555 circuit confusion

Thread Starter


Joined Aug 7, 2012
Here is block diagram of 555 timer:

I wouldn't analyse it, but under some circumstances (for some voltages at THRESH and TRIG pins), complemented output Q will be at high voltage level, with respect to ground (pin 1). And that's problem. For BJT to conduct, VBE is aprox. 0.7V, but in this case it will be much higher than 0.7V. What would happen in this case?


Joined Sep 17, 2013
Your diagram only illustrates the principle of operation and isn't intended to be an accurate portrayal of the inner workings of the IC. The not-Q output in practice would provide limited base current to avoid stressing the Vbe junction of the discharge transistor.


Joined Sep 9, 2010
The discharge pin 7 gives a path to ground. Current sinking is limited by the power dissipation rating of the particular 555 IC you are using. 200mA is typical.

Or perhaps you were asking about the base current, which Alec_t has answered.