I am trying to design a tone generator similar to those used in telecommunications applications (ex: http://www.tessco.com/products/displayProductInfo.do?sku=89724&eventPage=1) The probe (inductive amplifier) can be used on any tone generator, so I plan to use it with mine. So far, I have thought of using a 555 timer as the oscillator (since it is also adjustable, see astable mode http://en.wikipedia.org/wiki/555_timer_IC). I am having trouble figuring out how to design a circuit that creates a constant tone (should be easy). This constant tone (a constant voltage) will be AND'd with the output of the 555 timer, to make a pulsing sound/on-off sound. Any tips on generating a constant tone? Thanks!!!