hI,
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?

you might be missing something... people in our field seldom upload their work unless tested.. give us the reference to your work...

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...

look here for humidity sesnsor: http://www.sumeetinstruments.com/sensors.htm

LDR is good for light sensing. or a photo transistor for a better voltage o/p

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)

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..

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.

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?

do not use opamp... try connecting LDR between ADC and VCC directly... something like this..
               ,,,,
VCC |----/\/\/\----> ADC PIN
                LDR

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

hmmm il try that out... need to get to the lab for that. as of now the quad opamp gives me 3 leves. the reason i dint use the LDR as u mentioned is because i checked the voltage variations with multimeter under different lighting conditions, and the ldr just switched directly. il try this way u mentioned also tomorro.

can u pls help me on the humidity sensor! that is wats not happening now. temperature il be doing with LM35.

Hi Ajay,
I tried doing the light sensor the way u mentioned but i am not getting a varying voltage response! why is that? the voltage just switches from a low value to high and vice versa!! i used LM324 and summer to finnaly get 3 levels! a huge complicated setup!!!

Was having a quiery regarding LM35. Can i directly give its voltage o/p to the adc? i use resistance from 80k to 470k but my voltage readings dint cross 1V. Wont be too low as an i/p to adc? Pardon me forbeing so lame (I am yet to read the data sheets ) I am going bnkers now. been 2 weeks and just done 3 sensors, yet to test them properly and unable to get the IC for humidity sensor!

The LDR's resistance can reach 1M in dark conditions and 100 ohms or so in full brightness.So it naturally follows that you'll get an instantaneous swing between (almost) 0v and (near) Vcc under the dark\bright condition.
You may add a resistor in parallel to the LDR and another in series to limit the swing.
With the values shown and Vcc=12V you'll get ~4.85V if LDR=1K
~6.1V if LDR=2k
~7.7V if LDR=10K
~8.2V if LDR=1M
You may fine tune the resistor values to suit your needs.
Regarding LM35 you need to give it's output directly to the ADC;no resistors.It will output 1.5V at 150C.
As for the Honeywell humidity sensor,a friend had bought it from Emaar Impex for ~450 bucks.You may drop them a mail.....http://www.emaarindia.com/contact.html
(their website sucks)

thanks sashijoseph for this info i did not know this thing about LDR.

Hi shasi,
Thanks for the update! Il surely check it out today and let u know about the results Thanks for the link too. Il try getting the IC. Need it desperately.

Today have to start testing the bands and regions of operation after interfacing with the adc.

Il need all your help and Ajay and Shyam, yours too to code!!

Thanks again
Cya

Yes Amit, contact Mr. Paresh 09967022244 02223869472 of Precious Electronics Corporation, he placed at Mumbai, tall him you got this information from me - Chiranjib from Kolkata, he will help you out with humidity sensors. If he cannot give sensors of honeywell ask for tech details of the same.

To add some finer points for Amit with LDR and LM35

Most of the ADC chips works with 5V, so its better to power the LDR from 5V too. As for the circuit suggested by Sashi it is perfect, only change the 1K with 10K and remove the 2K2.

There should be small resistance like 1K at the OP of LM35 so as to prevent any short circuit conditions for the IC. You may also consider shunting the input of ADC with 0.1uf to bypass noise.

Shashi the ckt wrked like a charm! thanks a million. My board size has become 1/4th of wat i made!!!I am using it with 5V supply only.

Chiru, thanks for the details. I am trying through a person in Bangalore now, if its unavailable il surely use ur contact. And il surely make use of your tips in my project. I have already solderd the LM35 so cant make any changes to it. But most probably i will not be using the ckt because the HIH3605 humidity sensor needs a temperature sensor for temperature compensation in its calculation and a specific IC has been mentioned (DS2438) so i wil be probably using that only for my temperature readings.

Any more suggestions please do post it here. I shall try incorporating it.

Another problem i am facing now is that my faculty says that the project is too basic and that it is doing nothing great nor is it innovative!!! so i have to think of something more to add onto it!!! Pls help!!!!

