simulation circuit

Discussion in 'General Electronics Chat' started by Mark3722, May 31, 2015.

  1. Mark3722

    Thread Starter New Member

    Dec 26, 2014
    Hi All,

    Back again with another challenge.

    I am trying to build a conceptual solar panel demonstration with a small miniature house and a LED flashlight simulating the sun. Instead of using an actual solar panel, I will be using a more reactive photo resistor (, a 5v supply and some circuitry that simulates the generation of current. The flashlight will normally be pointing at the photo resistor when on display. By placing ones hand or a black card between the light and house, the current permitted to pass through the circuit should dynamically change.

    The circuit will feed into a tp4056 5v Lipo battery controller that normally requires around 400mA to charge a LiPo battery and power some attached appliances.

    Using my multimeter, the photo resistor seems to have values of around 3K ohms in the full light of the LED flashlight and 25K when the photo resistor is covered.

    I have been bread boarding a variety of circuits with various NPN transistors but have not had much luck. If the battery controller is hooked straight up to 5v, it will pull about 250mA. When I hook up the transistor and reduce the resistance between the base and the collector to 5ohms, the transistors seems to saturate and passes around 200mA through to the battery controller. With the photo cell hooked up in full light between the base and the collector, it only pulls through 15mA.

    Is there a clever way to create a circuit that will change the characteristics of the photo resistor so that it behaves like a 10K potentiometer? I have been experimenting with a combination of serial/parallel resistor circuits with a 2222 transistor but have not been able to get the desired behavior.

    Any tips or pointers are appreciated!

  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    Step 1.
    Get datasheets for all the components. If you had datasheet for the photo resistor, you would have known exactly how it behaves, what resistances it has at each setting.
  3. DickCappels


    Aug 21, 2008
    How does it feed the tp4056? Is this a logic signal or does it supply power to the tp4056? You might want to post a schematic of what currently does not work to get to a solution more quickly.