# How can I momentarily pull an pin Low with a trigger pin that is Active Low, using discrete components?

#### FranciscoB

Joined Feb 8, 2014
107
The following Falstad circuit (same as attached picture), shows something similar to what I want to do:

https://tinyurl.com/yxhcoaod

On the left there's the trigger pin, which rests LOW. When it becomes HIGH, the logic pin on the right is momentarily triggered from HIGH to LOW to HIGH again.

What I'd like to do is: if the trigger pin on the left rested HIGH, how could the circuit be changed to momentarily pull the pin on the right HIGH-LOW-HIGH?

I'd assume a pnp transistor would be involved but I'm not sure what's the most effective way.

Thank you

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#### crutschow

Joined Mar 14, 2008
34,692
If the load impedance is reasonably high than all you may need is a capacitor and a resistor.

#### DickCappels

Joined Aug 21, 2008
10,232
I think that loosely translated that means "What are you triggering and what is the source of the trigger pulse?"

#### djsfantasi

Joined Apr 11, 2010
9,183
You are simply negating the input. So, running the input through a NOT gate results in your desired operation. An NPN transistor acts as a simple NOT gate. Never used a PNP ad a NOT gate.

#### crutschow

Joined Mar 14, 2008
34,692
You are simply negating the input.
He shows an inverting output but I believe he wants a non-inverting output (low input generates a low output pulse).

#### Alec_t

Joined Sep 17, 2013
14,379
if the trigger pin on the left rested HIGH, how could the circuit be changed to momentarily pull the pin on the right HIGH-LOW-HIGH?
Edit: Replace the 5k resistor with a 10k (or greater) resistor connected between the transistor base and the 3.3V rail?
Is the resting state of the output high or low?
What is the duration of the trigger signal in relation to your 'momentary'?
What should happen when the trigger signal returns to high level?

Last edited:

#### FranciscoB

Joined Feb 8, 2014
107
Im using a Wemos D1 microcontroller and a DS3231RTC. The reason is I want to wake up the Wemos from deep sleep every 15 minutes between 7h30 and 18h30. The reset pin of the Wemos stays HIGH during deep sleep and it wakes up when the reset pin receives a short LOW pulse from and external source, in this case the DS3231RTC.

The DS3231 alarm pin goes LOW but stays LOW when the alarm time comes, keeping the Wemos from waking up. I need to create a short Low pulse on the Wemos when alarm time comes.

#### atferrari

Joined Jan 6, 2004
4,784
I do not know that Wemos. Is it correct that to wake it up, you put that signal in the RESET pin?

Are you waking it up or actually resetting it?

#### FranciscoB

Joined Feb 8, 2014
107
The Wemos D1 is a development board built around the ESP8266. This chip has at least two deep sleep modes: wake up until you trigger a reset (RESET pin is HIGH while sleeping, wakes up with a LOW->HIGH signal); wake up after equal interval of time, like every second, ever 3600 seconds, every 24h, etc (it has an internal rtc). It does it's thing and goes back to sleep.

Since my goal is to wake it up every 15min during the day but not the night, I decided to used an external RTC. This device, the DS3231, can be programmed by the esp8266 to set an alarm whenever you want. When an alarm is set, the alarm output pin of the DS3231 stays HIGH until the alarm time arrives, then stays LOW until a new alarm time is defined by the user.

If no new alarm is set by the user, it stays LOW, keeping the Wemos from waking up. Therefore I needed a short LOW pulse when the alarm time arrived.

#### BobTPH

Joined Jun 5, 2013
9,168
What is wrong with simply a capacitor from the output to the input and pull-up resistor?

Bob

#### FranciscoB

Joined Feb 8, 2014
107
I tried it but I think the LOW pulse was too short, hence posting here for help.

#### BobTPH

Joined Jun 5, 2013
9,168
if the pulse was too short, a bigger capacitor would lengthen it.

Bon