Latching reed switch

crutschow

Joined Mar 14, 2008
34,002
I would use AK's circuit in post #17.
It uses just one CMOS IC and one small MOSFET with no 555 required, and is very low power (only leakage current when off, and much less than a standard 555 when on).

Note that (if not apparent) the component labeled "reed switch" is your vibration sensor.
 
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crutschow

Joined Mar 14, 2008
34,002
Below is the LTspice simulation of AK's circuit.
The output starts pulsing the LED (red trace) when the Vib switch S1 closes (yellow trace).
It stops when the Reset switch closes (green trace).

The only change I had to make, was to use a lower max Vgs(th) MOSFET (logic-level type), since the 2N7000 can have too high a Vgs(th) to properly turn on from a 3V signal.
The one I selected was an arbitrary one in my library. It can be just about any logic-level N-MOSFET with a max Vgs(th) of 2V or less.

1690509788597.png
 
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Thread Starter

shawn19

Joined Jul 23, 2023
11
#21 looks simple and inexpensive. Thx. But I wonder if output is less than 3v. My Ckt needs min 3 v to function.
 

crutschow

Joined Mar 14, 2008
34,002
But I wonder if output is less than 3v. My Ckt needs min 3 v to function.
If you change R1 to 3kΩ, the 2N3906 should have a output saturation voltage of <50mV @ 10mA load, so the output voltage would be about 2.95V with a 3V battery.
If you need less drop you could change Q2 to a logic-level type P-MOSFET with a low on-resistance.
 

AnalogKid

Joined Aug 1, 2013
10,943
If you change R1 to 3kΩ, the 2N3906 should have a output saturation voltage of <50mV @ 10mA load
Yeah, I'm working on an update. I temporarily forgot that Vcc was so low. For that, a 3906 gets you Vcc switching rather than GND switching, and more consistent performance at such a low drive voltage.

The 2N7000 Rdson is around 5 ohms, maybe less. At that value, a load current of greater than 20 mA favors a 3906, with a saturation voltage of less than 0.1 V.

ak
 

Thread Starter

shawn19

Joined Jul 23, 2023
11
Such a great suggestion for using CPC1002N. It is cheap and small. I just hope the vibration sensor gives enough time for FET to close. It vibrates fast. Probably SW18030 (a slow or retarded sensitivity) would give more time. I should give that a try.
Thx
 

Thread Starter

shawn19

Joined Jul 23, 2023
11
Hi sghioto
The circuit board has a couple of modules, one of which is ESP12. I believe it requires 3.3V but it has been working fine with two AAAs at 3V. I don't think I can go much lower.
 

Thread Starter

shawn19

Joined Jul 23, 2023
11
Great questions. That has been the biggest issue, with saving battery. It pulls about 90 to 100 ma. To save battery we put the ESP12 to 'deep sleep' after 10 seconds. Preference would be to hold the power for 10 to 15 seconds using the latch to wake-up circuit, then actually disconnect the power until the next vibration is detected. I could use a CMOS 555 to do that. But yes, need 3V, and need to hold power for a few seconds and release to save battery.
 
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