Sounds like a plan to me. You need a way for the arduino to monitor engine RPMs. I'll be looking into this myself, over the summer, as I want to make a cruise control system.What vehicle are you running this system on? Does it use a cable throttle, or Drive By Wire?Off the top of my head, ideas to grab the engine revs.-Tach signal off the distributor (not sure if this would be at a high voltage though)-Voltage off of the gauge needle motor.-OBDII interface.Some sort of failsafe would be good. Perhaps a switch that kills power to the arduino unit? Something automated that kills Arduino power if the motor starts to race [Perhaps when you get fuel-cut at the rev limit?]In terms of powering the Arduino off of ~14.4v car voltage, I was planning on using a (LM7809CT-ND) to pull the voltage down to 9v, and have the Arduino voltage regulator drop it to 5v.Not sure if this is the best way though.
If I were you I'd follow the approach which is conventional for engine management systems to control idle speed - fit an idle control valve - basically a stepper motor controlled air bleed valve - to provide an air bleed past the throttle plate. This avoids interfering with the cruise control system. By designing the air bleed to provide a suitable restriction, you avoid any issues about giving the engine excessive throttle. (When it is off load and at small throttle openings the engine is extremely sensitive to small changes in throttle position.)The Megasquirt web site suggests some sources for the idle control valve and describes how to drive them. They are inexpensive, easy to plumb in for most installations, and electrically straight forward.
You may want to look at using a servo for the throttle control.
Can you elaborate on this, please?
QuoteCan you elaborate on this, please?A servo can be commanded to push/pull linkage to specific positions (or rotate to specific positions). May be easier than using a stepper motor.
Instead of monitoring the vehicles speed sensor to vary the throttle, I just want to monitor the tach signal, and actuate the throttle accordingly.
Have you got a reliable tach signal? That's something that can prove quite tricky to get accurate, because the coil LT signal is very dirty. This is another area where the Megasquirt site has some very useful suggestions and circuit diagrams.