Simon B

Hi All, I have a project I am working on (purely in the conceptual phase at the moment) which I hope will end up as a fully automated hydroponics control system. The system will have temperature, light, gas, and humidity sensors with which to sense it's environment. I also wanted the system to be able to mix nutrients in the right proportions, and be able to top up water or nutrient levels if they become too dilute or concentrated. The manual method is to use a PPM meter to determine the salt content of the water. The PPM meter just measures the electrical conductivity (EC) of the water, with two probes spaced 1cm apart, and a temperature sensor to compensate for the water temperature. I initially thought that just a couple of probes connected via a resistor to one of the netduino's analogue input pins would do the trick, but reading more about the process of calculating EC indicates that the measurement cannot be achieved with direct current -> it has to be alternating current, or the salts will all accumulate on one of the probe terminals. My background is mainly programming, my electrical engineering skills are limited and a bit dusty from not being used since uni. I don't want to go through the pain of trying to build a complex circuit that will generate AC current, so I am hoping to use the netduino and a PWM pin to generate the AC current with only a couple of components (capacitor/diode/resistor).. is this even possible? I do not know the frequency requirements, but I expect that 50Hz would be sufficient. So this could be achieved without using PWM, by just pulsing one of the GPIO pins 50 times per second. I know I can generate a waveform that goes from +0.01v to +3.3v, straight out of the netduino pin, how would I bias that wave so oscillates from -1.5v to +1.5v? I have a vague idea it could be achieved with diodes and two pins off the netduino (one for positive, one for negative) like a backwards bridge rectifier, but I can't quite work it out. Any suggestions/ideas appreciated. Cheers, Simon.