How to control secondary circuit using TTL
This is a very classic case to use a transistor as a switch.

Example:

Simulate circuit


The 300 Ohm resistor would be your load  
The 1k resistor limits the base current  
The 50Hz source simulates your 5V TTL output  


Note that there are some things to consider when dimensioning the circuit. The transistor must be chosen to  satisfy the collector current requirements. There are some other considerations, but this answer is not about transistor circuits in detail. There a plenty of resources in the WWW. Search for "transistor as a switch" and "emitter follower".