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Oops - output voltage swing not big enough

566 Views - Created 30/05/2018

30/05/2018

Posted by:
enovikoff

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After I ordered the ADC DAC Pi Zero card, I realized its output swing wasn't enough to control the fan speed on the fan I need to control, which requires 0-3.8v as a speed control signal.  I made the mistake of reading the DAC chip's specs instead of the card specs, which limit its supply to 3.3v.   Are familiar with any known ways to increase the swing?  Such as maybe an op-amp circuit based on a single-supply chip?   Or should I just order a Expander Pi?

30/05/2018

Posted by:
andrew

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Location:
United Kingdom

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You have a couple options for increasing the output range of the DAC chip.

You could use an op-amp to amplify the output voltage.  I found this page which has some useful information on making a single supply op-amp circuit.

The second option, if you don't mind having the ADC DAC Pi seperate from the Raspberry Pi is to use a buffer on the SPI bus and run the ADC DAC Pi at 5V.  On the Expander Pi we use a 74HC4050 buffer powered from the 3.3V supply on the Raspberry Pi.  The inputs on the 74HC4050 are tolerant of voltages up to 15V so you can power the ADC DAC Pi from the 5V rail on the Pi and the 74HC4050 will act as a level shifter for the SPI bus.  The Expander Pi schematic shows which pins need to be connected on the 74HC4050 to the SPI pins.

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