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https://www.circuitlab.com/circuit/9t68p2/ldrwithtwotransistors/ This circuit suppose to get the LED off or on AT A CERTAIN LIGHT VALUE (ie Resistance value). I need to get this value from simulation, but I need help

I replaced the LDR with a resistance and kept changing it and monitor the current in LED. I reached a value of 28K. I am not sure this is the correct way to use CircuitLab. The breadboard with the LDR installed, I got the LED to respond ONLY when I covered it (total dark) !!!!! The LED went off when total dark other wise it is on all the time with varing degree of light. I need Circuitlab to tell me the value that trigger the LED.

You gave me enough links to keep me busy for sometime. The LED is the 3mm type with 20ma and 2-3 volt. And it turns ON !!! when getting dark. Let us FORGET the LDR for a moment,let us say I have a variable linear resistance instead. Let us also forget about temprature effects. CAN I SIMULATE for LED on and get the value of the variable resistance at that time ??? Is that possible? if yes and the answer is NOT in the links (that I did not read yet), I hope you give me a way. I thank you for taking the time and the understanding to help me with this problem. Best regards and so long

OK.

Sorry I gave you lots of info but didn't actually answer your original question.

:)

You can get CL to sweep the resistor value for you while you monitor the current through the LED.

Then you just read off the X axis the value of the resistor at which the LED current changes from on to off.

This is done using a DC Sweep simulation.

You need to enter the part that you wish to sweep in the text box with the greyed out hint text in it.

When you click in it a list of sweepable elements will appear. As you type, the list will narrow down.

Note that CL is case sensitive.

These links refer to the technique:

https://www.circuitlab.com/forums/modeling-and-simulation/topic/ux8q7tbz/can-a-component-value-be-stepped/

https://www.circuitlab.com/circuit/z79rqm/leds-with-resistor-biasing/

https://www.circuitlab.com/forums/support/topic/k7cet7bb/how-to-run-a-dc-sweep/

Just rediscovered this thread too:

https://www.circuitlab.com/forums/basic-electronics/topic/88ma453y/basic-led-circuit-with-ldr/

More useful answer?

:)

Thank you for your follow up. So, the DC Sweep should do the simulation. I gave it a shot and I am at a loss. The R4 (The variable resistance) is the one I want to CHANGE so I can monitor the LED current But the R4 parameter is not part of the parameter slection so I wrote R4.R It accepted it but how can I vary the value between a range of Values ???? I hope I am pushing my luck with such naive questions ??

Read:

https://www.circuitlab.com/docs/the-basics/#dc_sweep_simulation

Then:

Open:

Open the Simulate panel.

Click on the DC Sweep option.

Look at how the parameters are set up in there.

This is an eye opener ....You cannot imagine what you just did. Really appreciated. In the DC Sweep the parameters are set Then the Time sweep did the "kill" How I can do this without help from people like you; beats me!!! Thank you...and thank you...

As the Sirius Cybernetics doors in "The Hitch Hiker's Guide to the Galaxy" say:

"Pleased to be of service!"

The official documentation in CL leaves a little room for improvement ...

... so you need to use the search box and try different search terms to try to find solutions and examples.

:)