I want to generate TTL pulses with adjustable on-off time....frequency not more than 500Hz....which is the best way of doing???
Using Crystal or something else???
I would use a 555 free-running multi (powered off 5V) with separately-adjustable charge and discharge networks. Pin 3 will drive TTL just fine. With two pots and diode, you can get any duty cycle from ~2% to ~98%. By switching in different capacitors, you can get it to cover a frequency range of ~0.01Hz to about ~1MHz.