Hey,
It seems the 3605 and 3610 are outdated and now only the 4000 series are available @ Rs. 750+VAT!!! Also i am unable to find a circuit for it online. I think its not wise to but the IC without having a ckt that is sure to work.
Wat do i do??? Any ideas? Pls help!!!! I really need to find a solution to this problem!And i am yet to start coding for the damn project

Dear Amit, the HIH4000 is smiler to 3605, find the same in the pdf, download it with search for HIH4000 pdf .
As for temp compensation, i don't think its required given the recommended operating temperature zone, the shift in o/p voltage with temp and the application .
You may add in your code some indications for comfort limit like hot, dry, wet, comfortable, etc., depending on the data collected, to make the project interesting .

Hi Chiru,
Thanks for the info.I did go thro the datasheets earlier but...they dint make much sense to me
I am able to get the HIH4000 locally @ Rs 750 shall i buy it? I am attaching a pdf document in which i found a ckt for the HIH3605. (humsensor 3605IC.pdf)Pls do go through it and let me know of i can do it. I shal only then proceed with buying the ic.
Also i found an interesting ckt done by someone using an open cap in an Astable multivibrator to sense humidity. Il upload that too. (Humidity_opencap.pdf) Pls do have a look
Il surely incorporate ur ideas in the code. Any more h/w or sensors u suggest could be added to the system? I am also planning to make it an sms based system, but that is Stage 2. I need to get this done 1st and foremost. Stage 3 is to make a multiprocessor system where i can use a master controller to control other controllers which will be interfaced with the sensors and actuators.
Do u think it is viable?
humsensor_3605ic.pdf
humidity_opencap.pdf

Hi...
with 3605 measuring humidity is a cinch.You don't need that exotic dallas chip.You can easily use LM35 for temperature compensation.
Just measure the 3605's output voltage with an ADC and the temperature with the LM and apply the fol. formulae for calculating RH

Sensor RH = (VOUT / VSUPPLY) – 0.16 /0.0062, typical at 25°C
To correct the calculation for temperature and compute true RH, use:
True RH = sensor RH / (1.0546 –0.00216 T)
where T is in °C

The open cap one could be used to detect changes in humidity,but to measure humidity you would need some kind of a relation between frequency(or capacitance) and humidity,which isn't provided.....so it's pretty useless.

Hey shashijoseph
Thanks for the post. Well the formulae for using with the 3605 is given in the document i had uploaded. But i am not getting the 3605 locally. The HIH4000 is available and much cheaper than the 3605 (from bombay) so i needed to know if i can use that instead of the 3605 in the same ckt.
That open cap thing seemed to be really useless to me too.
Pls let me know if i can use the 4000 in the same ckt?
Can i just read the voltage at pin2 of the HIH and keep my LM35 away from the ckt? and then do the necessary calculation in my code right?

Hi, sorry for being late to reply .
Sashi has already clarified something. But i think you can go straight and use HIH4000. No LM35. Just feed the pin 2 signal to ADC. The O/P voltage is proportional to RH (at reg 5V supply) and there is a little deviation with temp which may be overlooked for your kind of application .You may consult the data sheet for the same .
Want to add a barometer to your system , add one ASDX015D or ASCX15DN pressure sensor to your system (voltage o/p at 5V). You can also make overhead tank water level gauge etc. with this .
This sensors are from honeywell, and i am using one 1psi sensor for a critical medical equipment for a long time, and am more than happy (O/P directly connected to ADC).

