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At work, I have connected an Arduino to a security key (which has a 12v power supply) in order to give us a more versatile door lock. Unfortunately, people keep removing the key pad, disconnecting the wires and re-connecting them (defective keypad I guess.) In the process, I am afraid that they will accidentally send 12v into the Vcc, ground, digital in, and/or analog in pins. What is the best way to protect my Arduino from the 12 volts? I really need a component or two that I can to my pins.

Edit: it is difficult for me to describe this problem without mentioning the danger to all components. Please describe ways to protect digital outputs HERE

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There are some suggestions at Protecting Inputs in Digital Electronics .

Amended answer

Testing of my earlier answer involving clamping diodes gave unsatisfactory results. Due to the small current flow the diodes allowed a considerable voltage to reach the input pin (like, 9 V).

The amended circuit below, tested with 12V input, satisfactorily keeps the Arduino input pin to 4.9 V, even with 12 V from the device. The current through R1 is around 7 mA which is what you would expect:

I = V / R
I = 7 / 1000
I = 7 mA

The 1N4733A is a 5.1 V, 1 W zener diode which can therefore handle dropping 5 V at 7 mA.

In case you are wondering where the number 7 comes from in the calculations above:

  • The input voltage is 12 volts
  • The desired output is 5 volts
  • Therefore we need to drop (12 - 5) volts over the resistor
  • Thus 7 volts are dropped over the resistor

Input pin protection

Further reference material:

  • Is D2 and C1 connected to +5v or ground? I don't see a lable – Hoytman Jul 4 '15 at 3:38
  • As per Electrical Symbols & Electronic Symbols the downwards pointing triangles are common ground. So yes, D2 and C1 are connected to ground. – Nick Gammon Jul 4 '15 at 5:07
  • Thanks, great link for someone that is not a trained electrical engineer (me.) – Hoytman Jul 4 '15 at 17:22
  • Can this same circuit protect Digital outputs too, or should a different circuit be used? – Hoytman Jul 4 '15 at 17:33
  • A 10 ohm resistor, will mean around 0.7 A will go through the diodes if +12 volt is connected. That's 5Watts being dissipated by R1. Fine for 12v spikes, but not for continues 12v being applied. – Gerben Jul 4 '15 at 19:28

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