PIR MOTION SENSOR + LED problem

Discussion in 'The Projects Forum' started by Arhont, Jan 8, 2013.

  1. Arhont

    Thread Starter New Member

    Jan 8, 2013
    12
    0
    Hi,

    i'm new in electronics, so maybe i'll ask stupid questions, but really need your help. I'm trying to connect this PIR

    http://www.ebay.com/itm/370671048122?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

    to light up 3 led in parallel, but no luck. When i connect 1 led directly to +out of PIR, just getting 2,68V and that's not enough to light up led properly.

    With no load PIR is giving 3,3v.

    I understand that i should use trensistor and some resistor, but i need a circuit and value of resistors.

    regards and looking forward for your answers
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    What are you using to power the PIR?
     
  3. Arhont

    Thread Starter New Member

    Jan 8, 2013
    12
    0
    6v DC, also tried with 12v dc but output is 2.68v when directly connected led to pir output.
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Try this, and see if it works.
     
  5. Arhont

    Thread Starter New Member

    Jan 8, 2013
    12
    0
    i suck at reading diagrams...

    Please correct me if i'm wrong:
    is it OK to put 100k on pir output? Also, a 150 ohm will be enough for 6V (diagram shows 12v and 1k)? OK to use BC 547 NPN?
    And what i see in diagram - connect all 3 led's anote>cathode>resistor, then connest all 3 cathode to emitter, then connect PIR out to base, and connect collector to ground?

    sorry for bad english, it's not my native language
     
  6. Arhont

    Thread Starter New Member

    Jan 8, 2013
    12
    0
    This scheme works, but i'm only getting 8mah on entire circle. Is it maybe because of bc547 or 100k resistor i used on PIR output?

    waiting for answer

    regards
     
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    I was too hasty. The 100k resistor is too large. Try some smaller values; I will calculate after lunch.
     
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    You had a 10K in your diagram.. Which should be fine..
    He seemed to only have a 100k resistor I guess and used that..hence the problem.
     
  9. Arhont

    Thread Starter New Member

    Jan 8, 2013
    12
    0
    mcgyvr is right, i had only 100k resistor :)
    i'll buy 10k tomorrow and reasemble the circuit.

    Tracecom please do the math and post me the results so i can better understand the problem and avoid similar problems in future. Have a nice lunch :) I just had my dinner, in my country it's 8 pm now :) Sorry for the off topic.

    Another question - i guess a bc 547 is fine, and it's rated to 30V and 0.1a, i guess that's means that he can conduct only 100 mah of current (approx. 5 led)?

    regards
     
  10. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    What color are your LEDs?
     
  11. Arhont

    Thread Starter New Member

    Jan 8, 2013
    12
    0
    White 3.0 - 3.2 fv, 20ma
     
  12. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Then, given a 6V supply, you have correctly chosen the proper current limiting resistor value of 150Ω. And as you have stated the maximum collector current of the BC547 is 100mA. Three LEDs would present a 60mA load to the BC547, which seems like a safe working load.

    The value of the BC547 base resistor should be low enough to ensure that the transistor is saturated, i.e., fully turned on, since you are using it as a switch as opposed to an amplifier. Although the circuit does function with the 10k base resistor as shown in my original diagram, that limits the base current to only .6mA, which seems too low. I would prefer the base current be at least 6mA, and suggest that 1k for the base resistor would be a better choice.

    Now the disclaimer. I am not an engineer; I am a tinkerer, and math is my weak suit, so I cannot explain exactly how much base current is required to drive the BC547 into saturation. Maybe mcgyver, or some other forum member, who is more knowledgeable than I will jump in and correct where I have gone wrong.
     
  13. Arhont

    Thread Starter New Member

    Jan 8, 2013
    12
    0
    Thanks for quality reply, i'll try with 1k and then read the mah in circle.

    Mcgyver and other forum members, please comment our calculations and correct us if we get off the track

    Regards
     
  14. Arhont

    Thread Starter New Member

    Jan 8, 2013
    12
    0
    Can someone please give the calculation to put etc 60 mah (3 led) through transistor base? which tipe of resistor should i use?
     
  15. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    You don't want 60mA through the base. The rule of thumb when a transistor is being used as an amplifer is that the base current should be 1/10 of the collector current. In that case, you would want 6mA through the base, which is what a 1k resistor would give. However, you are not using the BC547 as an amplifier, but rather as a switch, which means you want the transistor fully on, i.e., in saturation.
     
  16. Arhont

    Thread Starter New Member

    Jan 8, 2013
    12
    0
    I think i understand what you are talking about...
    base resistor is not fixed value, and i should change his value according to desired/used current on the diodes (collector).
    6v/0.006=1k
     
  17. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Correct. 60mA collector current divided by 10 equals 6mA base current, which is provided by the 1k base resistor as you have calculated above.
     
  18. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Arhont,

    I built the PIR LED Trigger circuit today, connected the PIR, and experimented with different values of base resistor. The result is that 470Ω is the optimum value, allowing 56.8 mA total current through the 3 LEDs. However, the 1k resistor that I previously suggested allowed 56.5 mA total current through the 3 LEDs. From a practical perspective, there is no difference. At 3k, the collector current drops to 53.3 mA, and then begins to fall more rapidly. The finalized schematic is attached for your interest, or anyone else who might land on this thread.

    Good luck.
     
    Last edited: Jan 9, 2013
  19. Arhont

    Thread Starter New Member

    Jan 8, 2013
    12
    0
    Tracecom, thank you very much for your help with wiring up this cirle,
    i haven't tried anything yet duo to lack of resistor in my local store, so i simply must wait ebay shipping for the new one...

    Thanks again!

    Regards
     
  20. Arhont

    Thread Starter New Member

    Jan 8, 2013
    12
    0
    Just one more thing,

    how did you get/calculate 470ohm value for base resistor?

    I guess not by the formula 6v/0,094
     
Loading...