# Continuous low to momentary low

#### Andre Laurendeau

Dec 12, 2016
2
Hello all!

I’m a bit of a noob when it comes to electronics, so I need some help and I'm pretty sure it's a really simple thing ...

Context

I am building a small ESP8266 based light sensor. My sensor chip triggers an interrupt by lowering its int pin when the light level reach a threshold, keeping the int line it low until it is reset by software.

When that happens, I need to trigger a momentary low pulse on the reset pin of the ESP8266 to wake it up from deep sleep. The ESP8266 will then reset the sensor int status and do its things.

So simply put, what I need a small circuit to convert a continuous LOW to a momentary LOW. The circuit is running on 3.3v and is using surface mount parts, so it needs to be small and low power.

Searching the board (and the net), I've foud a lot of momentary to continuous circuit, but not the opposite :-(

André

#### Externet

Aug 24, 2009
883
A differentiator should do it. It is a series capacitor to the line that goes low and stays low.

Linegoinglow----------------||------------------lowpulse

#### AnalogKid

Jun 10, 2015
2,804
What you want to do is differentiate the negative-going edge. Sounds messy, but isn't. It would be nice to know the output characteristics of the sensor (sink current, fall time), but we can guess without harming anything. A 0.1 uF capacitor from the int pin to the reset pin and a 10 K resistor from the int pin to 3.3V will produce a negative pulse of approx. 1 millisecond. If that is too short, increase either component value.

Pulse width = R x C
= 10,000 x 0.0000001
= 0.001

This is an approximation, based on a guess of the voltage level that the int pin senses to determine when to act. As shown, the R-C time constant is 1 ms. This is the time it takes for the resistor to charge up the capacitor to 63% of the power rail. For 3.3V systems, the transition level is around 1.8 to 2.0 V, which is around 55% to 60% of Vcc. Close enough for a test.

ak

#### Andre Laurendeau

Dec 12, 2016
2
Thanks! I will make some tests over the weekend and report here when I'm done.

André

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