Switching relay with a PNP and TTL levels
If you have an open-collector or open-drain type logic output, you can drive a PNP with it, as long as the device is rated to take your supply voltage. In this configuration, use a pull-up resistor from the supply to the base of the PNP to ensure that the PNP defaults to the OFF state. Now tie the base of the PNP to the open-collector logic output through a second resistor, the value of which should pull just enough current from the base of the PNP to drive it to saturation. 

The values aren't too hard to work out. The pull-up usually wants to be something large-ish, like 50K or 100K. The logic resistor will see the supply voltage, less the saturation E-B drop of the PNP, less the Vce sat of the logic output. Basically, the supply voltage minus about a volt. From there, knowing the necessary base current to the PNP will let you work out the exact value. (Be sure that to saturate, the necessary PNP base current isn't actually greater than the current-sink capacity of the logic pin!)

If you do not have an OC/OD output, you can buffer a regular output signal with a small NPN transistor, like a 2n2222 or 2n3904 etc. A typical TTL output, driving the base of a small NPN through about 1K to 2.2K will give you the sort of open collector (the collector of the NPN) that I've assumed above.