I am working on an automotive general purpose ECU where I need to interface 12V digital inputs with a µC (3.3V).

The PCB space is limited, so I am searching for a single IC as an interface between the digital inputs and the µC.

The IC shall have the following features:

  • input stage operating range that goes down to about 7V
  • integrated input stage protection circuits (Clamping Diodes, ...)
  • at least 4 inputs

I have found an almost perfect IC: Infineon ISOFACE Input IC. The problem with this IC is that its designed for the 24V industrial domain, the input stage (VBB) operating range is 15V...35V (15V is the undervoltage threshold).

Does something comparable as the ISOFACE chip exist for the automotive 12V domain?

  • \$\begingroup\$ This is really a shopping question as opposed to design question. \$\endgroup\$ Nov 4, 2013 at 13:30
  • \$\begingroup\$ I would try something like a 4n37 opto \$\endgroup\$
    – user39547
    Mar 31, 2014 at 7:13

1 Answer 1


How about just using some transistors? Get an NPN transistor and put like a 10k or 100k current limiting resistor between the base and your input signal. Connect the emitter to ground. Connect the collector to an I/O pin and then add a 10k pull up resistor. You can get transistor array chips in various places that will do this, and you can support voltages much higher than 12 volts so long as you use a large enough current limiting resistor. As is, the threshold will be around 0.7 volts. Need a larger threshold? Use a voltage divider instead. A 100k series resistor with a 100k resistor to GND would make the threshold about 1.4 volts.

Alternatively, you might be able to get away with just voltage dividers on the inputs. A 10k series and a 3.9 k shunt resistor would get you pretty close to 3.3 volts with 12 volts coming in. Your microcontroller should have clamping diodes internally, you just need to make sure you don't accidentally overload those. If you're really worried about it, there are diode array chips that you can get that will either tie everything to ground with Zener diodes of the proper voltage, or clamp the inputs to power and ground.

  • \$\begingroup\$ I have about 16 inputs, using discrete components would require too much PCB space, so I am searching for an integrated circuit. \$\endgroup\$
    – sergej
    Nov 5, 2013 at 8:32
  • \$\begingroup\$ @sergej you can get 8-resistor arrays, and quad transistor arrays. With ~4 SMD resistor arrays and 4 SMD quad transistor and also 4 SMD quad TVS diode clamp arrays you can pack it in VERY close, and VERY cheap. \$\endgroup\$
    – KyranF
    Oct 3, 2014 at 18:35

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