Hi Chiru,
No issues with the late reply. My work's on hold anyway! I think i have developed RSI. Bad pain at the left back of my head, neck and shoulder.And was busy with seminar stuff.
But u have given me reasin to cheer I needed assurance on the ckt for the HIH4000 b4 buying it and now i can buy it in peace and try it out.
The ASD & ASC ics u mentioned also from honeywell?
And how do i use them for the overhead tank level gauge? that will be a good feature for the system. Pls do post the ckt for it if u have. els il google it
Wel my system aint critical at all... just need to show that something practical and usefull can be made

Hi Amit get wel soon .
ASCX and ASDX series pressure sensors are from honeywell (formally Sensym). ASCX is temp compemsated but costly. Both ASCX and ASDX are amplified voltage o/p 5V sensors, unlike normal 4-20 mA or mV o/p types. You can also use a cheap 4-20 mA type (14PC series) with a series resistance connected at the ground end and read the voltage against that res. There are also some cheap sensors in the market with reasonable stability, but ask for pin out details.
I shall try and give you the idea for cct of a tank lavel gauge. Its basically a rigid tube with one open end and the other end connected to the sensor, the open end facing down is dipped into the water. Initial or null pressure is measured with no water in the tank and set with the o/p voltage as zero lavel. Similarly fill the tank and take reading/s for other level/s (water goes inside the tube and the positive pressure gives a corresponding higher voltage o/p)
So go ahead...

Hi Chiru,
Thanks a ton for the good wishes and ur prompt replies really boosts my morale!!! il surely try and add the sensor for the water level indicator. I had read of another kind of ckt for this sometime earlier it was based on some varying resistance. dont remember it though now
Started working on getting digital o/ps from the adc for my sensors and i hit a roadblock! I get crap o/ps!!! tried with all the ICs (0808) in my lab still no success! anything i should keep in mind while using an ADC? I have connected al the unused pins viz. In1 to In7, EOC, SOC, etc to +5v I tried connecting the pot to In7 and In3 (as mentioned in the datasheet) to use only 8 or 4 bit o/p but the result was the same. Am i missing out on something???
Also i think its high time i started coding... wat do u think should be my approach? I also intend to have a RTC in teh system and also include a manual over-ride incase of system failure!

Hi Amit, I think you are familiar with ADC conversion. If not, first try with 0804, try the "free running connection" in the Harris Semiconductor datasheet .
Conversion time is very important. Use adequate delay. The clock should be within recommended frequency range. I think all analog inputs are within 0 to 5V DC, with v ref set to 5V. Follow the "timing diagram" properly. Check power supply for noise, minimal wire length ( if unavoidable) at analog input .
These should solve your problem (provided the code is OK, chip is not fake).

Hi chiru, il look into the datasheet of 0804 as u mentioned. I used the NS 0808 with i/ps in the range 0 to 5V either from my sensors or variable supply (both yielded no o/p). infact i encountered a peculiar problem of changing digital o/p everytime i turned ON the trainer kit but eventhough i changed nothing! I am not using it with the uC so i had clocked the ALE. I shorted the SOC and EOC pins. and the ckt i tried was almost similar to that given in the datasheet of NS 0808!
Il try wat u have suggested. gotta check if i have 0804 in the lab though
Roadblock after roadblock

Hi Amit,
The LSB of the digital o/p may drift a bit even with a stable analog i/p
A steady clock is required at the CLK i/p for the functioning of the ADC itself, why should you clock the ALE
I didnt find any SOC pin of 0808 in the National ds, is it START is it supposed to be shorted to EOC
You may follow the typical application at p12 of National ds, also check figure5 at p7
There is no ROADBLOCK if you are open in your mind

Hi chiru,
Thanks for the info.

Yes I did meant START (Pin6) by SOC. I had shorted it to EOC basically to use it free running mode (for continuous conversion just like how its done in 0804). I was trying to use the ckt mentioned in Fig. 9, Pg 9 of the NS datasheet. Il surely try out wat u have suggested tomorrow.

Oh and i forgot to mention...the ADCs in my lab were bad thats why i was not getting the o/p! I tried with the 0804 today and it worked so i have purchased 0809 (as 0808 was not available) and i shall try it out tomorrow. Could u pls tel me the diff b/w 0808 & 0809 i really couldnt figure it out from the ds!

I am facing another problem now i hope ul be able to give me a solution to it. In the ckt i uploaded for the HIH3605 in the 1st post of teh pg, they have used 2 diodes - BAT54S. It is not available here, i spent an hr today at the shops but couldn trace it! could u suggest an alternative. I shall look into its ds and try to find something.

