Analogue to Digital and Digital to Analogue Conversion

The PCF8591 is a 4 channel analogue to digital (ADC) and 1 channel digital to analogue converter (DAC). ADC and DAC have 8 bit resolution. The PCB card already contains 3 devices that can be connected via jumpers to the ADC inputs:

• a potentiometer on channel 3
• a photo resistor on channel 0
• a thermistor on channel 1
• channel 2 is free

The sensors are resistors whose resistivity changes with the parameter to be measured (temperature or light intensity) and, in conjunction with a 1 kΩ resistor they implement a voltage divider. This voltage is converted to a digital value by the ADC.

There are 2 LEDs on the PCB: a red one showing that the board is powered and a green one connected to the DAC output.

I used the free channel to feed back the DAC output.

pcf8591 front	pcf8591 back

You can distinguish the PCF8591 chip, the potentiometer and the photo resistor and thermistor. The pins on the left make the connections to external analogue signals and 1 pin provides the DAC output while the pins on the right implement the I2C interface with SCL, SDA, Vcc and Gnd.

Here is the circuit diagram of the PCS:

Unfortunately on the boards I purchased in China there is an error on the PCB: The thermistor is not correctly connected to ground. For this reason I only see the value 0xff. A little bridge between the ground pin of the photo resistor and the thermistor will do the trick.

-- Uli Raich - 2017-02-01

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