Fig. 03 · Robotics Lab · Logic drives power

Transistor motor switch

Arduino Nano

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Bench

No reading yet

Pin 9 (duty)

No reading yet

Current draw

0 mA

Build and break

1BuildNext up
Strip the Nano to its bare PCB
Start from the bare board so you see every part go on. The Nano is the controller here; the motor circuit lives on the breadboard.
2
Pick and place the SMD parts, then reflow
3
Place the two 15-pin headers
4
Solder the header pins
5
Seat the TIP120 and its base resistor
6
Fit the motor and the flyback diode
7
Connect the battery and the common ground
8
Plug in USB
9
Upload PWM 50%
10
Measure the gain
11
Unplug and open the exploded view
12
Break it: leave out the flyback diodeBreak it
13
Break it: fit the diode backwardsBreak it
14
Break it: put the TIP120 in backwardsBreak it
15
Break it: forget the common groundBreak it
16
Break it: leave out the base resistorBreak it
17
Break it: wire the motor straight to pin 9Break it
18
Break it: run the motor from the Nano's 5 VBreak it
19
Break it: put the battery leads on backwardsBreak it
20
Break it: load the holder with Li-ion cellsBreak it
21
Break it: a cold joint on the motor tabBreak it
22
Break it: hold the wheelBreak it

