1. Controlling a Motor with Cascading Switches
transistor relay DC Motor

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3. normally open contacts
Wiring
Setting the digital output on the Arduino to HIGH switches on the transistor
The transistor allows current to flow through the relay coil, closing the relay contacts
Power from Vin actuates the motor, allowing the shaft to spin 5V C B E
Arduino digital pin
1kΩ
diodes
normally open contacts M
coil
SPST relay + -
Vin

4. Flyback Diode Why Use the Flyback Diode?
When the digital pin on the Arduino goes HIGH, electricity is conducted through the coil of the relay, inducing a magnetic field in the coil
Energy is stored in the coil as a magnetic field
When the digital pin on the Arduino goes LOW, the decay of the magnetic field induces a current that can be harmful to our electronics (can arc across contacts or send a surge through the system)
The diode allows a circular current to be set up in the coil / diode loop so that the magnetic energy stored in the coil can be safely dissipated C B E 5V
Arduino digital pin
1kW C B E 5V
Arduino digital pin
1kW
Bad Wiring
Good Wiring

5. Power Considerations Power to switch transistor Power to switch relay
source:
max current per digital I/O pin:
Power to switch relay
source: max current from the voltage regulator: coil current for relay:
Power to motor
source: max current:
Arduino digital I/O pin
20 mA
5V from Arduino (from the on-board voltage regulator)
800 mA
40 mA
Vin (6 AA Batteries, 9V)
depends on battery type (often lithium ion or NiCad) and remaining life

6. Implementation + - M 5V Arduino digital pin normally open contacts
transistor
wire to digital pin 8 5V
Diode – silver stripe toward positive side
void setup() {
pinMode(8, OUTPUT); }
void loop() {
digitalWrite(8, HIGH);
delay(500);
digitalWrite(8, LOW);
delay(2000);
SPST Relay 5V C B E
Arduino digital pin
1kΩ
coil
normally open contacts
SPST relay + -
Vin M
Vin
coil leads
contact leads