I have used this method to reduce the output of an ipod when directly connected to the tape head for line in. The output of the ipod is two high so using a Voltage divider you have nice modulation of the volume control on the box and also volume control on the ipod (2 x 900 ohm resistors). This could also work for LED mods or Bluetooth add on's without having to use a separate power supply.
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Voltage Divider Calculator - Electrical Engineering & Electronics Tools
The voltage divider is a circuit used to create a voltage less than or equal to the input voltage.
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