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Circuit to determine whether vehicle is running based on battery voltage level
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HwAoRrDk:

--- Quote from: spec on October 19, 2018, 05:06:24 pm ---LT6703 chips are around £1.80 in one of quantities from Digikey and Mouser- is that expensive to you?

--- End quote ---

Well, I only took a quick look at Farnell and RS - both had prices of £3 in single quantities for SOT23 version.

But anyway, even at the lower price it's expensive compared to a jelly-bean £0.22 comparator. ^-^
spec:
Digikey UK have:

Microchip MCP65R41T-240 for 33p + VAT
and
Microchip MIC841 for 44p + VAT

Digikey UK also have quite a few other comparators with reference voltages for under £1
IDEngineer:
I'll cast another vote for simply reading the CAN bus, specifically the ECU's RPM parameter. Everything else will be some sort of guesswork, potentially compromised by the battery's state of charge, whether the alternator is switched, yadda yadda yadda.

I've done a lot of CAN work. The presence/absence of CAN data is not a reliable indicator of "engine running", even if you filter for the ECU's source address. For example, many marine engine ECU's continue to transmit CAN data for ~30 seconds after the engine is turned off. However, that data is accurate - specifically, the RPM parameter is zero. The absence of that parameter on the CAN network, or it being zero, is authoritative that the engine is not running.

I'd argue that this is the best way to truly know if the engine is actually running. While my work has been in the marine industry, we know the parameters for OBD are quite standardized so it should be straightforward to find the RPM info. CAN PHY chips are cheap, and there are lots of MCU's with an integrated CAN interface module (the logic side), so you could accomplish the mission with low cost and low parts count.

And who knows - once you have access to the CAN data, perhaps additional cool features will become possible for your project....  :-+
Adam60:
Would taking a signal from the crankshaft position sensor work? Engine signal would be digital and you would need to find a way to have a minimum to indicate it is running as opposed to cranking. I wouldn't mess with the CANBUS signal unless you really know what you are doing. It will get messy quickly. CKP is a sure sign of engine running.
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