Hi:
I'm trying to eliminate the need for the arduino UNO in using a cheap PIR (<$5.00) to drive a power relay (again the cheap one) directly. It works great when I use a UNO sketch that reads the PIR output pin, decides to fire an output pin, and turn on a 120 volt lamp via the relay. Am I asking too much of the output of the PIR to connect it directly to the input pin on the relay. I'm still using the power from the UNO for VCC and GND on both the PIR and the relay. It will not work. I've tried switching the Normally open and normally closed pins on the relay power side and it still does not work. Any thoughts?
Bob
That PIR has a 3.3volt regulator onboard, so 3.3volt logic.
When the PIR senses someting, the chip outputs a HIGH (3.3volt), but there is a 1k2 or 1k5 (seen both) current limiting resistor between PIR chip and output.
A problem if you want to use a relay module that needs 5volt/2mA.
If you have a common relay module, use a small-signal NPN transistor (BC54*, 2N3904, etc.)
PIR output directly to the base, emitter to ground, collector to the relay module (on active LOW).
Leo..
Will do. Thanks much.
StAugustine:
Will do. Thanks much.
And don't forget the back-EMF diode across the coil of the relay, or you will kill the transistor in short time.
// Per.
A relay module already has a relay drive transistor and kickback diode.
The extra transistor I was talking about is used as level shifter between PIR and relay module.
Leo..
Could you please give schematic? I have the output of the PIR going to the base, I have the emiiter going to ground. The collector should connect to the control pin of the relay module? The power for the relay coil still connects to +5 and ground?
StAugustine:
The collector should connect to the control pin of the relay module? The power for the relay coil still connects to +5 and ground?
Yes? If you are using a relay MODULE. You should stop talking in terms of 'relay coil' as if you have access to them directly.
Transistor is amping the PIR output along the way to the relay module input.
All of this is assuming the PIR is putting out the kind of signal the relay module needs.
There are a lot of active LOW relay modules, and I assume the PIR puts out active HIGH.
Works great. I had to change from NO to NC on the relay module but it works fine. Thanks much!
Incidentally, I used a 2N2222 transistor.
StAugustine:
Works great. I had to change from NO to NC on the relay module but it works fine. Thanks much!
That sounds very bad.
The relay and therefore lamp is only OFF while the relay is getting a signal. If you have the lamp connected to NC now, it means you're literally sucking power continuously to keep the lamp OFF.
Sounds like the logic is inverted as I suspected, you should really look into that. I assume the light will be off more than it is on. I.e., you're wasting a lot of power and there's a big area of misunderstanding that should be filled in.
To clarify:
On the relay, it has a COMMON, a Normally Open, and a Normally Closed.
"Normally" meaning the state the relay is in if you were to unplug everything from it. No power, no signal. A line run from COM to NC would be a completed/closed circuit. So having your lamp wired to NC means the light is on if you were to unplug everything. A signal input tells the lamp to turn OFF, instead of what you are likely wanting, where it stays off until a signal tells it to turn ON.
The Normally Open side only gets closed as long as the relay coil is energized, meaning the module has power as well as having the correct signal on the signal input (and in this case I'm suspecting a signal pulled LOW).