I'm trying to trigger eight different 5 V relays using an Arduino Atmega 2560. The relays (70Ω coil) are powered by an external 5 V source, and I'm trying to trigger them through a driving stage which contains for each relay an NPN transistor (2N3904) and a 1 kΩ resistor directly connected to digital pin of the Arduino.
The relays in my application are supposed to be driven 2 by 2, not all of them at once.
My problem is that I don't succeed in triggering the relay with digital pins of my Arduino; once I connect it to the resistor, the voltage of the digital pins drops from 5 V to 0.4 V or less. The transistor needs only 7 mA to be triggered, the output current of the Atmega2560 pins is 20 mA max.
I tried with an external 5 V source, it triggers correctly showing 10 mA maximum of current through the driving stage.