OR gate with momentary output

Question

I have 8 different boards that can be triggered via capacitive touch and I need to feed into one input of a microcontroller. Whenever one board goes HIGH, the microcontroller's input should also change state(I don't care if it is HIGH or LOW, as long as it detects the change).

After searching the web I think that CD4078 is what will cover it, but here's the catch:

Inputs can stay HIGH for anything between 1 second and 1 minute.

Every time an input goes HIGH the microcontroller needs to know it, regardless if another input had been HIGH already.

Ideally, the gate's output should change state momentarily regardless of the input's duration, and be able to change state again once another input goes HIGH.

I know that I can accomplish this with an Arduino, but I'm trying to keeps costs down as this circuit will have to be replicated 11 times(11 x 8 inputs).

Does such an OR gate exist, or can I accomplish it with only a few more components?

Answer 1

Each of the XOR gates below, with the RC delay at one of its inputs, will emit a 1ms pulse on each input edge:

^{simulate this circuit – Schematic created using CircuitLab}

These are some example inputs, A B and C:

This is the output of a single gate G1, at node X:

The OR gate G4 will simply go high whenever any of the edge detector outputs go high. This is overall output F:

That means that even if one of the inputs is stuck either low or high for an extended period, a change "notification" can still appear at F, if any other input event occurs in the meantime.

If you're worried about finding a 3 million input OR gate, you can diode-OR the XOR outputs instead:

^{simulate this circuit}

Answer 2

If what you're trying to achieve is a way to interrupt your uController with a event on your keys, consider a PCF8574; page 11, Chapter 8 on the datasheet PCF8574 - Datasheet - TI show a functional block, detailing the sensing operations and how you program it to generate an interrupt on pin change.

Following is an example on the datasheet.

Answer 3

Looking at all the options, I think the least complicated way is to use a microcontroller in-between.

Given I need 11x8 outputs -> 11x1 input, I can use 6 Arduino Nano clones(each handling 2 sets of 8 inputs and 2 outputs) that go for about 2,5 euros and have around 20 usable GPIO.I could also go for 1 nano with 5 MCP23017 but the cost and wiring would be the same, with more complicated coding.