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ULN 2004 protects 3.3v logic from higher voltages

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This is a method of protection 3.3v logic from higher voltages.
It uses one component a ULN 2004.
The higher voltage must be positive and less than 50v and the current limited to less than 500mA ( A 200 ohm resistor will limit the current for up to 50v and gets very hot).

Connect the 0v on the 3.3v logic to the 0v on the ULN2004 (pin8) and to the 0v on the external circuit. Leave COM (pin 9) unconnected
Connect the input to the output of one of the ULN2004 buffers. e.g. pin 7 to pin 10.
Connect a logic input of the 3.3v logic to pin 10 of the ULN2004 and to a logic output at a higher voltage (e.g. +5 or +12v)

Thats all! The ULN2004 as wired will hold a higher voltage down at 2.25v @1mA to 2.85v @ 500mA At 500mA the ULN2004 will heat up and the voltage fall (- 0.35v max). The 3.3v logic will read "1". For a logic input of "0" the 3.3v logic will read "0".

Use ULN2003/ULN2803 with a 6k8 ohm resistor pin7 to 10
  • 1 × ULN2004

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