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

[John Swindle]

Clay,
I assume when you say the resistors pull the output down a lot, that you mean it is pulled below 3.3 volts. If so, it seems the high resistor is too large or the low resistor too small. If the calculated ratio seemed good, remember that 10% tolerance resistors are NEVER the marked value, having been binned.
Conventional PCI used open drain, sustained tristate. When low value pullups are used, the signals restore faster but burn more power. If speed is not a concern  use large value resistors. The sensor's DC drive strength may not be able to overcome low values.
Good luck with your team!
John Swindle


-----Original Message-----
From: Clayton Timmons via DPRGlist <dprglist at lists.dprg.org>
To: DPRGlist <DPRGlist at lists.dprg.org>
Sent: Thu, Jul 11, 2019 10:31 AM
Subject: [Dprglist] 5v sensor to 3.3v GPIO interface?



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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