[SOLVED] Interfacing with a 3.3V uC on a 14V PWM signal

Hello all,

I would like to build my own PCB to monitor in parallel from an existing system (sensor + measuring equipment)

The sensor is supplied by the V(12V-14V typical), and the signal is a PWM.

I don't have any prior experience in electronics, but I know some basics. This is what I came up with:


Remarks:
- I will be using a 3V3 microcontroller to read the PWM from the sensor
- I think i need a faster switching time than what 4N28 can offer, but that was what I found in the LTspice library (just installed the SW this week)
- I also couldn't find a 3.3V zener diode, so I just add what I found
- Please advice on a PCB layout SW

Please give me some feedback and drive me in the right direction.

Thanks in advance!

No sense to use optocouple if you are using same ground.
No sense to use zener diode and e.t.c. Emitter goes to digital ground, collector directly to pin with pull-up resistor.
Zener protection actual only on left side. Not even zener, TVS! And ceramic capacitor to reduce ESD.

Thanks for your reply !

Actually I will connect this in my vehicle, so on the left side there will be interference like cranking, overvoltage as the sensor is connected to the car battery.

That is the reason I chose an optocouple. Do you think it would be ok connecting it directly ?

Hi,

Easyrider is correct with optocoupler.

IF .. you expect on left side overvoltage and peaks and noise, then protect the LEFT side. On the right side it makes no sense.

I recommend a bjt with protection on the base instead of optocoupler. The bjt can be used to protect the microcontroller.

We need more details about PWM:
* what is it´s frequency (range)
* what is it´s duty cycle (range)
* are you interested in measuring the duty cycle?
* if yes: What precision do you need, what resolution do you need?
* I assume the sensor signal will not be 12.000v and 0.000V. Therefore we need to know what source this is. (datasheet or other specifications).


I know these may be informations you you didn´t take much care about till now.
But try to find out as much as possible.
The more details we know the better will be our answers and the result for you.

Klaus

Hello KlausST,

The sensor has protection built-in, the sensor communicates with the car ECU.

I just want to come in parallel with a measuring equipment.

The sensor`s output is dependent on the pull-up in the ECU that is connected to the battery. So when the battery voltage changes, the signal changes itself.

I would like my equipment not to interfere with the current communication, just to monitor it, that is why I want only to protect the side on my micro-controller`s input.

I do not care to protect the sensor in my case, that is protected by the manufacturer.

My idea was to use the optocoupler to be somehow independent on the voltage changes (11V-16V) because my 3V micro-controller has a regulated voltage.

Is this OK just using the BJT ? Then i will do it like that.

Hi,

Read my post again.

--> I never said you should protect the sensor.
--> I said I recommend to use a bjt with protection on the base.

Klaus

OK, thanks

I will re-draw my schematic and maybe post it afterwards.

Typical PLC digital input:


**broken link removed**