Hi,
I'm trying to figure out how to design a simple circuit (RC + BJT) which could output a low pulse (10ms) whenever a switch is opened and closed.

Any idea?

 

You want this pulse to happen when the switch is closed and then another pulse when the switch is opened?
One problem to be solved is that some switches 'bounce' for longer than 10ms so when the switch closes you might get multiple output pulses triggered.

 

Hi, I will be using a reed switch to detect open/close of a gate and I would like to wakeup a micro to send the event.

 

Which micro?
Some have a pin change interrupt which could do the whole job for you.

 

I would like to use an ESP8266 and I should drive the reset line down to wakeup from deep sleep

 

Use schmidt trigger dual gate logic, this circuit (tiny packages)

https://www.diodes.com/products/logic/dual-gate/

Regards, Dana.

 

I will be using a reed switch to detect open/close of a gate

How will you distinguish between opening and closing? Or doesn't it matter?

 

Once the micro starts I could read the state of the switch.

I would like to use only discrete components which I have in lab, is it possible?

 

Could you use a changeover reed switch such as:
https://www2.mouser.com/ProductDeta...=sGAEpiMZZMvFdY0L2HfHxjINjBnDRefQwPzrzjiWFWQ=

 

Sorry, I only could use the installed one which is a N.O. one.

 

In this circuit V1, R1, Q3 and R7 are just for the simulation. Your reed switch would connect where Q3 collector and emitter are. The collector of Q4 would connect to the controller reset pin.
It would be a good idea if someone better at LTspice checked this out. I haven't tried using 1MΩ resistors in this type of circuit before. Given that this circuit is to wake up the controller from deep sleep clearly current consumption needs to be minimised.

 

Hi, thanks a lot for the idea, I will try it as soon as possible. I think I will need to adjust some values to minimize power. I'll let you know.

 

Assuming the reed switch closes when the gate is in the closed position, here's an alternative which draws only 5uA with the gate closed and virtually zero current with the gate open :

Only two of the four gates in the IC are used.
C2 suppresses switch-bounce. R2 limits the C2 discharge current. R3,C1 set the output pulse width.
This circuit should work on a 5V supply or a 3.3V supply; whatever suits your MCU.