Complete dashboard for an old car

What are you developing?
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Re: Complete dashboard for an old car

Post by RogerClark » Wed Nov 22, 2017 9:09 pm


Why does the power have to be physically removed from the STM32 by a relay ?

Won't just putting it into low power mode, reduce the current consumption considerably ?

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Re: Complete dashboard for an old car

Post by magflip » Wed Nov 22, 2017 9:14 pm

ahull wrote:
Wed Nov 22, 2017 9:01 pm
You could use a relay and a couple of optoisolators in parallel. The ignition switches on one optoisolator, this provides power to the relay coil, the relay then provides power to the 5V PSU/regulator for the STM, which in turn on power up, switches on optoisolator 2 which is parallel with optoisolator 1 (both being in series with the relay coil). (You may need a couple of diodes to stop the two optoisolator transistors from frying each other, I'm building this circuit in my head as i type.. so you will need your proof of concept needs to be breadboarded).

When the ignition goes off, so does optoisolator 1, but this doesn't kill power to the STM, since it is now keeping itself powered with optoisolator 2 controlling the 5V power supply relay.. When the STM has done its post "ignition off" housekeeping, then it switches off the second optoisolator an kills its own power. At this point, the system is completely disconnected from power. It only wakes up again when power is re-applied and optoisolator 1 fires up once more.

You can do the same trick with a 12V and a 5V or 3V3 relay with their normally open contacts in parallel. On ignition, the 12V relay closes contacts and provides power, PSU for the 5V for the STM and the 5V for the relay the STM wakes up, when the 5V comes on, the STM switches on the 5V relay and its contacts provide the alternative STM and 5V relay power route, since the 5V relay normally open contacts are in parallel with the 12V relay normally open contacts. Both the 12V and the 5V relays need to open before power is completely cut, and since the STM controls the 5V relay, it only kills its own power once its work is done.

If you can't get your head round my description, then let me know and if I have a bit of time, I'll attempt a schematic.

Once again, thank you for the collaboration ahull !

I even started to follow the reasoning but in the middle of the way I ended up losing myself and however much I have read a few times, I could not understand what that scheme would be like. I would appreciate it if you could help me.

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Re: Complete dashboard for an old car

Post by ahull » Thu Nov 23, 2017 1:06 am

Here is a similar (arguably better) idea using two mosfets. ... ing-on-off

The description of how it works should give you a few ideas.

My idea is a little simpler, one detail I forgot to clarify the 12v feed for the 5v supply is not switched by the ignition, the 12v feed for the 12v relay is.

I may get a chance to sketch up a schematic tomorrow, but it is looking like a busy day, so we will see how that goes.
- Andy Hull -

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