That was a short answer to a short question.
What exactly do you want to do? Do you want to supervise a voltage and turn on a signal when 5V is reached? Or do you want to turn on a circuit automatically if 5V are present? What is it the switch shall switch, e.g AC, DC? What power (aka current)?
Tell us your problem in detail and we may be able to find a solution - maybe even without using a switch.