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

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.


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iceng5 years ago
Here is a two voltage relay that is controlled by a PIR detector.

lonesoulsurfer (author)  iceng5 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.
Try this 5V coil  relay  from parallax for $2.75 see the spec.

lonesoulsurfer (author)  iceng5 years ago
Will a 5v relay work ok even if the voltage trigger from the PIR is only 3.3v?
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..
lonesoulsurfer (author)  iceng5 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.
lonesoulsurfer (author)  lonesoulsurfer5 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.
Try it on 6V and 12V with a voltmeter on the PIR output
to see if it only delivers 3.3v in both cases ??
lonesoulsurfer (author)  iceng5 years ago
Yep - tried that already. It always delivers 3.3v. The PIR has a little voltage regulator on it.

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 )

lonesoulsurfer (author)  iceng5 years ago
Thanks for all your help. Really appreciate it!
how much power does the motor take? Just use a transistor and a diode or two to drop the voltage for the motor.
mrmerino iceng5 years ago
I've seen that relay before....
iceng mrmerino5 years ago
good visual memory :-))
mrmerino iceng5 years ago