Converting Hall effect RPM to varying 5V voltage

Discussion in 'The Projects Forum' started by snizbatch, Mar 19, 2013.

  1. snizbatch

    Thread Starter New Member

    Jul 14, 2012
    Hi all,
    I'm new to the forum. I'm also new to digital electronics. I'm working on building a model aircraft Autopilot out of various break out boards connected to a Raspberry Pi computer. I've got most things working but my snag lies in the one device I'm building from scratch: An RPM translator. I have a US5881 (UA) hall effect sensor from Melexis. I've been able to get the sensor connected in a "2 wire" configuration using a 10K and a 150 ohm resistor to register 4.95v which falls to 0.25v when sensing a magnetic field. I'd like to translate the RPM pulses (between 0 up to about 12,000 RPM) into an analog voltage between 0 and 5v that I can measure using a Pololu mini maestro. For this purpose I've picked up a couple of LM2907N frequency to voltage converters and a couple of LM2917N models also incase they are better suited for the job. I'm afraid I'm going to fry them in trying to figure out how I should wire them up. The datasheet shows various schematics for miscellaneous uses but I have no idea where to start here.

    I'm powering the whole system using a single 6.6v LiFE battery with 2 Pololu voltage regulator boards (One 5v and one 3v3) Off of those run:

    Raspberry Pi Computer
    Mini Maestro 24 Servo controller & analog voltage sensor
    MPXV7002DP Differential Pressure Sensor (Airspeed)
    (Hopefully this RPM Sensor)

    MinIMU9-v2 Gyro/Accelerometer/Compass
    D2523T Serial GPS
    BMP085 Barometric Pressure Sensor (Altimeter)

    I've individually read each device from the raspberry pi except for the RPM. I have created a board layout for a GPIO expansion board that has headers for all of the above boards and sockets for the two separate voltage regulators. The last thing to add to the board is the circuitry for the RPM sensor so I can simply plug the hall sensor directly onto the GPIO expansion board and forward the varying voltage to the Mini Maestro. I've got all the tools and materials to etch my own double sided circuit board using toner transfer on parchment paper. If I could only figure out this LM2907N Chip I'd be golden! Thanks for reading.

    Jim Hunt
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    can't you just do it in software.. count the number of times it goes low and then output a voltage with that.