Adding Dark Sensor Circuit to LED wire string

Thread Starter

truwebs

Joined Apr 2, 2016
76
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
 

Sensacell

Joined Jun 19, 2012
2,582
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.
 

Thread Starter

truwebs

Joined Apr 2, 2016
76
Sensacell,

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

TRU
 

Bernard

Joined Aug 7, 2008
5,606
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.
 

Thread Starter

truwebs

Joined Apr 2, 2016
76
Bernard,

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

Thanks,
TRU
 

Bernard

Joined Aug 7, 2008
5,606
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
 

Thread Starter

truwebs

Joined Apr 2, 2016
76
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
 

Bernard

Joined Aug 7, 2008
5,606
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
 
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