The 3.3v output from a PIR movement detector doesn’t have enough current to energise a mechanical relay. With a little modification it is possible to make it work with a solid state Mosfet based relay.
For this project I used an HC-SR505 which are available from Ebay, AliExpress etc combined with a 3V Low Control High Voltage Mosfet Module (http://www.ebay.es/itm/291961255582)
Assuming your power supply is between 4.5 and 20v it might be possible to run the PIR and the load you want to activate from a single supply. The PIR requires a voltage between 4.5 and 20v but in my experiments it wasn’t happy with higher voltages. For this experiment I am using two separate power sources. Two button cells to power the PIR at 6v and a single 18650 battery to power the load (an LED) at 3.7v.
On the diagram above (1) is the PIR movement detector. When this senses movement it outputs 3.3v along the white cable (2) to the positive pin of the mosfet module (3). The current output by the PIR when activated is very small – 0.45mA and isn’t enough to overcome the resistance of the resistor and LED and then activate the mosfet to open the relay. There are two modification options to make it work. One is to solder a thin copper thread to bypass the resistor (4) or, if this isn’t allowing enough current to pass.. the same technique can be used to bypass the green LED.
The Mosfet relay allows switching of voltages up to 36v so if you are looking to switch on and off a voltage between 20 and 36v this setup will work well with the PIR being powered separately. The PIR has a very low current draw of less than 60uA (microamps) when it’s not activated by movement so it will run a long time on button cells. Maybe up to half a year with two cells as in this project.
If I've saved you some time/hair and you want to say 'Thanks!' you can buy me a coffee here...