Digital pin output voltage fall down when driving a relay
If the coil is 70 ohms and it's driven by 5V, it should draw about 5/70 = 71 mA.  That's pushing the 2n3904 a bit hard, and doesn't even account for inductive effects. (It's a small-signal transistor.)  Do you have an NPN power transistor to try, just as a test?  If its current gain (beta) is too low, use a Darlington configuration by combining it with your 2n3904.  (2n3904 base from digital out resistor, emitter drives power NPN base directly, both collectors are connected together to the bottom of the coil, and the power NPN emitter is the effective emitter of the just-created &quot;super transistor&quot; with high beta and power handling.)  Be sure to use an anti-kickback rectifier (1n4001)across the relay coil.