I`m new to electrical engineering, so it's probably a newbie problem.

I'm using a wall power supply(5V-2A) to power the DC Motor and the Attiny85.

A C1 470uF to make the power more stable. I don't know if it is necessary, but it helped.

A D1 green led to show if it's on.

A reset button connected to ground to reset(Duh).

One relay(833H-1C-C) connected to the digital pin 0 of the attiny to turn the motor on and off.

Two rotational potentiometer connected to two analog pins(2,3) to decide how much time the mini water pump will be on(0-1023 to 1s-30s) and the other to compare to the humidity sensor to decide how dry the soil will need to be to turn on the motor.

And one humidity sensor.

#include "kPot.h" // my library, just analogReads

#define HUMIDITY_PIN 1 //humidity sensor
#define LEVEL_PIN 3 //rotational potentiometer
#define TIMER_PIN 2 //rotational potentiometer
#define MOTOR_PIN 0 //connected to the relay switch

kPot humiditySensor = kPot(HUMIDITY_PIN, 10); //(pin, average of 10 reads)
kPot levelPot = kPot(LEVEL_PIN, 10);
kPot timerPot = kPot(TIMER_PIN, 10);

int humidity = 0;
int level = 0;
int timer = 0;

void setup() {
  pinMode(MOTOR_PIN, HIGH);  

  humidity = humiditySensor.Read();
  level = levelPot.Read();
  timer = timerPot.Read();

void loop() {
  humidity = humiditySensor.Read();
  level = levelPot.Read();

  timer = timerPot.Read();
  timer = map(timer, 0, 1023, 1000, 30000); //1s to 30s

  if(humidity < level){
    digitalWrite(MOTOR_PIN, HIGH);
    digitalWrite(MOTOR_PIN, LOW);

  delay(300000); // 5min Update

enter image description here

The problem: The mini water pump turns on and won't stop.
I removed the reset button to make sure that wasn't resetting, didn't help.
Maybe the motor is taking too much current and reseting the attiny?
I probably have to put a diode somewhere, but as I said, I'm a newbie, so...


  • \$\begingroup\$ Where is the decoupling capacitor? And why is there a resistor on the supply line? \$\endgroup\$ Jan 30, 2016 at 0:27
  • \$\begingroup\$ Is your reset switch designed exactly as drawn? If so you probably need either an internal or external pull-up resistor to keep that input from floating when the reset button isn't pressed. \$\endgroup\$ Jan 30, 2016 at 0:44
  • \$\begingroup\$ @RobhercKV5ROB pull-up is correct. A pull-down would hold it permanently in reset whether the button is pressed or not. \$\endgroup\$
    – brhans
    Jan 30, 2016 at 1:25
  • \$\begingroup\$ Lose the resistor. Don't try to drive the relay directly from your Attiny - use a transistor. Put a diode across the relay coil. \$\endgroup\$
    – brhans
    Jan 30, 2016 at 1:28
  • 1
    \$\begingroup\$ 20ohms is far too small for your LEDs series resistor. You should use one 10 to 20 times higher than that. \$\endgroup\$
    – brhans
    Jan 30, 2016 at 1:31

1 Answer 1


Ok, not sure this will fix everything, but it should at least improve several problems (and avoid some component damage).

Try adding 2 diodes, a transistor, and a decoupling capacitor to your circuit like this:
enter image description here

EDIT: Updated schematic to include decoupling cap.

  • \$\begingroup\$ I'm adding right now the Diodes and transistor. But the decoupling capacitor was already there...C1 on the original schematic. \$\endgroup\$ Jan 30, 2016 at 2:20
  • \$\begingroup\$ ok, but it needs to be on the other side of R2 from your orig. schematic (i.e. attached to the same end of R2 as your ttiny) \$\endgroup\$ Jan 30, 2016 at 2:24
  • \$\begingroup\$ It's working perfectly with the adding of a transistor, diodes, capacitor in this schematic and the pull up resistor on the reset button. Thanks! \$\endgroup\$ Jan 30, 2016 at 2:47
  • \$\begingroup\$ @KaiqueGomes: C1 is bulk, not decoupling. They serve different purposes. \$\endgroup\$ Jan 31, 2016 at 13:09

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