Let's Make Robots!

activate alarm using microcontroller

I have 2 circuits:

1. flame detection

2. alarm circuit

Flame detection circuit connected to the pololu robot controller.(atmel AtMega 328p) I am able to get the values from the ir sensor of flame detection circuit. I need to active an alarm when the value is, lets say = 100. I'm a little confused how can I do that using microcontroller... it dosn't output 5v, so I guess I need to use something that will work as a switch when microcontroller drives HIGH or LOW it would turn on/off the alarm circuit.

thanks