user7788: I am using the circuit diagram below to turn ON/OFF the lights using Photo-resistor. 
How can I use the Arduino to bypass the Photoresistor effect?
Let say I want to turn ON the lights even though Photoresistor doesn't want to.
Take note the 12V source is required by the LED. Knowing that Arduino must work with max 5V.


user1576: You could replace the circuit with these:

Use one Arduino analog input to measure the light.
Use a Digital Out (Preferable PWM) to control the LED.

Then you can apply whatever logic you want.

simulate this circuit – Schematic created using CircuitLab

7788: <p>I am using the circuit diagram below to turn ON/OFF the lights using Photo-resistor. </p>

<p>How can I use the Arduino to bypass the Photoresistor effect?
Let say I want to turn ON the lights even though Photoresistor doesn't want to.</p>

<p><strong>Take note the 12V source</strong> is required by the LED. Knowing that Arduino must work with <strong>max 5V</strong>.</p>

<p><a href="https://i.stack.imgur.com/QcoNh.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/QcoNh.png" alt="enter image description here"></a></p>
 1576: <p>You could replace the circuit with these:</p>

<ul>
<li><p>Use one Arduino analog input to measure the light.</p></li>
<li><p>Use a Digital Out (Preferable PWM) to control the LED.</p></li>
</ul>

<p>Then you can apply whatever logic you want.</p>



<p><img src="https://i.stack.imgur.com/mea5m.png" alt="schematic"></p>



<p><sup><a href="/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fmea5m.png">simulate this circuit</a> &ndash; Schematic created using <a href="https://www.circuitlab.com/" rel="nofollow">CircuitLab</a></sup></p>
