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I want to use Raspberry Pi to measure temperature using LM35 sensors (15-30 Celsius degrees). I can't power up the sensor with 3,3V because it requires at least 4V and also I need about 10 sensors even if I would change my sensor Raspberry Pi doesn't have enough power on 3,3V stick. Can I connect MCP3008 with 3,3V (VDD and VREF) and my sensors with 5V? Should I use some resistors with my sensors?

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You can power the sensors from 5V and also power the MCP3008 from 5V.

You need to use a voltage divider on the MISO (Master In Slave Out) line from the ADC to the Pi so that the 5V ADC digital signal becomes a Pi safe 3.3V signal.

The MOSI, SCLK, and CE (Chip Enable, also known as Slave Selects) are outputs from the Pi to the ADC and don't need any voltage conversion.

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    If I use voltage divider output voltage must be exactly 3.3V or can be lower? – mariusz2108 Dec 26 '15 at 22:14
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    It does not need to be exact. A 1k and 2k resistor would be good and give 3.3V. A pair of 1k resisors would give 2.5V which is still enough to be seen as high at the Pi. – joan Dec 26 '15 at 22:22

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