laser and PIR (Passive Infrred ) circuit in need of help

[bobgrif]

Need a little help in figuring out what resistor to use. Appreciate any help.

 

[andre_luis]

You should not use a NPN transistor, but a PNP instead.

 

[FvM]

Also should use current limiting resistors.

 

[bobgrif]

You should not use a NPN transistor, but a PNP instead.

Thank you. Can you tell me what else I should know in selecting the PNP transistor?

 

[bobgrif]

I need all the help that you are willing to provide. What you say absolutely makes sense. I clearly do not want
to burn out the lasers. How are these limiting resistors acquired? What else do I need to know when I purchase them? Grateful for your help.

Bob

 

[FvM]

Reviewing the SR-HC501 data sheet reveals that it has a 3.3V logic level output, active high with 1k current limiting resistor. A NPN transistor in common emitter configuration seems as an appropriate output circuit.

Laser diode current limiting resistor has to be chosen according to laser data.

 

[bobgrif]

Reviewing the SR-HC501 data sheet reveals that it has a 3.3V logic level output, active high with 1k current limiting resistor. A NPN transistor in common emitter configuration seems as an appropriate output circuit.

Laser diode current limiting resistor has to be chosen according to laser data.

I want to be clear. Are you saying that the SR-HC501 already has a limiting resistor? Do I still need one?
If the PIR has a 3.3v logic level output and my lasers are 3.0 Volts, why can't I use that power source and not use the transistor?
And, you are saying that the NPN transistor currently in use is ok and need not be replaced by the PNP?
You must see that I am over my head. Would you be interested in completing this circuit for compensation?

 

[d123]

Not sure what make the PIR is, I found this datasheet, below are the link and the datasheet:

I am guessing that post #6 may have seen a different datasheet, as this uploaded one shows no 1K resistor on the output.

You would need to know the model of laser diode, and say what it is, ideally uploading the specific datasheet, so that the current limiting resistor can be calculated.

For a LED, to calculate the current limiting resistor:
1) Input or supply voltage minus the LED voltage.
2) Result of this calculation divided by the LED current required in Amps.
3) The result of the second calculation is the resistance needed in Ohms.

e.g. 9V supply. 1.7V LED. LED requires 5mA current (5mA is 0.005A).
9V - 1.7V = 7.3V
7.3V/0.005A = 1460 Ohms

The resistor needs to be the closest "off the shelf" value available to 1460 Ohms.


You may be able to use the PIR output, but with two diodes that may be asking a bit too much and could need a transistor, either way you still need to know the current each laser diode needs and add the two together, and how much current the PIR can output safely before overloading it, and if those values fit calculate the resistor from there.

https://www.mpja.com/download/31227sc.pdf


...I've just seen R14, the 1K resistor mentioned in post #6 - for a moment I'd thought you only had the PIR, not the whole module, my apologies.

Here's the BISS0001 datasheet link, in case it's of any help...:
https://cdn-learn.adafruit.com/assets/assets/000/010/133/original/BISS0001.pdf

It says th BISS0001 can only output 10mA, so the transistor may be necessary.