555 oscillator circuit

I have successfully built a 555 timer circuit that with a 12-15VDC input switches between 0v and 740mV on the output every 3.3 seconds. I need to build an oscillator circuit using 555 timer IC, capacitors, resistors, and 12v, 3.3v, and 1.7v LEDs (those are the only components I will have available for some time). The oscillator circuit needs to cycle between approximately 250mV and 750mV approximately every 3-3.5 seconds.