Knowing when a capacitor finshed discharging

[jorx]

Hi!

I'm working on a digital project that have a sinking frequenzy using a voltage controlled oscillator (made of a 555). But I need a signal from the VCO when its done oscillating. So how can I get a signal from the capacitor that is producing the sinking voltage when it's done discharging?

I suppose 99% of the users can answer this

[Zero999]

If the capacitor is connected to a resistor (not a constant current sink) then it never finishes discharging. The voltage dies away exponentially, never falling to zero.

In reality, we consider the capacitor to be discharged, when the voltage falls below a certain level or after a number of RC time constants.

[jorx]

Thanks for the reply.

Let me put it like this. How can I make a circuit that gives a high bit when the voltage falls under a certain level (almost zero) without having an impact on the discharge?

[Zero999]

Thanks for the reply.

Let me put it like this. How can I make a circuit that gives a high bit when the voltage falls under a certain level (almost zero) without having an impact on the discharge?

Yes. You need a comparator such as the LM393 or LM311.