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Voltage path query for nfet/monostable switch circuit

Canobi

Jul 25, 2014
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Hi folks :)

I recently acquired a WT588D-16p audio playback module that I'm getting acquainted with as I have a few prop related fx systems that require sound fx:


I thought I'd start with domething simple and make a prop blaster fx board and I'm trying to work out how to integrate it so it triggers an nfet/monostable circuit for controlling a high current LED used for the mozzle flash effect.

My knowlage is still under development so I'm pretty much just going through a prelim to make sure I have my thinking straight but any suggestions for things I havent thought of, or a simple yay or nay would be most appreciated.


My Setup
----------------

The module is set to button trigger mode for the fx system I'm working on. The module is a 5v device and the button pins measure approx 4.5v when floating and 2.6v when pulled to ground.

I then got to thinking about tying a path to one of the the buttons directly but as the pins are only 2.6v when closed circuit, it isn't going to be enough for the 555 trigger, so I thought something like the LP2992 logic level nfet could be used to tap 5v from the modules power rail as a monostable pre trigger.


I made this quick schematic of that particular juction:



However, I have little faith in the above diagram as it looks like I've just gone and given the floating a pin a path to ground via the gate's pulldown resistor, which I'm guessing will automatically trigger the pin when power is applied. Useful to know for other projects but no good in this case.

The only place I can think of to go from there is on the buttons negative side to ground as indicated by the arrow in the pic below:



Again, my faith is a little shaky and not sure if it'll work as intended.

I hope so as I was then going to use the monostable to trigger a second nfet that controls the high current LED for the muzzle flash effect.
 

Harald Kapp

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As shown this will unfortunately not work.
The transistor shown is an NMOS transistor. It requires a positive gate-source voltage to become conducting. As the drain is at Vcc, the gate voltage would have to be above Vcc in this circuit, which is not possible by simple means. Also, the 10k resistor from the trigger pin to GND is in parallel with the pushbutton and will keep the gate voltage at 0V regardless of the state of the pushbutton.

I recommend the following changes:
  1. remove the 10k resistor
  2. replace the NMOS transistor by a PMOS transistor
  3. connect the source (arrow) of the PMOS to Vcc.
  4. connect the load to drain of the PMOS
Note that you will have to use a MOSFET with a low threshold voltage (Vth) which can be operated by the Vcc (i assume 3V...5V) of your circuit, These often come under the nickname of "logic gate MOSFET".

[Edit] As you are going to trigger a monostable multivibrator from the pushbutton, you may probably not need the MOSFET at all as long as the monoflop can be triggered by the falling edge of the input signal. If the trigger input pin of the sound module has an internal pull-up resistor, the pushbutton will siffice. If not, add an external pull-up resistor to ensure high level on the inputs of both monoflop and sound module when the pushbutton is inactive.


Regards,
Harald
 

Canobi

Jul 25, 2014
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Oops, my bad. I'm terrible for getting the source and drain the wrong way round in my diagrams, it was supposed to be an NFET so I've re drawn it (took your advice and removed the 10k as well

)
 

Canobi

Jul 25, 2014
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Needed to say thanks for your advice and input Harald, was posting during my short work breaks and didn't get the chance earlier.
 

Harald Kapp

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No, sorry, this won't work either. The source of teh PMOS is constantly at ground, turningthe PMOS on. The switch has no effect on the PMOS.
You need to connect the source of the PMOS to the switched side of the button.

Also your symbol still showsn an NMOS transistor.

upload_2016-4-29_7-20-26.png
 

Canobi

Jul 25, 2014
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Again, thank you for printing out my errors. It's not even like I haven't used fets or anything, I just suck at schematics big time (lol).

However, it did prompt me to trawl over the 555 datasheet again and your assumption was indeed correct, it looks like I don't need the initial fet as the trigger pin requires only +2v min, so should work right off the button.

Couldn't find any info in the modules datasheet regarding internal pullup resistors for the button pins though, most of it is badly translated Chinese and related to operation but I can stick one on the pin if it does need one.
 

Harald Kapp

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Couldn't find any info in the modules datasheet regarding internal pullup resistors for the button pins though,
It won't hurt to add an external pull-up, e.g. 100kΩ
 

Canobi

Jul 25, 2014
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Thank you, I was going to use 10k but will get some 100k apon your suggestion.


I'm interested to know a bit more about PFETs, I'm unsure where to connect the gate in a certain scenario I want in another setup but will start another thread for that one.
 

Harald Kapp

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The difference between NFETs and PFETs is comparable to the difference between NPN and PNP: The polarities are reversed:
NMOS: VGS > 0 V, VDS > 0 V
PMOS: VGS < 0 V, VDS < 0 V
 
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