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

[John Swindle]

Ron,
Good note that if there is voltage sourced at 4.7 volts, the output is not OC. That's a good diagnostic.
But... If Clay read 4.7v with a good high-Z voltmeter, he could have just read leakage into the OC pad structure, not a drive-high source. Need to set the voltmeter to Low-Z mode to see if there is any source current.

John Swindle

-----Original Message-----
From: Ron Grant via DPRGlist <dprglist at lists.dprg.org>
To: ctimmons <ctimmons at ieee.org>; DPRGlist <DPRGlist at lists.dprg.org>
Sent: Thu, Jul 11, 2019 4:24 pm
Subject: Re: [Dprglist] 5v sensor to 3.3v GPIO interface?

 Hello Clay, If the sensor output is OC, then you should be able to pull it up to 3.3V with say 1K resistor. It won't matter what the sensor power supply voltage is.  If you get a voltage with nothing attached to sensor then it is not OC.Ron.
 
 
-----Original Message-----
From: Clayton Timmons via DPRGlist <dprglist at lists.dprg.org>
To: DPRGlist <DPRGlist at lists.dprg.org>
Sent: Thu, Jul 11, 2019 11:31 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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