Break it on purpose

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

No flyback diode
missing part
Why it happens
Symptom: The motor runs, but the collector peak reads 60 V or more at every switch-off. In PWM that is 490 kicks a second: after a few seconds the TIP120 fails short and the motor runs flat out whatever pin 9 says.
A motor is a coil. When the TIP120 switches off, the coil keeps its current flowing for a moment and drives the collector up until something gives: here, the transistor breaks down at its 60 V limit and soaks up the energy. One switch-off it survives; hundreds a second wear it out, and a dead transistor usually fails short. With D1 in place the current circles round through the diode and the collector stops one diode drop above the battery.
Fix: Unplug, fit a new TIP120 and put D1 across the motor, band toward battery plus.
Flyback diode in backwards
part in backwards
Why it happens
Symptom: As soon as pin 9 goes HIGH the diode gets hot, the battery voltage collapses and the motor slows. With pin 9 LOW nothing happens.
Turned round, D1 points from battery plus to the collector. When the TIP120 switches on it pulls the collector to ground, and the diode then conducts straight from battery plus to ground: a dead short through D1 and the transistor, limited only by the cells. Several amps through a 1 A diode cook it.
Fix: Unplug the battery and turn D1 round: band toward battery plus.
TIP120 in backwards
part in backwards
Why it happens
Symptom: The LED lights, pin 9 is HIGH, but the wheel never turns and nothing gets warm.
Turning a TO-220 round swaps its outer legs: the resistor now reaches the emitter and the base sits on ground. The base-emitter junction is reverse biased, so no base current flows and the transistor stays off. Collector and emitter do not swap on this package, the base and emitter do.
Fix: Unplug and turn the TIP120 round: printed face toward you, base on the left.
No common ground
floating input
Why it happens
Symptom: The LED lights, pin 9 is HIGH, but the motor does not run.
Base current leaves pin 9, goes through R1, the base and the emitter into the ground rail, and has to get back to the Nano's GND to complete the loop. The battery's minus is not the Nano's ground unless a wire joins them, so no current flows and the TIP120 stays off. The LED has its own wire to the Nano's GND, which is why it still lights.
Fix: Run a wire from the Nano's GND to the battery's minus rail.
No base resistor
overcurrent
Why it happens
Symptom: The motor jumps on, then pin 9 stops working: the LED goes dark and the motor stops. The Nano gets warm near the chip.
With nothing but a wire, pin 9 drives the base junction directly. The junction holds the base near 1.3 V, so the only thing limiting the current is the pin's own output transistor: it tries to deliver well over 100 mA. The ATmega328P allows 40 mA per pin as an absolute maximum, and its output stage burns out.
Fix: Unplug and fit the 1 kΩ base resistor again. The Nano's pin 9 is damaged: use another PWM pin (3, 5, 6, 10 or 11) or a new Nano.
Motor straight on pin 9
overcurrent
Why it happens
Symptom: The wheel twitches or does nothing, and after a moment pin 9 is dead.
The motor needs about 160 mA to run and over 1 A to start. Pin 9 can give 20 mA comfortably and 40 mA at the absolute maximum. As it tries, the pin's voltage collapses to well under a volt, too little to start the motor, while the overloaded output stage overheats and fails.
Fix: Unplug, put the TIP120 back between the motor and ground, and use another pin: pin 9 is damaged.
Brown-out: motor on the Nano's 5 V
brown-out
Why it happens
Symptom: Every time the motor starts, the Nano resets: the LED blinks off, the serial banner repeats, the wheel jerks and stops.
Powered from USB, the Nano's 5 V comes through a 500 mA polyfuse and a Schottky diode. A starting motor gulps over 1 A, the 5 V rail sags below the ATmega328P's 2.7 V brown-out level, and the chip resets. Pin 9 goes low, the motor stops drawing, the rail recovers, the sketch starts again and switches the motor on: round and round.
Fix: Power the motor from its own battery and share only the ground.
Reverse polarity on the battery
reverse polarity
Why it happens
Symptom: D1 gets hot at once, whatever pin 9 does, and the cells warm up. The motor does not run.
With the leads swapped, the battery's plus is on the ground rail. Current now flows forward through the TIP120's built-in emitter-collector diode and then forward through D1: two diode drops straight across the battery, a dead short limited only by the cells' own resistance.
Fix: Unplug the battery and swap its leads: red to the plus rail, black to the ground rail.
Wrong voltage: Li-ion cells in an AA holder
wrong voltage
Why it happens
Symptom: The battery reads about 16 V, the wheel screams round at three times its speed, and after a while the motor smells hot and stops.
A 14500 Li-ion cell has the size of an AA but gives 3.7 V nominal and 4.2 V when full, so four of them make about 16.8 V instead of 6 V. The 3-6 V motor runs far over speed, its brushes arc and its windings overheat. The TIP120 (60 V) and D1 (1000 V) do not mind; the motor does.
Fix: Use four 1.5 V alkaline (or 1.2 V NiMH) AA cells: 4.8-6.4 V.
Cold solder joint on the motor
cold solder joint
Why it happens
Symptom: The wheel stutters: it runs, stops, runs again, though pin 9 stays HIGH.
The solder never wetted the motor's tab, so the joint is only resting on it. Vibration makes and breaks the contact, and each break is also a little switch-off that the flyback diode has to catch.
Fix: Reheat the joint until the solder flows onto the tab and forms a shiny fillet.
Stalled motor
overheating
Why it happens
Symptom: The wheel stops, the motor current jumps past 1 A, the battery voltage drops and the TIP120 gets hot.
A turning motor generates a back voltage that holds its current down. Stopped, only the winding's few ohms limit it: about 1.5 A at 6 V. The TIP120 then drops about 1 V and dissipates over a watt, close to its 2 W limit without a heatsink, and the cells sag under the load.
Fix: Let go of the wheel. In a robot, detect a stall (the current or a missing encoder pulse) and cut the power.

Meet the bench

a 20 mA pin switches a 160 mA motor

A Nano pin can give about 20 mA (40 mA is the absolute limit). A TT gear motor wants about 160 mA just to turn its wheel, and over 1 A for the instant it starts or when you hold the wheel. Wire it to a pin and the pin dies. Here pin 9 feeds about 3.4 mA through a 1 kΩ resistor into the base of a TIP120 Darlington, and that small current switches the motor's own supply: four AA cells, 6 V. The motor sits between the battery's plus and the TIP120's collector (a low-side switch), the emitter goes to ground, and the battery's minus shares that ground with the Nano. A 1N4007 diode across the motor gives the coil's current somewhere to go when the transistor switches off; without it the coil kicks the collector up past the TIP120's 60 V rating. A green LED on pin 9 shows what the pin is doing, half bright in PWM because your eye averages it, like the motor does.

