drive a relay from a 3.3v 2mA max output
Since you only need one channel, something like a ULN2803 is overkill. A single NPN level shifter should work.







simulate this circuit &ndash; Schematic created using CircuitLab

But I have to warn you that this is quite possibly marginal. It requires that the transistor be saturated at a gain of about 25, and this is a bit iffy. If the transistor is not quite in saturation, the relay voltage will not be a full 5 volts, and it may not operate reliably. In order to fix this, you might replace the NPN with a small N-type MOSFET with a logic-level gate threshold such as (for instance) a VN0104N3-G. These are available for less than $1.