I was given an interesting spec by a customer.
He wants a wall mount RH sensor with a resistive output which emulates a 3k NTC thermsistor curve. So 0% RH is about 10k, 25% 3k, 50% 1k etc.
However the only power available is 230V ac and there is no space for a transformer. And the design is cost sensitive as the quantities are good. The first batch of 5000 are needed in 8 weeks.
So The design needs a variable resistance output which meets SELV requirements.
My first thoughts are a capacitive dropper running a PIC which will provide a pwm output to an opto isolator which will control the resistance of a FET. Then all I need is a way of correcting the FET resistance but that is not easy as it's on the isolated side of the opto.