You used 3.3V as your numerator. The equation I supplied specifies that the numerator should be (3.3-0.7). Then, you’d divide the resulting 2.6V by 2.7mA, resulting in a resistor value of 963Ω. Or a 1K standard resistor.

You need to subtract the Vbe value from the input voltage. Vbe is the voltage drop between the base and emitter in a transistor. I used a general value of 0.7V, which is good enough. You could be more specific and use the Vbe value in the datasheet.

This is the kind of thing you have to be careful of. The equation (which you included in your post) specifically stated (3.3-.7). However, it looks like you just ignored the “.7” when you did your own calculation.

OH! I didn't realize that was a subtraction; the social scientist in me took over and I assumed it was a span (like 3.3 to .7)!! The voltage drop totally makes sense!! Now I understand!! Thank you!!!

 

Success # 2!! Ok, I ran the 2 LED's in parallel with 1K resistors to each. but I'm not sure what the current draw for each LED is so I will attempt to check that.

 

OK now I have a different question: I have the amp meter hooked up between the emitter of the NPN and ground, with the 1K resistors to the LED's it was reading 0.01 (on the 10 amp setting). When I tried to increase the load by replacing the LED resistors with 2K the reading dropped to fluctuating between 0.00 and 0.01?! I thought the current reading would go up? What did I do wrong?

 

OK now I have a different question: I have the amp meter hooked up between the emitter of the NPN and ground, with the 1K resistors to the LED's it was reading 0.01 (on the 10 amp setting). When I tried to increase the load by replacing the LED resistors with 2K the reading dropped to fluctuating between 0.00 and 0.01?! I thought the current reading would go up? What did I do wrong?

The resistance going up means the load (the current flow) went down, exactly as you'd expect.

So it sounds like your LED is switching on/off as you apply/remove a voltage to the base resistor?

 

Oh ok. Thanks Wayneh!! So how should I go about trying to match the 27.4 mA draw of the mp3 with LED's? I added a 3rd LED/1k resistor and I'm up to 12.5mA do I just keep adding LED's in parallel?

 

Or should I put in lower reisitors!? Did I just have a breakthrough?

 

Oh ok. Thanks Wayneh!! So how should I go about trying to match the 27.4 mA draw of the mp3 with LED's? I added a 3rd LED/1k resistor and I'm up to 12.5mA do I just keep adding LED's in parallel?

Three LEDs can safely handle 30mA, 10mA each. Do you have other resistors to use? With 5V you should be safe going down to any value over 150Ω. Try 330 or 390Ω if you have them, even 470Ω. As you are seeing, typical LEDs are plenty bright at even 4mA. They'll be much brighter at 10mA but still safely below their maximum rating.

But if your switch is working, I'd consider this a learning opportunity but not an effort likely to uncover your problem. I don't think the transistor is the cause of your problem per se but is a victim of something else. Nothing wrong with confirming that.

 

Oh ok. Thanks Wayneh!! So how should I go about trying to match the 27.4 mA draw of the mp3 with LED's? I added a 3rd LED/1k resistor and I'm up to 12.5mA do I just keep adding LED's in parallel?

You should know the current requirements and forward voltage of the LEDs you are using. Many common LEDs (5mm red or green) typical current requirement is 15mA. Hence, my suggestion for two in parallel, which would draw 30mA. Or you can calculate current limiting resistors for your LED to draw 14mA. That way, your test setup will draw 28mA.

Typical forward voltage (Vf) for a red LED is 1.8V. Using this value and a desired current of 14mA, use this equation. Vs is supply voltage and is 5V in your circuit. Vf is 1.8V. Current or Iled is 0.014A.

R = (Vs - Vf)/Iled
= (5.0-1.8)/0.014
= 3.2/0.014
= 228Ω
The value is not critical, so since you appear to have 220Ω resistors, I’d use two of them.

 

Again, wayneh responded while I was typing. I agree with his assessment.

 

You folks are amazing!! Thank you! Yes this is a (much needed) learning experience. OK I have set it up with 3 LED's, in parallel, running 330 Ohm resistors. It is pulling 32.7mA.
So I think the next test is with an MP3 player and hope that there is no smoke!

 

Whoo hoo!! It works!

 

MP3 player is working with the PIR!! So it was a combination of my ignorance of resistors and an (apparently) faulty MP3 module!!
You folks are the best!!

 

Congratulations! Glad you got it to work.

 

I would say the kudos are yours! I was absolutely baffled! Now I have a few more tools in my shed for understanding this stuff! Thank you, thank you, thank you,!!!!!