Adding a PIR sensor to a string of LEDs

[sentry7]

Hey guys,

I have a string of decorative LEDs that's powered by 3 AA batteries. There seems to be a series resistor as well (looks like 5.1ohm, 2 watts?...I can't see it very well). I need the lights to turn on when people come into close proximity, so I want to add a pir sensor. The sensor is 3.3v, but I'm not sure about the current draw.

So...something like this?

[georges80]

You say PIR sensor, but is that a raw sensor device or a module.

Raw sensor needs amplification and lens etc...

If a module then it must have some basic specs.

cheers,
george.

[sentry7]

It is a module. Sorry, not at the bench ATM, will get back with some specs.

[SeanB]

If it is a 3V3 module simply put a 3v tap in the battery pack, so it will get a 3V supply. Many though are 5V modules so will run direct of 4V5. A 1000uF 16V capacitor across the power leads to the PIR will help it as the batteries age. These typically are a low power draw though, so should last quite a while on a set of AA cells. You can increase the series resistor for the leds to 10R without much light reduction, but with a good increase in battery life.

[ajb]

If the LEDs are blue, green, or white, then current draw is ~300mA.  I = (Vsupply-Vled)/R = (4.5-3)/5.1 = 0.294, or if they're red, I = (4.5-2)/5.1 = 490mA. 

You haven't shown the LEDs in your diagram.  Assuming R1 is the current limiting resistor for the LEDs, then the LEDs should be in the same spot.  So Vcc->Resistor->LEDs->collector of the transistor.  This will work as long as the PIR produces a positive pulse when triggered; if it's inverted then you'll need to flip everything around and use a PNP transistor (or add a second transistor as an inverter).

[edavid]

If the LEDs are blue, green, or white, then current draw is ~300mA.  I = (Vsupply-Vled)/R = (4.5-3)/5.1 = 0.294, or if they're red, I = (4.5-2)/5.1 = 490mA. 

It's extremely unlikely that a battery powered LED string would draw that much current.

They could be lower voltage LEDs, with 2 in series.

Or there could be resistors in series with the individual LEDs in the string.

Or OP misread the value (OP, do you have a multimeter?).

[sentry7]

Sorry for the delay. So I did a meter check, and the resistor is indeed 5.1 ohms. I'm not sure about the power rating, I have a pair of calipers and can check the diameter.

[edavid]

Did you try measuring the current draw of the LED string when you connect it to the batteries?

[sentry7]

Did you try measuring the current draw of the LED string when you connect it to the batteries?

Okay, I didn't measure the current directly, because I have a non-operational ammeter, but I got a voltage of 1.35v across the series resistor, so I = 1.35v ÷ 5.10ohms ~ 260mA. I'll find some specs for the sensor.