And yes...as u truly mentioned ... "There is no ROADBLOCK if you are open in your mind ".

Thanks bye for now

Hi amit, you can take some info from tutorial section if you want.. and keep working for learning you are doing a good job.

Thanks Ajay il surely look into the tutorials. i hav exceeded my bandwidth for this month so wil do it on 1st march only. u guys are really helpful and motivating ... thanks a lot!!! hope il gain some knowledge n wil be help some more noobs lik me sometime in the near future bye for now

Hi Ajay and Chiru,

Pls read the 1st post on this pg.

Tried with the 0808 today as u mentioned, but i used it in free running mode and not with uC. so i had to separately connect ALE (tried it both ways i.e. toggle and also permanently high) and as i had seen on another site online i had shorted START and EOC. Clock = 400kHz - 700kHz (mostly 650kHz).

I did get desired o/p ... but not always. infact below 1V i dint get a correct o/p. Above that it was not consistantly correct o/p. sometimes i would not get it. and generally i was getting double than wat i was supposed to get (i.e if my i/p was 1V the digital o/p was 2v).

Can u pls tel me wats wrong! this is getting really frustratimg. Cant move ahead!!! unless i am sure it works only then i can test it with my sensors and get the operating bands and only then can i start the biggest part of the proj - the coding!

Also am unable to get BAT54S for my humidty sensr and the ds shows it as a 3 terminal device (iv not studied it properly) but in the ckt dia its been shown as 2 terminal only...

Pls help asap!!! :'(

Hi Amit,
ADC0808 has max error of +/- 1/2% against 1% of 0809, it is a superior converter
BAT54S is dual Schottky SMD diode, you may use two BAT42 or BAT85, they are available
I am confused with what you mentioned in your last post, will work on it and let you know shortly.

You don't need those Schottky diodes.They are used for supplying voltage from the 1-wire interface.You wouldn't be using that Dallas chip(in the attached pdf) would you?
Simply provide 5v regulated voltage to the HS and monitor it's output voltage.

Thanks Chiru and Shashi,

I dunno wats wrong, the ADC doesnt seem to be consistant at all! Behaves wierd!!! Today i tried it agin. Started with 1.5V (increased till 4.5V ) and got near perfect o/p but i tried for 0.5V and got garbage. after that i dint get a single correct o/p even for the same i/ps i tried before!!! did i spoil the IC? i dont think i did. I faced this same peculiar problem yesterday also but today i used the same IC and got correct o/p initially!

I will not be using the Dallas chip in the humidity sensor ckt. Should i use an LM35 or il just stop at the o/p of the HIH4000? eliminating the RC ckt also? So in that case i needednt use the BAT54S diode? or can i replace it with a zener?

Maybe a noise issue for the ADC.......have you used long wires at the i\p?Have you used a regulated power supply and proper bypass caps at the ADC Vcc pin?

As for the HS you may use LM35 for temp correction(if you want greater precision) and no need for the diodes.Just use a regulated ps.

Hi Amit, i don't know if the 0808 is bad , but in case you plan to buy a new one, shift to serial ADC (saves space) and buy MCP3208 . It is 8 ch serial 12 bit ADC from Microchip.
Enclosing a 8051 code for the same to scan the 8th channel whenever the ADC is CALLed (i use it with a 2051 ), the result is stored in R2 for MSB and R3 for LSB. It is a guideline for you and modify it for other channels .

Hi Chiru....how much does it cost buddy?

What cost, Sashi

Hi shashi n chiru,

Well i am using a IC trainer kit in my college lab and using the power supply n gnd from that only. i have not added any bypass capacitors. Yes the clock was not a clean square wave it had a bit of distortion at the +ve edge. could that be the problem? Il try using a capacitor for that tomorrow.

Yes how much would that ADC cost? I have just spent Rs 150 on 2 new 0809s. I dont think i can buy another expensive IC

Are you sure about not using any diodes? The IC is very expensive @ Rs 750 + VAT. I have not yet bought it.

No improvement in the project still. Had a photo exhibition so was busy with that past 2 days. will get back to work tomorro onwars ... but i dunno wat to do. just sitting and there is no +ve result!

