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Aha...!
http://www.amazon.co.uk/6V-90V-Pulse...pr_product_top
...only 15kHz, so not strictly ultrasonic, but I personally can no longer hear 15kHz at all (snif) so it should be OK. One of the Amazon reviewers there had good luck using this unit to control the blower motor on his Landrover so I have a good chance of success with it. I do like the fact I can control the output with a 0-5v signal rather than having to use a potentiometer.
By the way echo-francis, I just noticed what mpg you're getting there. Impressive. In theory you should be able to squeeze a couple more mpg out with the blower motor PWM mod.
Another test I can recommend is to measure mpg at various electrical load levels. (Do you have a real-time mpg meter of some kind in the car?) That will give an idea of the mpg gains you get by reducing the load on the alternator. I would imagine that a small, petrol-driven car (?) which already gets high mpg would get the most mpg gains from reducing electrical load because the alternator is using a higher proportion of the engine's output. Also because petrol engines tend to be less efficient than diesels in absolute terms of power produced per gramme of fuel burned, so all that electrical power is going to cost more fuel than in a diesel.
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Ha ha - i just found your thread. Loads of good stuff there. I'll be back for a good long read soon. Also I just realised the 'perfect' PWM I just found is the exact same one you found. Looks good to me.