The 555 timer and its follow-ons have got to be some of the most useful ICs ever invented. The 556 is a dual and the 558 is a quad. The TLC and 7555 are low power, CMOS versions of the same. You will learn a lot playing with them. 2 resistors and a cap, get you an astable multivibrator.(>50% duty cycle) Adding a diode gets you any duty cycle that you want. Feeding a variable voltage to the control voltage pin gives you basic PWM capability. I first used one to build a wiper delay for my Chevelle back in the 70s. I have 2 going into my B-17 now, to pulse 100CD LEDs as strobes, and to brighten and dim to appear like a rotating beacon. There are actual function generator chips out there, but most have been discontinued. I remember that Harris and Exar had them. Digi-Key only shows a couple and they show zero stock. I just showed a friend at work a circuit that spanned <2% to >99% duty cycle, while staying at roughly the same frequency. I did it by connecting the wiper of a linear, multi-turn pot to Pin 7, with each end going through low value fixed Rs to Pin 2,6 and V+. You use the fixed Rs because you really don't want to short your Discharge Pin 7 to V+ OR the Trigger(2) or Threshold(6) pins. Adding a plain small signal diode (1N400x, 1N914, 1N4148,...) Cathode to 2,6, Anode to 7, gets you the <50% duty cycle. If you were to substitute a linear, dual pot for the 2 fixed R's, it would be a frequency adjustment. If you then added a rotary switch to change capacitors between 2,6 and ground, you would have a range switch. I attached a PDF that I generated to help fellow RC club members, strobe their LEDs. Joe
