Is it possible to connect a sensor (hygrometer) to a digital input that has a resistance of 16MOhm and more? Example: So that at 85% hygro state changes from 0 to 1. Or is the resitance to high and must I use analog inputs?
You can use an analog input but a cheaper option is to use a setpoint relay to switch your digital input. Your hygrometer would be connected to the input of the setpoint relay. The output of the setpoint relay would switch the PLC input.
To select the type of setpoint relay you need to buy, you first you need to get the electrical characteristics of your hygrometer.
For example, it might vary in resistance from 0 to 1000 ohms and have a resistance of 600 ohms at 85% humidity, or it might be a 4-20mA device and give you 17.6mA at 85% humidity.
Once you have that, you can buy a setpoint relay to suit. In the USA a well known brand is Red Lion.
You could also use a Yokogawa UM330 or a Shinko or Shimaden controller from Japan.
But, a tiny PC board with analog comparators and pots will do the interface to the digital inputs with extreme precision and at far lower cost than analog inputs. Adding this to my automation gadgets list.
One caution I would add is that a bit of hysteresis is a good thing when using comparitors with slow changing signals. They sometimes like to give you a burst of noise when kept in their very small active region. The hysteresis gives you clean switching at a slight loss of accuracy.
