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How do I use a PIR (Passive infrared sensor) motion sensor to turn on a 3v battery? Answered

Hi All,
I want to use a PIR to turn on a small 3v DC motor.  I know that I can't just directly attach the motor to the PIR as there isn't enough power to run the motor.  I have done some research and it looks like I could use a 3.3V relay as a switch to drive the higher power needed for the motor

what I want to know is how to wire it all up?  Do I have to supply a separate power source to the relay to run the motor?  If anyone has a schematic that would be great.  I found this relay on ebay which I thought might work but I'd like to make one myself if possible.

thanks for your help.

lonesoulsurfer

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iceng
iceng

8 years ago

Here is a two voltage relay that is controlled by a PIR detector.

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PIR Rly.GIF
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lonesoulsurfer
lonesoulsurfer

Answer 8 years ago

Thanks so much iceng. Makes perfect sense.

Can you recommend a relay to use? I really only want to use 4.5v to trigger the PIR so the relay would need to trigger at around 3.3v, the same as the PIR.

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iceng
iceng

Answer 8 years ago

Try this 5V coil  relay  from parallax for $2.75 see the spec.

A

5-PIR Rly.GIF
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lonesoulsurfer
lonesoulsurfer

Answer 8 years ago

Will a 5v relay work ok even if the voltage trigger from the PIR is only 3.3v?

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iceng
iceng

Answer 8 years ago

Might not work, i found this 4.5 VDC unit.with different pinout 
maybe you can find a lower voltage ////

How do you make a 3.3 v supply. ?

Or show the PIR schematic so I can engineer a transistor instead..

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lonesoulsurfer
lonesoulsurfer

Answer 8 years ago

Hi Iceng, What about this relay I found on ebay - it triggers at 3V - would this work?  Sorry - I couldn't find a schematic of the PIR sensor that I have.  It's one I brought from Ebay and are very common.

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lonesoulsurfer
lonesoulsurfer

Answer 8 years ago

The power supply to the PIR can be anything from 3.5v to up to 12v. it will give out 3.3V (on) once it detects heat for a period of time then drop down to a low voltage (off) state once it has finished.

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iceng
iceng

Answer 8 years ago

Try it on 6V and 12V with a voltmeter on the PIR output
to see if it only delivers 3.3v in both cases ??

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lonesoulsurfer
lonesoulsurfer

Answer 8 years ago

Yep - tried that already. It always delivers 3.3v. The PIR has a little voltage regulator on it.

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iceng
iceng

Answer 8 years ago

Looking at the eBay relay and this thread.

YES ...  they will all work together.

Run the PIR and relay on 4.5V ( 3 AA in series )
Run the motor and contacts on 3.0V ( 2 AA in series )

A

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lonesoulsurfer
lonesoulsurfer

Answer 8 years ago

Thanks for all your help. Really appreciate it!

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frollard
frollard

Answer 8 years ago

how much power does the motor take? Just use a transistor and a diode or two to drop the voltage for the motor.

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mrmerino
mrmerino

Answer 8 years ago

I've seen that relay before....

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iceng
iceng

Answer 8 years ago

good visual memory :-))

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mrmerino
mrmerino

Answer 8 years ago

Thanks.