A circuit without Arduino to shut off DC current, using a PIR sensor

Thread Starter

doronb

Joined Jan 28, 2009
17
HI,

I wish to build a simple circuit, using a PIR sensor (fed by 5v dc) to shut off DC current to the load.
The idea is that when somebody approaches a designated area, the PIR sensor will notice movement,
and the load will shut off.
After the automatic delay of the PIR sensor ends, the signal from it will drop back to zero, and the load will turn on again.

the load is 2A, 12V.

The components that I already own (and off course, most of the basic components in electronics kits):
1. IRLB8721 N-channel MOSFET (but I think that is used to do the opposite of what I want to do)
2. PIR sensor that is outputting 3.3v when triggered.

I do know to make it happen with an Arduino, but my question is whether it's possible to get the same result with a simple circuit without an Arduino ?

I will appreciate an example of the circuit.

Many thanks

Doron
 

KeithWalker

Joined Jul 10, 2017
1,216
Do you understand enough about electronics to try to design the circuit yourself? We will help you with your design but we are not a free design team.
Regards,
Keith
 

ebeowulf17

Joined Aug 12, 2014
3,274
I'm not familiar with PIR sensor timing, but if the load switching just needs to be the inverse of the PIR output, this should be a pretty simple circuit.

Your N channel MOSFET can be used as a low-side switch to control the load (load connects to drain pin and source pin connects to ground,) and it will be on as long as its gate pin is at least 3V or so higher than its source pin.

Since you want the load on when the PIR output is off, you just need an inverter, which could be as simple as a little NPN transistor, like a 2N3904.

You'd need a current limiting resistor for the NPN base, as well as a few pull up and pull down resistors to set the default states for each transistor, but should be pretty straightforward.

See if that description makes any sense to you (or if you can make sense of it after some online reading.) If not, let me know what questions you have.
 

ElectricSpidey

Joined Dec 2, 2017
1,232
Controlling loads from those PIR modules does seem to be the holy grail these days.

The drive voltage for the MOSFET you listed is 4.5 to 10 (Max Rds On, Min Rds On)

So, be sure to tie the collector of the inverting transistor to the 12 volt rail, not the 5 or 3.3.
 

ElectricSpidey

Joined Dec 2, 2017
1,232
BJTs are current devices that will go into cutoff as long as no current is flowing on the base, but it does take time to shutoff, so if higher speeds are needed, then I would say a pull down would be in order, but not here.

MOSFETs on the other hand "MUST" discharge the gate to turn off.

Some people feel more comfortable using pull up/downs with BJTs but many times they are just a waste of current.

The OP can choose to use one if they feel it's necessary.
 

Thread Starter

doronb

Joined Jan 28, 2009
17
GREAT guys,
this is exactly what i wanted, and i tried to build the circuit by myself, and got confused....
that's how it is when you are a tour guide, ex engineer, into drones, guitar, electronics and more...
every time when i start or coming back to a project, i have to re-learn what i already know but don't use often.
I will try it tomorrow.

many thanks to everybody

doronb
 

Thread Starter

doronb

Joined Jan 28, 2009
17
Does the NPN base need a pull down?

I'm not too familiar with PIR output stages, but if there's any chance that it's floating at any point (for example, if 12V supply is present, but 5V supply isn't) then the pull down would provide a little insurance.

Maybe overkill though, I'm not sure. It really is a question, not a statement on my part.
I gave tried this circuit, but with 12v input to the PIR (As I have realized that it is 12v capable) and as a load, I put a 5v diode + a resistor.
The outcome of this circuit was a blinking diode.
Does a pulldown resistor mean a resistor between the base of the transistor and the ground ? or between the emitter and ground ?
The outcome might also be due to the settings of the PIR sensor itself, I will check the circuit while playing CW-CCW with the delay knob, and also with the bridge that it has.
 
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