555 circuit adjust pulse/height

Thread Starter


Joined Jan 2, 2012
I have the following circuit, an oscillo connected at J1 J2 for visual and you can adjust the pulse width and height. I understand what happens in the left side of the circuit. The problem is after Pin 3 of the 555. How exactly is working the pulse height adjustment and the R3 role.

I would appreciate any help!!!
thank you


Joined Jul 9, 2011
How exactly is working the pulse height adjustment and the R3 role.
That is a simple resistor divider. The output voltage will change between 100% and 7.6% depending on the potentiometer wiper position. If you remove R3 and connect the potentiometer directly to ground it will adjust to zero. The problem is that the output impedance of that circuit varies a lot and is quite high. Depending on what you intend to drive with this circuit it may not be suitable.


Joined Dec 26, 2010
R3 prevents the output from being short circuited to ground when the potentiometer is adjusted to 0Ω
Unless my eyes have got even worse than I think, R3 does not do that - it simply imposes a non-zero minimum level.

The slider of the pot RV2 is connected to the output only, not to the bottom end of the pot. As shown it is wound to the top, for maximum level. Winding it down would not create extra loading, even with R3 absent.

If RV2 were connected in the same way as RV1, R3 could prevent a short.