ω = 45 mA, half as much again as the LED's 30 mA maximum, and 0.45 W in a 1/4 W resistor. The simulator burns the LED out after about 3 s; a real one may last longer, getting dimmer as it dies. Either way the resistor was sized for 5 V, not 12.
Fix: Unplug, set the supply back to 5 V (or work out a bigger resistor for 12 V) and fit a new LED.
Supply current limit at 10 mA
brown-out
Why it happens
Symptom: The LED is a little dimmer, the meter reads 10.0 and the supply sags to 4.2 V.
A bench supply holds its voltage only until the load wants more than its current limit. Then it lowers the voltage to hold the current at the limit: 10 mA here, which leaves 2 V + 10 mA × 220 Ω = 4.2 V. That is a brown-out: nothing breaks, but the circuit no longer gets what you set.
Fix: Raise the current limit above what the circuit needs (20 mA is plenty here), or use a bigger resistor.

Meet the bench

Build the trainer, move the jumper, predict the current and read the meter, then break it on purpose

The first circuit of the course has no chip at all: 5 V, a switch, one resistor and a red LED, with a multimeter in the loop to read the current. A jumper picks the resistor: 220 Ω, 1 kΩ, 10 kΩ or a bare wire. Work out each current on paper first, I = (V − V_F) / R with about 2 V across the LED, then move the jumper and check the meter: 13.6, 3.0 and 0.30 mA. Then break it on purpose: no resistor, the LED or the capacitor in backwards, a missing USB-C pull-down, a cold joint, swapped supply leads, 12 V instead of 5 V, a supply that runs out of current. Every LED, pull-up and sensor divider on a robot runs on this one formula.

Sources

On the bench

Selected

Instant: switches at once, no upload

Code: 220 Ω

# Jumper on 220 Ω: Ohm's law for the LED, I = (V - V_F) / R
V_supply = 5.0     # volts from the USB-C charger
V_F = 2.0          # a red LED drops about 2 V when lit
R = 220            # red red brown gold

I = (V_supply - V_F) / R      # 3.0 / 220 = 0.0136 A = 13.6 mA
P_R = I * I * R                # 0.0409 W = 40.9 mW: heat in the resistor (a 1/4 W part)
P_LED = V_F * I                # 0.027 W = 27 mW: heat in the LED (75 mW at most)
# 13.6 mA: bright and safe. Run a 5 mm red LED at 5-20 mA, never above 30 mA

Practice tasks

0/2 tasks

Write the code, then run the checks. The run uses a virtual clock and a scripted world, so the same code always gives the same result.

Ohm's law in a sketch

Arduino C++

Make an Uno do the paper maths and print the current for each jumper row (220, 1000 and 10000 ohm) with a 5 V supply and a 2 V LED, in the form "220 ohm: 13.64 mA". The starter prints 0 mA for all three rows: find out why. Then add one line with the smallest resistor that keeps the LED at 20 mA: "20 mA needs 150 ohm".

Checks

Not run yet220 ohm: 13.6 mA
Not run yet1000 ohm: 3.0 mA
Not run yet10000 ohm: 0.30 mA
Not run yetPrints 20 mA needs 150 ohm

Hints

In C++ an int divided by an int is an int: 3 / 220 is 0, and 0 × 1000 is still 0.
Make the numbers float (5.0, 2.0, 1000.0) and the division keeps its decimals.
R = (V − V_F) / I, with I in amps: 3.0 / 0.020 = 150. Serial.print(x, 0) prints a float with no decimals.

Code for Ohm's law in a sketch

Set the average current with PWM

Arduino C++

Wire the trainer's 220 Ω and LED to pin 9 of an Uno: pin 9, then the resistor, then the LED, then GND. Fully on, pin 9 sits at about 5 V, so the LED draws 13.6 mA. PWM switches the pin fully on and fully off about 490 times a second, so the average current is the duty times 13.6 mA, and that average is what the meter shows. Step the LED through 13.6, 6.8 and 3.4 mA on average, one second each, and print each step.

