Turn on Led using photocell to control N MOSFET.

Thread Starter

mrel

Joined Jan 20, 2009
178
Hello
Found this circuit on youtube this circuit turn on Led using photocell .
Anyone know what the 100,000 ohm resistor in the circuit for?
Is this to protect the led or mosfet or the photocell?
Looking at circuit look like there no resistor to limit current to led.
Led rating is:12 volts ,watts 20 ,current 1800ma..
If i run led at 11.50 volts and current at 1.500 ma would my resistor be 7.6 ohm?
should i put the resistor in series with the led?
In the youtube the guy use 10 watts ,12 volts led,i want to change 20 watts 12 volts led.
Thank for the help mrelIMG_20190906_020737.jpg
 

Alec_t

Joined Sep 17, 2013
12,004
If the LED is genuinely rated at 12V then it has a built-in resistor and if run from a 12V supply will draw 1800mA.
The 100k resistor is to pull the FET gate up towards 12V to turn the FET on if the photocell is a photo-resistive type and is in the dark, hence has high resistance.

Edit: The circuit has no hysteresis so has the disadvantage that there is no sudden switching from on to off. This means that the FET could be in a half-way state for a prolonged period and overheat.
 

ericgibbs

Joined Jan 29, 2010
12,511
hi,
Its a poorly designed circuit, I would not waste time and components in building.
The term Photo Cell in general usage is a photo voltaic, which is misleading for that circuit description, also as pointed out, the MOSFET could be only partially biassed On and possible cook.
No limiting resistor is shown for the LED, also the TS wants to double the LED current.
E
 

MrChips

Joined Oct 2, 2009
23,248
My bad. From the circuit diagram I made the assumption that the photocell was an LDR (light dependent resistor).
 

bloguetronica

Joined Apr 27, 2007
1,431
Hi,

Think of that resistor as a pull-up, not as a protection, although if does limit the current to the photocell as well (side effect). Notice that the IRFZ44 tolerates a gate to source voltage up to 20V. In your case, even if you replaced your resistor with a short, you would have a Vgs of 12V, so the transistor would work within its limits (unless there was an inductive spike, but that's matter for another topic).

Adding to my answer, the photocell is definitely an LDR. Notice the symbol, and the need of a pull-up resistor. The LED light switches off when the LDR gets illuminated, and switches on when the LDR is not illuminated. Street lights work in a similar fashion.

For me, the circuit looks correct. It should work, although its sensitivity will vary depending on the type of LDR you use.

Kind regards, Samuel Lourenço
 
Last edited:

ElectricSpidey

Joined Dec 2, 2017
1,566
Yea, that circuit is problematic.

I would replace the MOSFET with a lower power version, replace the LED with a relay, and let the relay switch the LED on and off.

The advantage of using a relay in this type of circuit is as follows.

1. lower power transistor/s.
2. Provides the needed hysteresis.
3. Can turn devices on or off.

I have built quite a few circuits like this over the years, and always used a very high gain stage before the relay, for best results.
 

DickCappels

Joined Aug 21, 2008
7,278
If your sole purpose is to experiment with a light dependent resistor and then the circuit you posted will be fine.

If you are going to invest some "real money" with the intention of making a reliable darkness-operated lamp, then the circuit below is much better suited.

upload_2019-9-7_22-18-18.png

R3 provides hysteresis to help keep the MOSFET out of its linear region so it runs cool and also helps prevent flickering.

(Typo corrected because of input from ElectricSpidey)
 
Last edited:

ElectricSpidey

Joined Dec 2, 2017
1,566
The circuit in post #9 needs a series resistor for the LDR depending on its resistance and wattage rating.

Also I think the text under the image is referring to a different circuit?
 

Thread Starter

mrel

Joined Jan 20, 2009
178
If the LED is genuinely rated at 12V then it has a built-in resistor and if run from a 12V supply will draw 1800mA.
The 100k resistor is to pull the FET gate up towards 12V to turn the FET on if the photocell is a photo-resistive type and is in the dark, hence has high resistance.

Edit: The circuit has no hysteresis so has the disadvantage that there is no sudden switching from on to off. This means that the FET could be in a half-way state for a prolonged period and overheat.
Purchase from Amazon ,amazon website said by colorfulpeal .
Type : high power led chip,rated power : 20w.35mil emit colors: cool white.
wide viewing angle 135-140.
forward voltage (fv):dc12-13v,forward current (if) 1800ma
color temperature 6500 kelvin.
That is all information amazon website list on this Led.
Does not mention anymore built in resistor to the led
If there is no current limit resistor,what ohm resistor should use 7.5 ohm would that be right if hookup direct to 12 volts battery?
mrel
 

ericgibbs

Joined Jan 29, 2010
12,511
hi mrel,
I am using Amazon 12Vdc 20Watt Security light LED's, directly from a 12Vdc 2A supply.
If you examine the LED face you will see its made up of a number of single LED's and there are resistors on the LED mounting PCB.
E

Just counted the LED's on the 20W light , there are 24 set as a 4 * 6 Array.
 

ericgibbs

Joined Jan 29, 2010
12,511
hi mrel,
This is a clip of the MOSFET d/s for the Vgs.
We need details of the LDR [ photo cell] .
I would not recommend that simple circuit, it needs additional components to make a reliable On/Off action.
E
021 Sep. 08 09.59.gif
 

MrChips

Joined Oct 2, 2009
23,248
Mr chip
If i increase or decrease the resistor value will this keep the led from come on until dark?
mrel
Treat the pullup resistor and the LDR as constituting a voltage divider.
The voltage at the node of the two will change depending on the amount of light falling on the LDR.
The critical point is determining the threshold voltages required to turn ON the MOSFET and the threshold voltage to turn OFF the MOSFET.
Note that these two threshold voltages are not necessarily the same.

It is common practice to add a variable trim resistor in series with the fixed resistor. This will allow adjustment in order to set the threshold voltages for a particular LDR and lighting conditions.
 

Thread Starter

mrel

Joined Jan 20, 2009
178
hi mrel,
I am using Amazon 12Vdc 20Watt Security light LED's, directly from a 12Vdc 2A supply.
If you examine the LED face you will see its made up of a number of single LED's and there are resistors on the LED mounting PCB.
E

Just counted the LED's on the 20W light , there are 24 set as a 4 * 6 Array.
ericgibbs
The led got from amazon only have 20 leds see enclosed picture the seller say don't need current resistor.
Seller say only heat sink.
You mention in your post there resistors in the led don't notice the resistors,is this led same one you think of.?
mrelIMG_20190916_022933.jpg
 

ericgibbs

Joined Jan 29, 2010
12,511
Hi mrel,
Mine has a different LED layout.
There are so many Chinese versions, if the supplier says no resistor I would go ahead and use 12Vdc.
E
 
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