I'm trying to trigger 8 different 5V relays using arduino atmega 2560. the relays (70Ohm coil) are powered by an external 5V source, and im trying to trigger them through a driving stage which contains for each relay a npn transistor (2N3904) and 1kOhms resistor directly connected to digital pin of the arduino.
the relays in my application are supposed to be drived 2 by 2 not all of them at once.
My problem is that i dont succeed triggering the relay with digital pins of my arduino, once i connect it to the resistor the voltage of the digital pins falls down from 5V to 0.4V or less. The transistor needs only 7mA to be triggered, the output current of the atmega2560 pins is 20mA max.
i tried with an external 5V source, it triggers correctly showing 10mA maximum of current through the driving stage.