Hello all,
I am trying to develop a new type of aftermarket product that will connect to a vehicles battery. Ground return will be through the chassis. I desire no other electrical connections. This will help simplify installation for the typical consumer. Do you know many people who can't find their vehicles battery? Well, I'm sure you could name a few. But, the majority of us can. What I want to avoid is telling someone to splice into some signal within their vehicle. This can get difficult. The type and location of such signals will vary from one manufacture to another. Who knows, it may not exist at all in some vehicles...
Oh yeah, one more thing. This product will be mounted in the engine compartment so any electronics specified will need to be available in extended temperature ranges.
When activated this product will draw about 5 amp. I'm trying to devise a way to allow activation of this product only while the engine is running (for obvious reasons). Therefore the product will need to sense engine running. If this criteria is met, it will be allowed to switch in its load. It will also need to detect when the engine has stopped. At this point it will disconnect its load and go into a low power mode. It will sit powerd up indefinitely in low power mode. Therefore, whatever sensing scheme we come up with will need to draw minimal current. Preferably in the neighborhood of 500uA or below. I don't want to drain someones battery!
I have already tried electrical system voltage sensing with no success. One would think that when a vehicle is at rest the battery would sit at approximately 12.6VDC. When the engine is running, system voltage climbs up to charging levels (typically 13.0VDC or greater). However, what I find is that system charging voltages range all over the map from one vehicle manufacture to another. For instance, one vehicle manufacture in the USA typically charges at
14.00VDC. However, they also have a mode that will drop charging voltage down to approximately 10.8VDC in cold weather. Another US manufacture charges at 13.2VDC and also has its own version of low charging voltage mode. Some European vehicles charge at 12.8VDC. Then there is the inconsistancy in rest voltage of a vehicles battery. A fellow co-worker has a brand new vehicle who's battery rests above 13VDC! As you can see, this is all over the map. Where would I set a comparator to allow activation of my circuit?This got me thinking about alternate options. One thing I do know is that the raw power feed on a vehicle is REAL noisy when the engine is running. Is there a way I can use this to my advantage? Essentailly what i would need is a noise detector. At rest the VBAT power buss is quite. While the engine runs there is all kinds of crud superimposed on it. One interesting nibblet is the pulse generated on the power buss whenever a cylinder fires. Could I detect and latch on in the presence of ignition pulses? A potential problem... The shape and amplitude of these pulses varies from vehicle to vehicle. In one vehicle we tested, pulses were about 600mV high. The highest peak was about 2uS wide. The period between pulses was 50mS with engine at idle. In another vehicle, the pulse was approximatley 1V in amplitude but only a few nanoseconds wide! The period between pulses was about
25mS with engine at idle.Can you think of a highly reliable way to detect engine noise, or better yet engine running, through sensing of the vehicles battery feed? Something that perhaps would throw a logic signal high in the presence of engine noise and then bring it low when the engine is shut off? This circuit would need to reject random EMI coming from other sources than the vehicle. I don't want someones ipod setting off my product while the car is parked in an airport long term parking lot!
Any help you can offer would be greatly appreciated.
Ge0