Yes Amit, all serial ADC's are dear. A 2ch will be around 120/- and 8ch as high as 400/- in Kolkata .
Give me some time, i shall get back to you with a working ckt of 0808 .
Diodes are not required, stick to the simple ckt in the DS. Supply should be neat and not exceeding 5V DC .

You love photography? i do. For that we can talk elsewhere, maybe. My id is ckarmakarams@gmail.com

OMG!!! Well the cost of my proj is going up!!! Well, today i tried again with the 0809 and guess wat at IN7 i got precise digital o/p!!! exactly the same as expected. even for 0.2V. And then comes the bad news... it dint happen for any other i/p lines! we are not going wrong wit the address lines am sure of that. now i dunno wats wrong!

Is it possible that i have spoilt bot my ICs? Well i had 1st tried them using IN0 and IN3 only and the o/p data lines to the LEDs on my college IC trainer kit. Is it possible that they tried to draw more current and in an attempt to drive them my IC got spoilt? But i always checked the IC from time to time nad it never heated up!

Wat is wrong? Tomorro i have planned to put it onto a pcb rather than get stuck with the bread board or trainer kit. that way can avaoid unnecessary noise also.

Well i am not a photography lover. Few of my friends are and i was just a volunteer helping to organise the event. I do love scenic and macro photography but... have never tried though. Will surely add u on gmail

Pls tel me wat do i do about the ADC!!! Cya for now

Well Amit, since one of the i/p is working, the ADC is not bad .
You say the address is ok, still once more check it as, for ch7 it should be #7 (all high), ch6 = #6 (high high low), etc. Remember for each analog ch to be scanned, there should a complete cycle to get a valid o/p .
If it still doesn't work, then the input logic or the mux is spoiled somehow .

And never mind about the photography thing.

Hmmm, i did give it enough time for the conversion and yes, i did follow the addresses correctly. will try it out again els i have got a new 0809, wil try with that.
N u must check out the link http://www.flickr.com/photos/shakirfm/
Its my friends. hes an amazing photographer. got interested in it cos of him. hes a macro guy too. Cya for now. do let me know wat do i do to troubleshoot the damn thing

Take it cool, start it afresh

Checked flickr, excellent, thanks and regards to your friend.

Tried afresh with the new adc but the same prob :'(

But found something interesting... the IN7 works even in the new IC. also the junk o/p is the same in all the 3 ADCs. is it possible that i am making a mistake? i have checked and rechecked the address lines and also tried al kinds of permutations and combinations but to no avail

Dunno wat to do now. planning to interface it to the 8051 and try directly wat happens. probably am making a mistake while using it in a stand alone mode.

Any idea on AVRs? i heard that the ATmega has built in adc. but how about programming it? i have no skills in embedded C and had planned to code the 8051 in assembly only since i have learnt it.

Wat do i do now?

Help Help!!!!

Take the plunge buddy.For now stick to the 8051 since you learnt 8051 programming.Try out a few projects with ADCs,I2C,SPI,LCD etc etc and then you can migrate to any other platform.The transition won't be too difficult.

Hi Amit, follow Sashi's suggestion In case you need to change, shift to PIC, start with 12F675 There are lots of circuits and projects available on the net, moreover Microchip gives good application support for its chips.

Thanks Shashi and Chiru,

Il keep that in mind. I did start reading on the PIC 16F74. But there are no good books on them infact few ppl i asked said that u can only learn bout pics from their datasheets! is that true? Pls do suggect books, if any for them.

Yeah and another good news! The adc finally worked! All i did was to clock the ALE also with the same signal given to CLOCK of ADC. and it works. perfect o/p at 6 channels and a slight 1 t0 2 bit error at 2 channels. will check it for lower values also tomorrow.

While using the LM35 with ADC should i use an opamp? I think you told me not to. I had already put it on the board but. So should i pull it out now? But in this link http://www.freewebs.com/maheshwankhede/sensors.html (pls check it out.) he mentions that an opamp is required? Wat do u suggest?