Hello I'm doing an electronics project and I've come across an issue when building my circuit on a digital breadboard, what I'm trying to accomplish is for the 555 timer to output a low voltage when ldr is detecting high levels of light and vice versa, it then goes through a transistor where the collector is connected to an LED which should turn and stay on when LDR is at low light. My circuit works perfectly but the breadboard simulation isn't because the 555 timer outputs a high voltage whatever the input is(So LED is constantly on), I've tried but I really don't know how to fix this I would really appreciate some help, I have attached the circuit and breadboard design. Thank you

 

You already have IC2 as a comparator for the signal.
Base upon your description of the circuit operation, I see no purpose for the 555.

 

I've tried but I really don't know how to fix this I would really appreciate some help, I have attached the circuit and breadboard design. Thank you

Must have a wiring mistake on the breadboard. I verified the design of the 555 circuit.
Post a good photo of the breadboard not a drawing.
SG

 

It's always good practice to assemble the circuits individually.
Start with the comparator and make sure it is working properly.
Next, construct the 555 circuit and verify it's operation.
Finally, connect the two circuits.
SG

 

What type is IC2?

 

What type is IC2?

The sketch of the breadboard shows a CA3140
SG

 

Breadboards are horrible... check all the connections. I've chased all sorts of problems to find intermittent connections on breadboards.

 

Have you determined the ohmic range of R4? You have created a 2.5V reference into the inverting input, and so the question becomes, what is the possible range of values from the LDR? You've biased the non-inverting input by 100K-Ohms, so you've already lowered the entire voltage input ranage on the non-inverting input--- if it is never higher than 2.5V, your LED will always be on.

 

Could also eliminate the opamp and just use the 555, fewer parts.
SG

 

Here's an Ltspice simulation of the basic 555 circuit used as a comparator with hysteresis:

 

The original circuit has nothing to limit the LED current so the LED will quickly burn out.