I firstly just want to make sure that I am going to use the right circuit. I basically am building an on/off timer that I want to trigger on every 30 seconds. I was looking at just using a simple 555 timer as an astable multivibrator. I don't want to use a switch to trigger it, I just want to have it start when I put 5VDC to the ciruit. All I want it to do then is constantly switch on every 30 seconds. Am I using the right ciruit or should I be using a monostable circuit? I also need to be able to change the time that it triggers on every once in a while to maybe a minute. Do I need a relay switch for this ciruit? Also, I am going to add an LED to light up when it turns on so how do I figure out what resistor to use so that I don't burn it. And last question is should I use a transistor in the circuit?