# 3.3v Signal - How to delay the rise time of the signal, and use a pull-down resistor still?

#### Mahonroy

Joined Oct 21, 2014
332
Hello, I have a switch connected up to a 3.3VDC source. When the switch activates I am wanting to have the voltage increase slowly (up to ideally 3.3v), but when the 3.3v is removed, I am wanting the voltage to immediately snap down to 0 volts.
This is an example circuit that I made showing what I was trying to accomplish... but the problem is the 2 resistors create a voltage divider, so the voltage never raises high enough.
Is there a simple way of doing this that I am missing? I was hoping to do this with just some simple resistors and capacitors, and not have to incorporate any IC's?

#### crutschow

Joined Mar 14, 2008
26,040
When the switch activates I am wanting to have the voltage increase slowly (up to ideally 3.3v), but when the 3.3v is removed, I am wanting the voltage to immediately snap down to 0 volts.
Below is the LTspice simulation of two circuits to do what you want.

The top circuit (Out1, yellow trace) uses a Schottky diode to rapidly (≈0.2s) pull the voltage down to near zero (about 200mV).

The bottom circuit (Out2, red trace) uses two transistor to more rapidly (≈2.5ms) pull the voltage to within a few mV of zero.

Last edited:

#### AnalogKid

Joined Aug 1, 2013
8,817
I think that "instant snapdown" is going to tace semiconductors of some kind. The problen is that in an all-passive circuit, the low impedance path (needed for that "instant"part) is open, not closed.

What you want can be done with one transistor; no IC's needed. I'm travelling and cant do schematics. Basically, the 3.3 v signal turns off a PNP transistor, allowing the R-C to make the slow positive ramp. When the 3.3 V source is switched off, a much smaller resistor discharges the cap.

ak

#### MrChips

Joined Oct 2, 2009
22,535
Remove the 1k resistor.
Put a diode in parallel with the 100k resistor, anode at the capacitor.

Edit: as suggested in post #2.

#### Mahonroy

Joined Oct 21, 2014
332
Below is the LTspice simulation of two circuits to do what you want.

The top circuit (Out1, yellow trace) uses a Schottky diode to rapidly (≈0.2s) pull the voltage down to near zero (about 200mV).

The bottom circuit (Out2, red trace) uses two transistor to more rapidly (≈2.5ms) pull the voltage to within a few mV of zero.