How to accept switched GND or switched Positive on a 3v3 MCU input

Question

I am making a device based on a STM32F105 (3v3) that has to accept inputs that I am not in control of, for example in a vehicle or other wiring that I cannot change.

The device is to have 16 inputs.

I want the input to be selectable between active low and active high. Because sometimes I might come across a switch that is switched to GND and sometimes one that is switched to say 30v.

All inputs have to be 30v tolerant.

I would like to have the option of selecting in firmware whether the input is connected to a switched Ground or switched positive.

How can I go about accepting inputs that could be switched GND or on other occasions be switched 24v?

For example I may come across either of the following so I need my circuit to be able to accept both:

Answer 1

How about:

This assumes a 5V microcontroller, common ground between 5V and 30V circuits and suitably chosen mosfets.

Note that M1 (P-Channel mosfet) is installed with the source facing the 5V rail.

When your microcontroller outputs a logic high, the N-Channel turns on and the P-Channel turns off (simultaneously) which enable the pull-down R3. A logic low will do the opposite and turn on the pull-up R2. D1 and R1 are the protection circuitry to drop the 30V level down to something the micro will be happy with.

Additional info: This circuit can be modified slightly to put the pull-up/down on the high-voltage side if you want that instead, just move D1 & R1 to be on the microcontroller side and use mosfets suitable for 30V switching.

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Answer 2

Figure 1. Opto-isolators such as the LDA202 are bidirectional which eases terminal configuration.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 2. Typical common + or common - input configuration for PLCs.

This arrangement has several advantages:

• Full opto-isolation of micro from the external power sources.
• If the opto-isolators are socketed then repair is simple.
• The inputs can have a common ground or common 24 V. Group optos together to allow flexibile connection of both types of input by the user.

^{simulate this circuit}

Figure 2. Wiring arrangements for switched negative and switched positive connection.