I have a K8055 connected to a universal temperature sensor. I have disconnected the Jumper SK2, as the sensor is powed by a 12v transformer and the sensor is connected to analogue input 1 and I can adjust the sensor using the little brass adjustment screw on the sensor. But I notice that turning the potentiometers ATT1(RV1) & ATT2(RV2) on the k8055 board also alters the reading.
So, what position should the potentiometers be set to when not in use? Full on, Full off or half way, I’m guessing Full on.
The potentiometers are building a 100K resistor between the analog in connector and ground, with the input of the PIC connected to the wiper. That way it acts like a variable voltage divider.
Well, the input of the PIC isn’t connected directly to the wiper, there is an op-amp in between. But let us ignore that for a moment because it is simply a voltage follower unless you installed R8/R9.
The A/D converter of the PIC needs 0…5V as input and represents that in 8-bits as a value of 0…255. So when your input signal has a range of 0…5V, you want the pot full clock-wise in order to get 0…5V to the PIC. If your input signal has a range of 0…10V however, you want to set the pot to half, so that it divides the voltage in half and again 0…5V reach the PIC.
To accurately adjust it you would turn it full counter-clock-wise, apply the maximum input voltage your signal can have, then slowly turn it clock-wise until your multimeter reads 5V on the wiper or the input reads 255 in your application or the demo.
Now what if your input signal has a lower range, like 0…2V? This is where R8/R9 come into play. As mentioned above, without these resistors installed, the op-amp IC1A/IC1B acts as a voltage follower. Its output will simply match the input voltage. If you however install R9 for example, it completes the “voltage divider” with R11 that then causes the op-amp to be configured as a non-inverting amplifier. You could install a 4K7 resistor as R9, which will cause IC1B to roughly triple the input signal, then attenuate it with the pot ATT2 so that your 0…2V input signal again results in a 0…5V signal to the PIC, this time measured at pin 7 of IC1.
"The A/D converter of the PIC needs 0…5V as input and represents that in 8-bits as a value of 0…255. So when your input signal has a range of 0…5V, you want the pot full clock-wise in order to get 0…5V to the PIC. "
thanks for that, I sort of worked it out straight after I posted the question, the universal temp sensor sends out 0…5V so the potentiometer is set to least/no resistance.