How to calibrate
1 channel 8 Bit DA 4 channel 8 Bit AD
AIN3<-AOUT (both connected to Nano A0) from about i==100(1.8V): Diode D2 will light proportional to value AIN2<-Input2 = thermistor (connected with switch) AIN1<-Input1 = photoresistor (connected with switch) AIN0<-Input0 = potentiometer (connected with switch) LED2 wil
What is it good for ?
Arduino Nano/Uno has only PWM for analog output, but sometimes you may want a real DA converter. For less than 1$ you can get a PCF8591 true 8 Bit DA converter And it has 4 channels 8 Bit AD converter. But Nano/Uno already have 7/5 channels 10 Bit AD converter. For that some manufactorers add sensors to the PCF8591 channels:
-
Photoresistor Inp0 You can set a switch to get values from a photoresistor from Inp0. It makes it easy to switch a light if it gets dark
-
Thermistor Inp1 You may calibrate it to get information about temperature
-
Potentiometer Inp2 You may set the potentiometer to a stable voltage - for Instance 1.5V With the switch set you can read the set voltage Then remove the switch and you may use this voltage at Inp0 for instance for a comparator or to set the base voltage of a transistor (to gain capavitive coupled AC voltage)
But it is not so easy to calibrate this device !
First Trick: I had problems to set an output voltage and read it back. When I read Ain3 connected to AOUT it gets down to 0. You have to set Bit6 in the read command !!! Wire.write(4+64); // 64: sets the PCF8591 into a AD mode - without 64(bit6) the AD-value is reset to 0 !!!!
Second Trick: My Nano shows 4.68V at the 5V pin - where the PCF8591 is conected to to get power. If I put 255 to AOUT I only get 4.55V Even that in mind I only get linear voltage between output 0 to 215. Here is the sketch to set the voltage as estimated - see Image.
The voltage set with PCF8591 DA is measured with Nano and PCF8591 AD (and with an instrument).
The resolution (1 bit) is about 0.018V The precision is +/- 2 bit
Time needed for setting 1 voltage (AOut) and reading 1 channel: <1ms
Conclusion: This device is not so fast and precise - but it is cheap and enough for a lot of measurements. And the LDR, Thermistor and Potentiometer may be useful AddOns.
Connections:
Nano GND to GND
Nano 5V to VCC
Nano A4 to SDA
Nano A5 to SCL
Nano A0 to AOUT
Nano A0 to AIN3