Activate mosfet with transistor and pir

Thread Starter

christiannielsen

Joined Jun 30, 2019
298
Hi,

Im completely green, have only been watching around 100 videos and are now starting to experiment myself.

Im starting out with making a startling scarecrow to scare away the deers from my rose garden.

So I thought I would make it with a PIR which sends voltage to a transistor which activates a mosfet for a larger current to run a motor or a solenoid of some kind.

The following circuits PIR does work (blue LED lights). It lights up the LED for 3 sekunds and turns off.
The circuits MOSFET also works fine (green LED lights) if I activate it manually with the wire floating in the air.

My question is, and I have tried several places, where do I connect the wire to activate the mosfet, and do I have a circuit that actually can do that?

The first photo should give you some information about the circuit.

It is suppose to run on 6-12 volts, I will adjust it when I figure out what its going to run.

from
Christian Nielsen, Denmark
 

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ericgibbs

Joined Jan 29, 2010
10,452
hi,
Please confirm what the PIR output voltage is when the PIR is activated and while at rest [ not activated]
The PIRs I use are +3V when activated and 0V at rest.

E
 

ericgibbs

Joined Jan 29, 2010
10,452
hi,
Take out that 1K resistor from the Gate to 0V, so have only the 1K from the PIR out to the MOS Gate. OK.
Lets get the MOS working then we can increase the Gate voltage by using the transistor.
E

EDIT:
Your MOS is incorrectly wired.!
The S goes directly to 0V
the D goes to the resistor for the LED
and the G goes to to the 1K to the PIR..

Do you follow OK.?
E

Updated: added the basic circuit

fet1.PNG

PIR2.PNG
 
Last edited:

Thread Starter

christiannielsen

Joined Jun 30, 2019
298
hi,
Take out that 1K resistor from the Gate to 0V, so have only the 1K from the PIR out to the MOS Gate. OK.
Lets get the MOS working then we can increase the Gate voltage by using the transistor.
E

EDIT:
Your MOS is incorrectly wired.!
The S goes directly to 0V
the D goes to the resistor for the LED
and the G goes to to the 1K to the PIR..

Do you follow OK.?
E

Updated: added the basic circuit
stupid question: are you sure its connected incorrectly?
because now the mosfet cant be switched on manually with the wire.
 

ericgibbs

Joined Jan 29, 2010
10,452
hi christiann,
Sorry you have wired incorrectly.:(

1. Remove all the wires to the MOSFET.
2. Connect the Source pin directly to 0V [ no resistor]
3. Connect the Gate pin to the PIR output via 1k
4. Connect the LED and a resistor from the Drain pin to +Volts

Look at my diagram in post 8.

You do not want the LED in the Source line.

E

EDIT:
Look at this image.

View attachment 180724
Breadboard.jpg





 
Last edited:

Thread Starter

christiannielsen

Joined Jun 30, 2019
298
I can't post more than 5 posts per hour...sorry

I followed your drawing and this should be it
fullsizeoutput_30e.jpeg

The blue LED lights on PIR detection.

Now the green LED lights for 2 sec just like the blue LED does on PIR detection.

Shouldnt the green LED keep lit because its all about voltage?



I dont get it
 

ericgibbs

Joined Jan 29, 2010
10,452
hi,
Thats good progress.
The Green light will only be Lit when the PIR is activated, for that 2 second period.
The Blue LED and BC transistor are wired incorrectly, do you want me to post a sketch.?
E
 

Thread Starter

christiannielsen

Joined Jun 30, 2019
298
Do you know if the 5 post per hour limit will disappear at some point?

hi,
Thats good progress.
The Green light will only be Lit when the PIR is activated, for that 2 second period.
The Blue LED and BC transistor are wired incorrectly, do you want me to post a sketch.?
E
I thought the mosfet was turned on by the PIR and would stay on because of its capacitans?

Is it really? damn, yes I would very much like that..
 
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