Essentially, each pnp transistor compares a reference voltage on the npn emitter with the voltage across the lower limit switch circuit resistors R4, 5 and 6, directing current to the led if the limit ...

To create the DIY remote shutter button, [NirL] used a spare limit switch, resistor, and cannibalized an old set of earbuds for the cable and 4-conductor 3.5 mm audio plug.