Sources

On the bench

Selected

Sketch uses 1,568 bytes of program storage

Code: Input HIGH

// Input HIGH: pin 9 holds the TIP120 fully on
const int IN = 9;          // to R1 (1 kΩ), then the TIP120's base

void setup() {
  pinMode(IN, OUTPUT);
  Serial.begin(9600);
  digitalWrite(IN, HIGH);  // about 3.4 mA into the base...
  Serial.println("pin 9 HIGH: TIP120 on, motor runs");
}

void loop() {}             // ...lets the battery's 160 mA flow through the motor
// a pin gives 20 mA at most: never wire a motor to it

Practice tasks

0/4 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.

Pulse the motor without delay()

Arduino C++

Switch the motor on for 1 s and off for 1 s, forever, using millis() instead of delay(), so the sketch could still read buttons or sensors in between.

Checks

Not run yetPin 9 toggles with a 2 s period
Not run yetThe motor is on at 1.5 s
Not run yetThe motor is off at 2.5 s
Not run yetUses millis(), no delay()

Hints

Remember when you last switched: unsigned long last = 0;
In loop(), when millis() - last >= 1000, add 1000 to last and flip the pin.
digitalWrite(MOTOR, on ? HIGH : LOW) writes a bool flag to the pin.

Code for Pulse the motor without delay()

Soft start

Arduino C++

Starting at full power makes the motor gulp over 1 A for an instant. Ramp pin 9 from 0 to 255 over 2 s with analogWrite() and millis(), then hold full speed.

Checks

Not run yetAbout 25% duty at 0.5 s
Not run yetAbout 50% duty at 1 s
Not run yetAbout 75% duty at 1.5 s
Not run yetFull on at 2.5 s
Not run yetUses millis() and analogWrite(), no delay()

Hints

t = millis() - start tells you how far into the ramp you are.
The duty is t * 255 / RAMP_MS while t < RAMP_MS: with unsigned long it cannot overflow here.
After the ramp, analogWrite(MOTOR, 255) holds the pin fully on.

Code for Soft start

Speed knob

Arduino C++

A 10 kΩ potentiometer on A0 sets the motor speed. Map its 0-1023 reading onto 0-255 for analogWrite(), and switch the motor fully off below a duty of 40, where the TT motor only hums and heats instead of turning.

Checks

Not run yetKnob at zero: motor off
Not run yetKnob at half: about 50% duty
Not run yetKnob at full: motor fully on
Not run yetKnob nearly at zero: motor off, not humming
Not run yetReads A0 and writes pin 9 with analogWrite()

Hints

map(raw, 0, 1023, 0, 255) scales the reading.
if (duty < 40) duty = 0; gives the dead band.
analogWrite(9, 0) and analogWrite(9, 255) give a steady LOW and HIGH.

Code for Speed knob

Toggle button with debounce

Arduino C++

A push button from D2 to ground (INPUT_PULLUP) toggles the motor: press once for on, again for off. The contacts bounce for a few milliseconds, and the starter sketch counts every bounce as a press. Debounce it with millis().

Checks

Not run yetThe first press switches the motor on within 60 ms
Not run yetIt stays on after the bounces
Not run yetThe second press switches it off within 60 ms
Not run yetIt stays off
Not run yetUses millis() and INPUT_PULLUP, no delay()

Hints

Note the time whenever the raw reading changes.
Accept a new state only after the reading has stayed the same for 30 ms.
Toggle on the change to LOW only: that is the press, not the release.

Code for Toggle button with debounce

Build and break

1BuildNext up

Strip the Nano to its bare PCB

Start from the bare board so you see every part go on. The Nano is the controller here; the motor circuit lives on the breadboard.

