NE555 in a light detecting circuit
I'll assume your intention is to use the 555 as a kind of snap-action (hysteresis) for your LDR, and to use the .step statement to parameterize the LDR behavior. That is, you want to show proficiency with LTSpice.

The first thing I can help you with. You can change your circuit to use TRIG and TRHESH together to do the snap-action. Tie a voltage divider with your LDR to both TRIG and THRESH, as shown below. What should happen is:


light on, voltage is above THRESH (2/3 VCC), output goes low (reset)
light off, voltage is below TRIG (1/3 VCC), output goes high (set)








simulate this circuit &ndash; Schematic created using CircuitLab

A couple of things.

I ran a DC sweep (the CircuitLab equivalent of .step) on the LDR and it worked fine. I haven't yet figured out how to make CircuitLab to use a .step statement directly, but you can try this modified version with your params you set for the LDR in LTSpice.

Oh, and no need for the NPN. The 555 has plenty of drive for an LED. But if you want to add the NPN, you can simply change the polarity by swapping the LDR and the 5K resistor.