Checks

Not run yetUses analogWrite()
Not run yetThe full 13.6 mA in the first second
Not run yetAbout half, 6.8 mA, in the second
Not run yetAbout a quarter, 3.4 mA, in the third
Not run yetPrints the 6.8 mA step

Hints

digitalWrite() knows only HIGH and LOW: the full 13.6 mA or nothing.
analogWrite(9, duty), with duty from 0 to 255, sets the share of each cycle the pin is on: 128 is about half.
duty = target / 13.6 × 255, rounded: 255, 128 and 64.

Code for Set the average current with PWM

Build and break

1BuildNext up

Unplug it

The trainer comes plugged in, its LED lit at 220 Ω. Unplug the USB-C cable before you touch a part: never work on a live board.

Strip it to the bare PCB

Fit the low parts

Fit the tall parts

Solder every joint

Jumper cap and meter

Predict, then plug in

Move the jumper to 1 kΩ

Move it to 10 kΩ

Back to 220 Ω

Break it: no resistorBreak it

Break it: the LED in backwardsBreak it

Break it: C1 in backwardsBreak it

Break it: leave out R4Break it

Break it: a cold joint on the LEDBreak it

Break it: swap the bench supply's leadsBreak it

Break it: 12 V on the 5 V inputBreak it

Break it: a 10 mA current limitBreak it

Back on USB

Meet the bench

Build the trainer, move the jumper, predict the current and read the meter, then break it on purpose

On the bench

Selected

Instant: switches at once, no upload

Code: 220 Ω

# Jumper on 220 Ω: Ohm's law for the LED, I = (V - V_F) / R
V_supply = 5.0     # volts from the USB-C charger
V_F = 2.0          # a red LED drops about 2 V when lit
R = 220            # red red brown gold

I = (V_supply - V_F) / R      # 3.0 / 220 = 0.0136 A = 13.6 mA
P_R = I * I * R                # 0.0409 W = 40.9 mW: heat in the resistor (a 1/4 W part)
P_LED = V_F * I                # 0.027 W = 27 mW: heat in the LED (75 mW at most)
# 13.6 mA: bright and safe. Run a 5 mm red LED at 5-20 mA, never above 30 mA

LED + resistor

The bench

Build and break

Unplug it

Strip it to the bare PCB

Fit the low parts

Fit the tall parts

Solder every joint

Jumper cap and meter

Predict, then plug in

Move the jumper to 1 kΩ

Move it to 10 kΩ

Back to 220 Ω

Break it: no resistorBreak it

Break it: the LED in backwardsBreak it

Break it: C1 in backwardsBreak it

Break it: leave out R4Break it

Break it: a cold joint on the LEDBreak it

Break it: swap the bench supply's leadsBreak it

Break it: 12 V on the 5 V inputBreak it

Break it: a 10 mA current limitBreak it

Back on USB

Break it on purpose

No resistor (jumper on NONE)

LED in backwards

C1 in backwards

R4 left out (a USB-C pull-down)

Cold joint on D1

Bench supply leads swapped

12 V on the 5 V input

Supply current limit at 10 mA

Live status

Current draw

Check yourself

After: Solder every joint

After: Predict, then plug in

After: Move it to 10 kΩ

After: Break it: no resistor

End of bench quiz

Meet the bench

Sources

On the bench

Practice tasks

Ohm's law in a sketch

Checks

Set the average current with PWM

Checks

LED + resistor

The bench

Build and break

Unplug it

Strip it to the bare PCB

Fit the low parts

Fit the tall parts

Solder every joint

Jumper cap and meter

Predict, then plug in

Move the jumper to 1 kΩ

Move it to 10 kΩ

Back to 220 Ω

Break it: no resistorBreak it

Break it: the LED in backwardsBreak it

Break it: C1 in backwardsBreak it

Break it: leave out R4Break it

Break it: a cold joint on the LEDBreak it

Break it: swap the bench supply's leadsBreak it

Break it: 12 V on the 5 V inputBreak it

Break it: a 10 mA current limitBreak it

Back on USB

Break it on purpose

No resistor (jumper on NONE)

LED in backwards

C1 in backwards

R4 left out (a USB-C pull-down)

Cold joint on D1

Bench supply leads swapped

12 V on the 5 V input

Supply current limit at 10 mA

Live status

Current draw