Light Sensor Rebuild

Thread Starter

truwebs

Joined Apr 2, 2016
76
Hey All,

Looking for some assistance in building a light sensor circuit to power 12 LED lights.

I thought I could do a tear down on a 120v night wall plug night light.

Well it appears the circuitry needs more power to be effective as when I power the pictured circuit board with a 12v supply I am getting less than 2 volts on output.

It appears the entire circuit includes and LDR, MCR100-6-P86 semiconductor, and resistor.

Feel free to correct me if my terminology is wrong. Learning as I go here. :)

My question is what do I need to replace the MCR, and resistor to make this work with low voltage so that I can get 12v DC output?

See included images for your reference and check if I identified items correctly.

circuit1.jpg circuit2.jpg mcr 100-6.jpg transistor.jpg

Thanks in Advance!

TRU
 

DickCappels

Joined Aug 21, 2008
8,151
The MCR100 is an SCR (Silicon Controlled Rectifier), which is a switch that operates with AC. You need something else between the phototransistor and the LEDs. How are the LEDs wired, what color are they?
 

Thread Starter

truwebs

Joined Apr 2, 2016
76
The MCR100 is an SCR (Silicon Controlled Rectifier), which is a switch that operates with AC. You need something else between the phototransistor and the LEDs. How are the LEDs wired, what color are they?
4 green. 8 yellow wired in parallel. each has an inline resistor
 

DickCappels

Joined Aug 21, 2008
8,151
No, in the circuit the 2N3905 is labeled "2N3905", which is very similar.

The N-Channel MOSFET should be selected based on the total amount of current your LEDs will draw. It would also be good if the gate threshold voltage is less than 6 volts.

By the way, in the original night light if the power applied to the LEDs and their series resistors was not 12 volts, then you will need to calculate new resistor values for each color LED.
 

AnalogKid

Joined Aug 1, 2013
9,618
What is the forward voltage rating (Vf) for each of your LED types?

I think the resistor is reversed, and the real value is 4.7 M. Here is a probable schematic, designed with a different LDR:

http://niotronic.blogspot.com/2013/07/auto-night-lamp.html

Did the original nitelight have an incandescent bulb or an LED? If it had an incandescent bulb, the circuit can be use to run LEDs directly off the powerline very simply. However, that violates the rules on this forum because that kind of circuit work is dangerous for pros, and lethal for others.

ak
 
Last edited:

Thread Starter

truwebs

Joined Apr 2, 2016
76
What is the forward voltage rating (Vf) for each of your LED types?

I think the resistor is reversed, and the real value is 4.7 M. Here is a probable schematic, designed with a different LDR:

http://niotronic.blogspot.com/2013/07/auto-night-lamp.html

Did the original nitelight have an incandescent bulb or an LED? If it had an incandescent bulb, the circuit can be use to run LEDs directly off the powerline very simply. However, that violates the rules on this forum because that kind of circuit work is dangerous for pros, and lethal for others.

ak
Original had a 4w incandescent bulb.
 

AnalogKid

Joined Aug 1, 2013
9,618
I believe I have found a working solution. I have tested and it appears to work the way I wish. It seems to give the exact brightness I was looking for from the 12 LEDs with inline resistors.

https://www.instructables.com/id/Dark-Detecting-LED-Decoration/

Does anyone see any flaws or shortcomings in this design? If so, please advise.
It seems that I no longer get alerts for this thread. The circuit is typical for an Instructable - a basically sound approach full of implementation issues.

If you are running LEDs in parallel, each needs its own current limiting resistor to act as a ballast and even out minor variations in the LED characteristics.

100K is high for a transistor trying to supply up to 60 mA. If the intent is to starve the base intentionally so the transistor acts as the current limiter, the brightness will vary significantly with room temperature changes and from one build to another (device parameter variations).

ak
 
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