Summer is coming, in the heat, the aquarium water quickly evaporates.
These little device check the aquarium water level every ~12hours, and refill if need.
Size: 20x25mm / ~0.8x1 inch
Powered from 5V USB mini.

Step 1: The Parts

- A small PCB
- Attiny45 SU (1x)
- BC847 smd transistor (1x)
- Diode smd (1x)
- 3,5mm 1x2 terminal block (2x)
- 100K resistor 0805 (1x)
- USB mini connector (1x)
- G5V1 relay (1x)
- 9-12V brushless water pump (1x)
- 5V adapter (1x)
- 9-12V adapter (1x)
- Some aquarium air tube
- wire for sensors. (im using breadboard cable)

To programming Attiny
- Arduino panel
- 10uF capacitor
- SOP to DIP adapter to programming smd Attiny

Step 2: Circuit and PCB

The PCB too small, i solder the USB to the other side of the PCB, so mirrored VCC and GND,
but not necessary i see this now.

The pump wiring:
-from the 12V adapter to the pump -
-from the 12V adapter to the relay terminal block one pin
-from the other pin of the realy terminal block to the pump +

Step 3: Arduino Code and Testing

Arduino code is very simple.

Check the water level every ~12 hour, and if water level low, the pump fill, until required water level reached.
Then stop measuring for the next ~12 hour.

This little device run over a month with my 30L aquarium and it works perfectly.
Using a 1L jug with a cover for the refill water.
I see no sign of electrolysis on sensor!

The code:

int relayPin = 2;
int sensorPin = 1;
int triggerPin = 0;
int sensorState = 0;

void setup()
{
pinMode(sensorPin, INPUT);
pinMode(relayPin, OUTPUT);
pinMode(triggerPin, OUTPUT);
}

void loop(){
digitalWrite(triggerPin, HIGH);
sensorState = digitalRead(sensorPin);
if (sensorState == LOW){
      digitalWrite(relayPin, HIGH);
}
      else if (sensorState == HIGH){
        digitalWrite(relayPin, LOW);
        digitalWrite(triggerPin, LOW);
        delay(43200000);                    //~12hour delay.
}
}

I think lot of arduino programming attiny video on instructable, but here my short how-to:

http://youtu.be/IMkKb0Vu55A

And the device test: