MicroPython relay and motor on a Pico: a button toggles it
MicroPython on a Raspberry Pi Pico: each press of the button on GP15 flips a relay through a transistor on GP14, and the relay runs a DC motor. The whole build, a Raspberry Pi Pico and 12 more parts, runs here in your browser on the firmware below; open it in the editor to change the wiring or the code and run it again.
Teaching? Assign it to your class in one click.
Intermediate About 25 minutes to build and run. Runs in your browser. Free, and no account needed.
The parts retail for about $11.05. On Mokxi they're free.
What you'll learn
- Toggling an output on each button press
- Switching a relay coil with a transistor
- A flyback diode across a coil
- Debouncing a button in software
- Powering a motor from a separate supply
- Difficulty: Intermediate
- Time: About 25 minutes
- Board: Raspberry Pi Pico
- Parts: 14
How it works
A Raspberry Pi Pico runs MicroPython. Each press of the button on GP15 flips a relay, and the relay's contact starts or stops a motor. The button has a 10k pull-up to 3.3 V, so the pin reads 1 until you press it.
GP14 cannot drive a relay coil directly. It goes through a 1k resistor to a transistor, and the transistor switches the coil. A diode across the coil absorbs the kick when it lets go, and an LED lights with the coil. The relay and motor run from a separate battery and MB102 power module.
The program prints motor on or motor off at each press.
Parts list
14 parts, plus the jumper wires. They retail for about $11.05; on Mokxi they're free.
- 1 × Raspberry Pi PicoIn Fight Stick Kit.
- 1 × Resistor, 1k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
- 1 × Transistor (NPN or PNP)In LED Cube Kit.
- 2 × Resistor, 10k ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
- 1 × PushbuttonIn Starter Kit, Christmas Lights Kit, Logic Lab Kit.
- 1 × Battery: AA, coin cell, LiPoIn Handheld Console Kit, Laser Sword Kit.
- 1 × Breadboard power supply, MB102
- 1 × DPDT relay
- 1 × DiodeIn Pinball Table Kit.
- 1 × Resistor, 220 ΩIn Starter Kit, Claw Machine Kit, Halloween Prop Kit.
- 1 × LED, redIn Starter Kit, Halloween Prop Kit, Logic Lab Kit.
- 1 × DC motor
- 1 × Power, 3.3 V
Kits that can build it
This starter kit has every part in the circuit. Each kit's page lists what else it can build.
Wiring table
Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.
| Part and pin | Connects to |
|---|---|
| Resistor, 1k Ω pin 1 | Raspberry Pi Pico pin GP14 |
| Resistor, 10k Ω (2) pin 1 | Raspberry Pi Pico pin GP15 |
| Transistor (NPN or PNP) pin B | Resistor, 1k Ω pin 2 |
| DPDT relay pin COIL2 | Transistor (NPN or PNP) pin C |
| Diode pin A | Transistor (NPN or PNP) pin C |
| LED, red pin C | Transistor (NPN or PNP) pin C |
| Transistor (NPN or PNP) pin E | Ground |
| Resistor, 10k Ω (1) pin 1 | 3.3 V |
| Pushbutton pin 1a | Resistor, 10k Ω (1) pin 2 |
| Resistor, 10k Ω (2) pin 2 | Resistor, 10k Ω (1) pin 2 |
| Pushbutton pin 2a | Ground |
| Breadboard power supply, MB102 pin VIN | Battery: AA, coin cell, LiPo pin V+ |
| Battery: AA, coin cell, LiPo pin V- | Ground |
| Breadboard power supply, MB102 pin GND | Ground |
| DPDT relay pin COIL1 | Breadboard power supply, MB102 pin 5V |
| DPDT relay pin COM1 | Breadboard power supply, MB102 pin 5V |
| Diode pin C | Breadboard power supply, MB102 pin 5V |
| Resistor, 220 Ω pin 1 | Breadboard power supply, MB102 pin 5V |
| Breadboard power supply, MB102 pin BOT+ | Breadboard power supply, MB102 pin TOP+ |
| Breadboard power supply, MB102 pin BOT- | Breadboard power supply, MB102 pin TOP- |
| DC motor pin 1 | DPDT relay pin NO1 |
| DPDT relay pin COM2 | DPDT relay pin NC1 |
| DPDT relay pin NO2 | DPDT relay pin NC1 |
| DPDT relay pin NC2 | DPDT relay pin NC1 |
| LED, red pin A | Resistor, 220 Ω pin 2 |
| DC motor pin 2 | Ground |
The code
The MicroPython code exactly as the editor opens it. Change a line there and press Run: the board runs it on the official MicroPython firmware, with nothing to compile.
# A relay and a motor: each press of the button on GP15 flips the relay,
# which GP14 drives through an S8050, and the relay's contact runs the
# motor from the power module's 5 V. The LED lights with the relay's coil.
from machine import Pin
import time
relay = Pin(14, Pin.OUT, value=0)
button = Pin(15, Pin.IN)
while True:
if button.value() == 0:
time.sleep_ms(20)
if button.value() == 0:
relay.toggle()
print("motor", "on" if relay.value() else "off")
while button.value() == 0:
time.sleep_ms(10)
time.sleep_ms(10)
Troubleshooting
The usual mistakes with this circuit, and the fix for each.
- The relay clicks twice or flickers on one press This project
Mechanical buttons bounce. The code checks again after 20 ms and then waits for release, so check that the button's wiring is sound. - Nothing happens This project
The transistor's emitter must go to ground and the Pico's ground must be shared with the power module. - The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure. - A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground. - The load never turns on, or is always on, because the legs are in the wrong order. Transistor (NPN or PNP)
Check the pinout in the datasheet for your part. A 2N2222 or BC547 can have emitter, base, collector in a different order. Flat side orientation is easy to get wrong. - The base is wired straight to the pin with no resistor, or the motor has no flyback diode. Transistor (NPN or PNP)
Put about 1k between the pin and the base. For motors and relays, add a diode across the coil with the stripe toward the positive side. - The input reads random HIGH and LOW when the button is not pressed. Pushbutton
A floating pin picks up noise. Add a 10 k pull-down or pull-up resistor, or use INPUT_PULLUP and wire the button between the pin and GND. Then pressed reads LOW. - The button seems always pressed, or never changes. Pushbutton
A 4-leg tactile switch has legs joined in pairs. Place it across the breadboard center gap so the pairs sit on different rows. Wired along one side, the legs are already connected.
Change it and keep it
Open it in the editor, change the circuit or the code, and keep your version in a free account.