HC-SR501 Pir Motion IR Sensor

My first impression is that these are great little sensors. They seem to detect motion well and hookup is really easy. They work perfectly to drive the 5 volt input of a relay module; when I get time to experiment I'll see what they can really do.

Hooked the VCC and ground to a 12v A/C adapter and it worked perfect. The 3.3v "trigger" pin (the "OUT") even had enough juice to directly power a red LED (tho a little dimmer than normal). Very wide sensitivity pattern. Tho description says "< 100 degrees" I thought it was greater than 120. At default sensitivity it would trigger easily from 8-10' away. I never adjusted it as that was the perfect distance for my needs. Holding it with the sensor "dome" facing down and the adjusting (orange) "screws" facing you, the left one is the sensitivity, the right is the duration that the OUT pin will stay "on" (i.e. hold at 3.3v). Minimum seems to be around 4 seconds.

These modules are perfectly good for most applications they are intended for. If you want very close movement (2 meters to 2 millimeters) you need to sonic ones. I tested these with an ESP-12 and have great results. For those complaining about false positives, and unreliable readings... When you switch a GPIO pin to an input and hang a long wire off it, that wire can pick up stray signals and cause the voltage on the GPIO pin to vary. This can cause the system to think the switch has changed. To fix this, there are several things you can do. 1. add a pull-up resistor 2. add a bypass capacitor 3. shielding on the wire 4. use twisted pair wiring A pull-up resistor is a resistor connected between the GPIO pin and 3.3v. The exact value of this is not critical, 4.7k is a common value to use, as is 10k. This ensures that when the switch it open, the GPIO pin will go high. A bypass capacitor is a small (pF range) capacitor that is connected between the GPIO and ground. This provides a path for any radio signals that are picked up by the wire to go to ground and not confuse the system. Shielding or using twisted pair wiring are other ways to reduce the effect of radio signals on the system.

So far I have used two of these and both worked right of the bat. Very easy to hookup with ample code available on the net. I have these hooked to an ATTINY85 programmed to turn on when motion is detected. I used Pin7 as the light output which goes straight to a 2N2222 via a resistor. The transistor accepts the signal, turns on and activates a relay which the lights are connected to through COMM. Love um....a lot of fun to fool with.

I first bought a pre-built 'smart' light switch with a motion sensor that was WAY too sensitive and couldn't be adjusted. I decided to go the DIY route and bought these. I connected one to a NodeMCU, which I set up with Home Assistant using MQTT (along with a simple 'smart' switch sans motion sensor). It's awesome, gives me so much more control and cheaper. I do get some false-positives, but not nearly as much as with the pre-built unit. I suspect it's due to being in the garage and the sensor picking up sunlight/shadows coming in through cracks in the garage door. Overall, very happy with the purchase.

I'm providing a middle rating because documentation is not good. I don't know if I misread the specs when I purchased but I thought these PIR sensors would work with 3.3v input/source voltage and output 0v low and 3.3v High on output. I now know that they certainly need 5v source input voltage and output 3.3v. When I provided 3.3v input power they acted erratically and would not go predictably low. Using 5v input the 3.3v output consistently dropped low to 0v. In Summary: 5v In = consistent 3.3v out when triggered, and 0v when clear/not triggered.

Work as advertised. No docs but they are standard and worked with some example code i found on Internet (Arduino project). If you get weird reactions double check the voltage and connections. I had mine on a really long wire and had some issues at first, but due to a faulty contact in my own soldering. You can measure the reaction easily with a volt meter..

Great price, amazingly sensitive and reliable. I have this hooked up to a project that makes sound and it is in the other room, the only time it ever make sounds when I walk in. Can be adjusted for sensitivity, I have it set to max.

These are useless as motion detectors due to the false reports. Yes I tried the pF capacitor from Signal to Ground and I shortened the signal wire. It did reduce the false motions but they still happen few minutes. Unfortunately I can't send them back due to adding the capacitor to the circuit. I would not recommend these to anyone. I will try to keep fussing with them. I am trying now on a Wemos D1 Mini with less than 1 cm of signal wire from PIR to data pin. UPDATE: Still bad on the wemos with <1 cm wire. Tried pull up resistor - no effect. Increased and decreased capacitor size - no effect. I've ordered a different type. Hope they work.