Discussion in "General help Guidance and Discussion" started by    Amit Radha    Feb 11, 2008.
Mon Feb 11 2008, 04:47 PM
How can i make a sensor for checking moisture in soil and air humidity? ckts on the net not working! I am working on a control system which needs these parameters where can i get help on this?
 jone like this.
Mon Feb 11 2008, 06:52 PM

kts on the net not working! I am working

you might be missing something... people in our field seldom upload their work unless tested.. give us the reference to your work...
Tue Feb 12 2008, 04:37 PM
Hi Shyam,
Finally my soil moisture sensor is working. I have not tested it fully. Will upload the ckt and resposnse voltage ranges for it. I need to make a humidity, temp and light sensor now and pH if possible. Temp is easy using LM35. Humidity is a problem. unable to get ICs here for it. Need todevise another way. Light can be done using LDR but i want a varying voltage or resistance response just as LM35 gives. Any suggestions? Pls help me out...
Tue Feb 12 2008, 06:23 PM
look here for humidity sesnsor: http://www.sumeetinstruments.com/sensors.htm
Wed Feb 13 2008, 06:18 AM
LDR is good for light sensing. or a photo transistor for a better voltage o/p
Wed Feb 13 2008, 03:20 PM
Shashi thanks for the link. I tried mailing them as no details are given on the site but the mail couldnt be sent, probably the company has shut down! Om Technologies is the only shop having a himdity sensor kit @750 Rs and he says he has not tested it. No one has any of the ICs i searched online and felt that it could be used! Does anyone have any suggestions?
Ajay I tried LDR but its respons is On-Off kind but i need a varying response. Atleast 4 levels(On Off givem me 2). Am trying using a opamps may work. not completed it at the lab. Photo transstors am unable to bias them right. need to get some ckt for it. (I got QT120 photo transistor)
Thu Feb 14 2008, 04:44 AM
LDR is actually a variable resistor who's value changes according to intensity of light falls on it. Resistance of LDR is inversely proportional to light intensity.

you can connect LDR's o/p to ADC and get 256 levels more than what you wanted..
Thu Feb 14 2008, 05:00 AM
Maybe I can help you with the humidity sensor, Amit, it is HIH3605 from Honeywell. Linear O/P 0.8-3.9V at 5V supply. Details at Honeywell site. Product is available in market, else try RS components. Thanks.
Thu Feb 14 2008, 02:58 PM
Hi ajay i know about the LDR but the problem is that the voltage across it switces instantly in a dark/light condition. so it effectively gives only 2 values. Have u tried it with the ADC directly and got 256 levels? I have managed to get 3 descrete levels out of it by sing the quad opamp LM324 and then to a summer today! Not yet soldered and tested properly.

Chiru, Thanks for the info. I found about the HIH series of Honeywell and i got circuits based on 3605 and 3610 also but in Bangalore I am unable to get a dealer! no shop on SP road has it! RS components only sells finished goods and i dont want a device that gives me the humidty readings on a display because i need to give that as an input to my microcontroller.

Can u name any dealers or any ither ideas for making the sensor?
Thu Feb 14 2008, 04:00 PM
do not use opamp... try connecting LDR between ADC and VCC directly... something like this..
VCC |----/\/\/\----> ADC PIN

make it work like a variable resistor, usually with opamp it act like a switching device

[ Edited Thu Feb 14 2008, 04:02 PM ]

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hello! newbie here. i am kind of a guy who hates using ready - made existing libraries, so this helped a lot as i was trying to control and write to a character lcd display. however, i can't figure out how to set the cursor position. for example, i want to set it to the 5th column in the first row, so position 5. so according to the table, i am sending 10000101 on the 8 data lines of the lcd (register select pulled low). the cursor does not move there however. and i cant move the cursor left or right too. is it something wrong that i am doing? thank you.



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