I want solar panel to turn light on and off. When above 0.75V solar panel light off.

See post #20

 

See post #20
View attachment 361902

In real life solar panel don't work like what you show as V2 in snapshot. They are not voltage sources.

I modeled in post #1 snapshot circuit but that is not working either. See post #1

 

If they are not voltage sources then what are they?

 

If they are not voltage sources then what are they?

 

In a perfect world.
Can't have current if there isn't any voltage.
Are you going to change R2 to 1K or keep going around in circles debating why it won't work?

 

In a perfect world.
Can't have current in there isn't any voltage.
Are you going to change R2 to 1K or keep going around in circles debating why it won't work?

Its not a perfect voltage sources. So no if you load it too much voltage drops.
Now that you insists changing resistor, I have to desolder and take out 9.9 ohms and put 1k in. Takes time. Let you know what happened.

 

Modified version using the TL431 as a comparator.
No load on the solar cell with R1 at 10K.
R6 provides a small amount of hysteresis.
Operation verified on a breadboard with a small solar cell for V2.

 

Modified version using the TL431 as a comparator.
No load on the solar cell with R1 at 10K.
R6 provides a small amount of hysteresis.
Operation verified on a breadboard with a small solar cell for V2.
View attachment 362390

I dont get it how does TL431 act as comparator here? Can someone walk me thru the voltages to see how?

 

I dont get it how does TL431 act as comparator here? Can someone walk me thru the voltages to see how?

The internal reference pin on the TL431 is appx 2.5 volts. When the LED is ON the voltage at the reference pin is appx 2.68 volts from feedback resistor R6.
As the voltage on the solar cell reaches appx 0.8 volt Q1 begins to conduct lowering the voltage at the collector until it falls below the 2.5 volt reference which turns OFF the TL431 and LED.