Can somebody tell me the operation of the below 555 circuit.It is deiffer from traditional astable mode since it is not used pin7.Then how the cap discharges?
The output pin has a full output driver. Meaning is has both a high side and a low side transistors.
This means the capacitor is both charged and discharged through R1 producing roughly a 50% duty cycle.
When the output goes high, it charges the capacitor through R1 until it reaches the upper threshold on pin 6.
Once that happens the output goes low, discharging the capacitor through R1 until it reaches the lower threshold of pin 2 (trigger) which causes the output to go high again.
The advantage of this circuit is fewer parts.
The downside is that you can't have a duty cycle of anything other than roughly 50%.
Yes it's a 50 percent duty cycle oscillator, that's why the different connections. Another slight drawback is the frequency stability with supply voltage and temperature is slightly worse than with the more traditional connections...it's not too much though, but it is worse. Probably works in most applications just ok.
I have made circuits like this before and while it will deliver a 50% duty cycle the frequency stability will be rubish. The one that I made was on a bread board so that may have contributed to the issue, but as time went on, the frequency off the 555 decreased. There are beter ways of driving a 555 with a 50% duty cycle.