Adding Dark Sensor Circuit to LED wire string

Discussion in 'The Projects Forum' started by truwebs, Apr 27, 2016.

  1. truwebs

    Thread Starter Member

    Apr 2, 2016
    54
    2
    Hey All,

    I am tinkering with a dark sensor circuit. I have it working as noted in the schematic which I borrowed from a YouTube video. A couple questions so to better understand and add to what seems like a simple project.

    I understand the function of R2 is to prolong life of LED. Correct?

    What is the function of R1?

    I have seen many schematics similar to this with the major difference being R1 and R2 values. Are these resistor values dependent on the source and load?

    My goal is to add this circuit to this LED string.

    Besides the Source changing from 3V to 4.5 volts and the single LED load changing to an LED string. What else if anything needs to change to have the circuit integrated into the LED string?

    Thanks,

    TRU

    LDR Dark Detector.jpg
     
  2. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,131
    267
    R1 provides bias for the transistor, without this resistor, the transistor will never turn on.
    R2 is the current limiting resistor for the LED, LEDs must always be operated at a controlled current appropriate for the part.

    While this simple circuit works, its far from optimal.

    The point of switching is poorly defined, it will slowly turn on as the illumination of the LDR dims.

    Depending on how much current your LED string requires, you may need to change the circuit to supply more current, this circuit is designed to drive the LED at a feeble current, about 3 mA.
     
  3. truwebs

    Thread Starter Member

    Apr 2, 2016
    54
    2
    Sensacell,

    Thanks for the reply and pointing out the shortcomings. What changes would you make to improve the circuit?

    TRU
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    No time today but quick tip: 4.5 V supply, R2 = 15 ohm ( one string of 15 LEDs ), transistor - darlington or logic level FET driven by a comparator with a bit of positive feedback.
     
  5. truwebs

    Thread Starter Member

    Apr 2, 2016
    54
    2
    Bernard,

    Appreciate your response. When you have time would appreciate even more a schematic.

    Thanks,
    TRU
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
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    What do you have on hand in N ch. LL FET or darlington transistor , or 2N 2222A or sim. good for 1/2 A ?
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    I tried to not use a comparator but did not like results, so maybe this will work. R2- 5 ohms, was with 3 old alk.AA.s, 1 W white LED , 3.1V, @ 200 mA, D1326 Darlington. Be back Monday. Dark SW 00000.jpg
     
  8. truwebs

    Thread Starter Member

    Apr 2, 2016
    54
    2
    Bernard,

    As a rookie hobbyist I have a beginner inventory. Not seeing a comparator.

    http://amzn.com/B00E3JGGF2

    Each resistor kit contains 1075 total resistors. 25 each of the following standard E6 values: 1, 1.5, 2.2, 3.3, 4.7, 6.8, 10, 15, 22, 33, 47, 68, 100, 150, 220, 330, 470, 680, 1k, 1.5k, 2.2k, 3.3k, 4.7k, 6.8k, 10k, 15k, 22k, 33k, 47k, 68k, 100k, 150k, 220k, 330k, 470k, 680k, 1M, 1.5M, 2.2M, 3.3M, 4.7M, 6.8M, 10M. Resistors are 1/4 watt, 5% tolerance, carbon film resistors. Each value is individually bagged, labeled and packaged in a reusable box. This is the perfect starter kit for any electronics hobbyist.

    http://amzn.com/B00GPZQ5B2


    Joes2.jpg Joes1.jpg

    TRU
     
  9. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,172
    397
    Company's on the way, chicken's in the pot. so I'll steal a minute for next version. A 555 IC may be more available than comp. Radio Shack ? Works well on breadboard driving 1 W white LED @ 225 mA.
    It uses one of the comparators in the 555. The LDR , 1 meg. to 1 k ohms, is assumed to be about 100 k at desired switch point which with R1 gives about 1/3 VCC at pin 2, trigger, the normal trip point, but trip has been shifted down a fraction of a V by R4 for a bit of positive feedback. I did not have MJE 182, NPN, or close so used a surface mount,SMT, FZT849 NPN, 7A, 80V, B 100. You could probably use a 2N3904 & MJE 182 as a Darlington pair if necessary. Dark SW # 2 00000.jpg
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Why dont just tie 2,6 together as normal ditch pin 5 and it will work as a photo switch with its own hysteresis.
     
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