Pick and place the SMD parts, then reflow

Place the two 15-pin headers

Solder the header pins

Seat the TIP120 and its base resistor

Fit the motor and the flyback diode

Connect the battery and the common ground

Plug in USB

Upload PWM 50%

Measure the gain

Unplug and open the exploded view

Break it: leave out the flyback diodeBreak it

Break it: fit the diode backwardsBreak it

Break it: put the TIP120 in backwardsBreak it

Break it: forget the common groundBreak it

Break it: leave out the base resistorBreak it

Break it: wire the motor straight to pin 9Break it

Break it: run the motor from the Nano's 5 VBreak it

Break it: put the battery leads on backwardsBreak it

Break it: load the holder with Li-ion cellsBreak it

Break it: a cold joint on the motor tabBreak it

Break it: hold the wheelBreak it

Meet the bench

a 20 mA pin switches a 160 mA motor

On the bench

Selected

Sketch uses 1,568 bytes of program storage

Code: Input HIGH

// Input HIGH: pin 9 holds the TIP120 fully on
const int IN = 9;          // to R1 (1 kΩ), then the TIP120's base

void setup() {
  pinMode(IN, OUTPUT);
  Serial.begin(9600);
  digitalWrite(IN, HIGH);  // about 3.4 mA into the base...
  Serial.println("pin 9 HIGH: TIP120 on, motor runs");
}

void loop() {}             // ...lets the battery's 160 mA flow through the motor
// a pin gives 20 mA at most: never wire a motor to it

Transistor motor switch

The bench

Build and break

Strip the Nano to its bare PCB

Pick and place the SMD parts, then reflow

Place the two 15-pin headers

Solder the header pins

Seat the TIP120 and its base resistor

Fit the motor and the flyback diode

Connect the battery and the common ground

Plug in USB

Upload PWM 50%

Measure the gain

Unplug and open the exploded view

Break it: leave out the flyback diodeBreak it

Break it: fit the diode backwardsBreak it

Break it: put the TIP120 in backwardsBreak it

Break it: forget the common groundBreak it

Break it: leave out the base resistorBreak it

Break it: wire the motor straight to pin 9Break it

Break it: run the motor from the Nano's 5 VBreak it

Break it: put the battery leads on backwardsBreak it

Break it: load the holder with Li-ion cellsBreak it

Break it: a cold joint on the motor tabBreak it

Break it: hold the wheelBreak it

Break it on purpose

No flyback diode

Flyback diode in backwards

TIP120 in backwards

No common ground

No base resistor

Motor straight on pin 9

Brown-out: motor on the Nano's 5 V

Reverse polarity on the battery

Wrong voltage: Li-ion cells in an AA holder

Cold solder joint on the motor

Stalled motor

Live status

Current draw

Check yourself

After: Pick and place the SMD parts, then reflow

After: Solder the header pins

After: Connect the battery and the common ground

After: Upload PWM 50%

After: Break it: leave out the flyback diode

After: Break it: wire the motor straight to pin 9

After: Break it: run the motor from the Nano's 5 V

End of bench quiz

Meet the bench

Sources

On the bench

Practice tasks

Pulse the motor without delay()

Checks

Soft start

Checks

Speed knob

Checks

Toggle button with debounce

Checks

Transistor motor switch

The bench

Build and break

Strip the Nano to its bare PCB

Pick and place the SMD parts, then reflow

Place the two 15-pin headers

Solder the header pins

Seat the TIP120 and its base resistor

Fit the motor and the flyback diode

Connect the battery and the common ground

Plug in USB

Upload PWM 50%

Measure the gain

Unplug and open the exploded view

Break it: leave out the flyback diodeBreak it

Break it: fit the diode backwardsBreak it

Break it: put the TIP120 in backwardsBreak it

Break it: forget the common groundBreak it

Break it: leave out the base resistorBreak it

Break it: wire the motor straight to pin 9Break it