[Dprglist] 5v sensor to 3.3v GPIO interface?

[Clayton Timmons]

I'm helping a solar car team with telemetry.   Using a Raspberry Pi with
touchscreen on the car's dashboard to display data, log it, and push to the
cloud.  Thanks for help from DPRGlist,  James LeRoy pointed me to
io.adafruit.com which was very helpful.  We had the example up and running
in just a few minutes.  We have a cellular WiFi hotspot with dual antennas
on the car.

We are in the last few days of work before the multi-day race event.   The
last big crunch trying to get everything working.   We've added a rotation
sensor on the drivetrain and hope to get distance and speed from it.

There was one issue connecting the rotation sensor to the Raspberry Pi.
 The rotation sensor is an open collector type device which pulls down to
ground when the magnet is sensed.   The rotation sensor works from 4-12v.
 We decided to run it off 5v which is available on the Raspberry Pi
connector.
The output of the rotation sensor is normally about 4.7v and the GPIO on
the Raspberry Pi is 3.3v  not 5v tolerant.     I was hoping a simple
resistor divider could do the trick to interface the sensor to the
Raspberry Pi GPIO.   It seemed like the loading of the resistor divider was
pulling down the sensor output quite a bit.


Any recommendations for a really simple circuit to interface the 5v open
collector sensor to the Raspberry Pi GPIO?

Thanks,

-Clay Timmons-
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