1

I'm trying to wake up an ESP-12F from deep sleep mode by pulling the RST pin low for a brief moment (~10ms or less). I thought I needed a monostable (one shot) circuit so I bought an LMC555 timer to take a low pulse and output that as a fixed 10ms low pulse to the RST pin.

Reading and understanding more about the LMC555 timer I realized it may not work for my situation. My wake up signal source could last anywhere from 100ms to several hours (moisture sensor), but I need to generate just a single 10ms pulse and can't have it repeating. I need the ESP-12F to wake up just once, do some work and then go back to sleep. I don't think the LMC555 will work for me because the trigger pulse needs to last for a shorter period of time than the output pulse. So, if I'm looking to generate a 10ms high pulse out the output pin of the LMC555 my trigger pulse needs to be less than 10ms. Unless I'm reading this section of text from the TI datasheet incorrectly "During the timing cycle when the output is high, the further application of a trigger pulse will not effect the circuit so long as the trigger input is returned high at least 10 µs before the end of the timing interval.". In my case the trigger input will NOT be returned to high before the end of the timing interval.

Hopefully, I've explained this properly. To simplify it I'm trying to figure out how to take a low (ground) pulse that could last anywhere from 100ms to several hours and have it generate a single 10ms low pulse. My power source is an 18650 battery which could be anywhere from around 4V to 3.3V. If I can still use the LMC555, great, otherwise I'm back to square one.

How do I generate a regular 10ms pulse with a 555 timer from a Vbat voltage that varies?

Voltage Spike
  • 75,799
  • 36
  • 80
  • 208
Chris H
  • 11
  • 1
  • Sounds like a differentiator (capacitor) circuit, followed by a one-shot to shape the pulse duration as needed. You'll get an upward spike out of the differentiator on the rising edge of your signal (if fast) and a downward spike out of the differentiator on the falling edge. (You can use a diode to ground in order to clip that downward spike.) How fast are the edges of your moisture sensor output? – jonk Apr 24 '17 at 19:06
  • The moisture sensor isn't really a "sensor" so to speak so I wouldn't know how to describe how fast the edges are. It's just a PCB board trace that when it gets wet would complete the circuit (albeit high resistance) and pull the Trigger pin on the LMC555 low (with the help of a transistor if needed). It would probably be a very slow edge? And since it's wet it would take hours to dry out and break the circuit. I'm very new to electronics so trying to learn it all. – Chris H Apr 24 '17 at 19:23
  • 555 is an inverter it can't make low from low. – Jasen Слава Україні Apr 24 '17 at 19:33
  • 1
    Um.. why reset the micro? Why don't you wake it using the IO interrupt pin, GPIO16, that is intended for that function. – Trevor_G Apr 24 '17 at 19:35
  • @ChrisH Okay. I get it. Then you ***also*** need a conditioning circuit for the "wetness on traces." I actually built such a thing from a pair of BJTs (and multi-megohm resistors) and have one right here in front of me, fully tested and working. But now this sounds like you need a complete circuit design (or else a shopping pointer to buy something -- as such sensors exist as COTS sensor systems.) If this is shopping, the question isn't appropriate. If it is circuit design, you are probably also out of bounds as you aren't asking a small question of clarification anymore given your knowledge. – jonk Apr 24 '17 at 19:41
  • @ChrisH Or can you point to the "complete the circuit" part that you are currently using to condition the wet traces to a signal you can apply to a 555? – jonk Apr 24 '17 at 19:43
  • I don't have any conditioning on the wet traces. I'm ok with the traces taking time to soak in the water until their resistance drops enough to trigger the LMC555. The LMC555 takes very little current to activate and I've been able to get it to work just playing around with it on my bench. Putting the sensor inside a sponge with table salt speeds up the process but isn't necessary. Looking at this question https://electronics.stackexchange.com/questions/180716/555-timer-one-shot-trigger it seems I might need to incorporate a differentiating input? – Chris H Apr 24 '17 at 20:01

0 Answers0