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I had a chance to play with an LM2907 circuit on the bench. The circuit I had laying around to play with was the circuit (fig. 24) right out of the app note with the addition of some input protection components and a 30K load.
I tested the circuit as low as 3 volts so 5 volt operation should be fine.
Being careful to adjust the input signal so there were zero crossings to detect I was able to detect a signal as low as 6mv P-P. The ability to detect the small signal as the speed approaches zero is the prime reason for using this part.
Some voltages I measured for output high and low at different excitation voltages: (5V exc, low 0.84V, high 3.08V), (12V exc, low 2.44V, high 8.50V), (14V exc, low 2.94V, high 9.98V). I expect these levels can vary a bit form one chip to the next so your mileage may vary.
From the data sheet for the ATmega the logic thresholds for 5V operation are max voltage for low is 0.3V and the min voltage for high is 0.6V. So this confirms your problem is a miss match of logic levels. You just need to get the low logic level low enough for the ATmega chip to see it as low.
The following suggestions assume you are powering the LM2907 with 5 volts.
A simple voltage divider of about 3:1 or 4:1 on the output of the LM2907 should do the job. A 10K and 20K resistor would be about right for a 3:1 divider. A 10K and 30K resistor will give you a 4:1 divider. Be sure to take into account or remove the input pull down resistor if present on the VSS input.
Another approach would be to use a pull up resistor on the VSS input and a FET to switch it to ground. The Gate input of the FET would be driven by the LM2907 output. You would want to select a FET that is 3.3V logic compatible, the gate threshold voltage should be around 2.0 to 2.5 volts.
A logic chip from the 74AHCT family would work as well. 74AHCT04 inverter would be a good choice but AND, NAND, OR, NOR and others can be used by tying other inputs high or low as appropriate. The AHCT logic chips have input thresholds of 2.0V for high and 0.8V for low. The output levels are 4.5V for high and 0.1V for low.
Mike
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