TFT display ON when car key is ON

[maharzan]

Hi,
I am doing a project with nano every in a car. I have successfully integrated nano every to pull BMS data via RS485 and show some elements needed on a TFT display. I am working on next steps, and this one is to put arduino in sleep mode and wake up when few things happen. First I am trying out when the car key is on.

I don't want to turn the nano every off completely as it would stop checking any data coming from BMS. I just want to wake it up from IDLE mode or other modes when the car key is on. There is a wire in a dashboard that is 0V when key is off and 13V when key is on. This is from a small EV car. The problem here is for a sec, that voltage shoots up to 20V. I am trying to separate this voltage from feeding into nano. So, I thought about using an optocoupler but since the grounds will be same (from the car/arduino since they are using same power supply) I read suggestions of using a transistor. So, here is what I have come up with.

When car is off

When car is on

I pass 13V in base, 5V from a PULLUP pin say D3 to collector and after a resistor we connect another D4 pin as well. So, when key is off, D4 will be 5V and when key is on D4 will be 0V so depending on the D4 state, we can wake up arduino/turn on the TFT display. The problem is base voltage can spike and greater than collector which doesn't seems to be recommended. I am not sure of any other way to do this.

Any suggestions are highly appreciated. Thanks.

[gilshultz]

@maharzan

Any suggestions are highly appreciated.

Hopefully you have protected your project from the nastiness of the automotive electrical system such as reverse battery. There is many good app notes such as AN2689 by ST on automotive electronics. reading it will help you a lot. After all the work I would not want to see your project trashed. In place of the transistor I would use a 74HC14, less standby current. Also a good interface chip to the automotive electrical. I usually connect with a resistor to the input 50-100K.
https://www.st.com/resource/en/application_note/cd00181783-protection-of-automotive-electronics-from-electrical-hazards-guidelines-for-design-and-component-selection-stmicroelectronics.pdf
Also take a look at these: https://www.analog.com/en/technical-articles/distilled-automotive-electronics-design.html and
Transient Voltage Suppression in Automotive Applications

[maharzan]

Thanks for the suggestion. I will take a look at them. There is already a dcdc converter in the car (12v) that runs all its current electronics. I am just trying to replace its non functionalities to a newer system thus trying to use a nano every. So, I hope there are no reverse polarity here. The car runs on 48v system.

I will also put a diode for safety. Is that logic i am trying to add good?

[gilshultz]

The reverse battery comes from towing companies that use 24V batteries to jump start cars and connect backwards. If that cannot happen you will not need the diode. Have fun and keep us posted.

[maharzan]

Ah, that will not happen with this car as it doesnt have a separate 12v battery.

[maharzan]

I tried this today and it worked except we just need to connect to one pin and say INPUT_PULLUP instead of that 5V and resistor.. I have another issue though which I will post later.