  # Open Collector Pullup Resistor Calculator

## Note: this is not a complete treatise on the calculating open collector pull up resistor values. It is a simplified substitute.

Terms:
• RPU - Pull up resistor
• VPU - Pull up voltage
• IIH - Minimum input current required to drive the input high.
• IOL - Maximum output low current the sensor output transistor can sink. This is 100mA for most Motion Sensors open collector outputs.
• VIH - Minimum input voltage required to drive the input to a high state
• RPUmax - Maximum pull up resistor value
• RPUmin - Minimum pull up resistor value
• RPUmax = ( VPU – VIH ) / IIH
• RPUmin = VPU / IOL

• The pull up resistor must be between RPUmin and RPUmax. Where in between depends on the application. The lower the input resistor, the greater the drive current which is useful when driving capacitive loads and improving signal rise time but also requires larger resistors to dissipate the power. The higher the resistor, the lower the power dissipation requirements of the resister but with a slower signal rise time.

Example:
For a pull up voltage of 5V with a minimum input voltage at 3.5 at 100uA:
RPUmax = ( 5V – 3.5V ) / 100uA = 15K ohms
RPUmin = 5V / 100mA = 50 ohms