what would you pay for a accurate digital thermostat custom built?

[crossedwires]

ok heres the deal........i have read alot of posts on here with thermostat problems etc, i build thermostats using micro controllers and accurate temp sensors. i wouldnt mind making theese to sell at a very small profit but i would like to know what people would consider to be a reasonable price to pay, so give me a price and the features you would like for example lcd readout or not. and i will cost it out.

 

[Bloomie]

Quote:
Howdy from Northern Nevada

Yer thread caught my attention because just 2 days ago I went to town looking for a pet store so I could locate a decent thermometer/hydrometer.
I ended up paying $32 for a Zilla.....how accurate it reeeeeally is is beyond my know-how....had to rely on the shop owners experience.
So I reckon $32 was acceptable to me, so that'll be my input.

 

[aprophet]

I have been using Johnson controls electronic T-stats for my homebuilt bators they have a 1 * diff I theink the ones I buy run me about 50-52 $ I have an account at a local supply house I do refrigeration repair . what I have been setting mine at they swing between like 99-100. Or what ever I set them for. the last time I checked on accurate humidity sensors ( 2%-3%)they are totally outrageous on price

 

[EliteTempleton]

I bought two of these about a year or two ago, one for me, one for my dad. My dad's still kinda works but it's limping along. Mine have seen harsher conditions and neither the outdoor sensor nor the station work anymore(that gave up it's ghost when I put it in the freezer to see what the temp was when I used different # settings). *Resisting urge to rant about fridge/freezer chill settings being #'s instead of temp settings...* They go for $25-$30.

So....

If you built me a custom one I'd want:
-Accurate to +/- 5 degrees Fahrenheit
-Could read the outdoor one wirelessly
-Low reliance on power(Read: I don't want to be replacing batteries and re-syncing the W signal quarterly), to the point I'd pay more for sustainable power source feeds, like a solar panel or something.
-The inside the house one should look decent(I don't expect it to be an art exhibit, but I don't want people asking me if I bought it at a school fair either...)
-Your promise it will be far more durable than that Chinese set.

Hook me up with all that and I'd pay $30-$50, maybe more with some convincing, and I'd buy two!(one for my dad).

 

[Blooming chicks]

I'd pay between 35-45 dollars and I am a beginner at the incubation thing. Also--I live on the east coast--sometimes location can play a big part in price.

 

[amyrains]

i am also very new to this and i can already see the importance of having good temp control...id also pay between 30-50.....

 

[Redcatcher]

I already use this for my incubators. It is external and requires no rewiring. The incubator plugs into the thermostat and that is it. I would never go back to using anything else.

http://www.spyderrobotics.com/products/herpstat.html

 

[dsowens15]

I use the ranco ETC-111000-000 digital thermostat. I got it for i believe $60 it works great but I had to wire it myself.

 

[crossedwires]

ok so lets work this out...............$50 is about £32

so what would it cost to actualy build one...

lets assume we go down the vereo board route as this would save faffing about making the pcb board wich although not costly in materials is alot of messing about to actualy make the board and accurately drill it.

so we allow roughly £3 per board for the vereo
next we need the chip my preference would be a pic , for just temp controll i would go for a 18 pin pic so lets say a 18f1330 http://uk.farnell.com/microchip/pic18f1330-i-p/8bit-mcu-8k-flash-256b-ram-18f1330/dp/1324377

its listed as £2.85 but you have to add vat on that so its £3.35

next we need the sensor , i would sugest a dallas 18s20 now they have a accuracy of 0.5C and 9bit resolution. in my experiance you can have 3 in exactly the same place but they all read slightly different, however i use them alot and apart from the dallas 18B20 they are probaly as accurate as your going to get for the money, the 18B20 has better resolution but to be honest you dont need 12 bit resolution its a bit pointless with 0.5C accuracy.
again from farnel http://uk.farnell.com/maxim-integrated-products/ds18s20/digital-thermometer-18s20-to-92/dp/9724761 its £5.09 and with VAT thats £5.98. notice on the farnell site it say accurate to 2C, thats an error on the farnel site. the datasheet specs say 0.5C and is correct.

next we need a relay
for the purpose of this lets assume we are going to be using light bulbs as a heat source it dosnt matter if its 1 or 2 bulbs 1 relay will do.also we need to think about what voltage we are going to use to run the circuit and relay switching coil, lets assume 12v as a 12v power supply is easy to find such as a old mobile phone charger or pc power supply 9yes i know the chip we chose is 5v i will get to that in a mo) the cheapest on farnel that will fit the bill is this one http://uk.farnell.com/multicomp/hrm1-s-dc12v/relay-pcb-dpco-12vdc/dp/9479953 and with VAT is £1.29

Because we are using a relay we have to add a small signal diode to stop the back EMF thats generated when the relay switches from destroying the delicate chip, for this a pretty standard 1N4148 diode is the chappy for the job http://uk.farnell.com/vishay-semiconductor/1n4148/diode-small-signal-100v-do-204/dp/1612346 with VAT £0.06

we are using 12v supply for the circuit board so we need to cut the voltage down to 5v for the chip, easiest and probaly cheapest route is a zener diode (electronic Buffs will frown and say use a regulator!...ignore them for this

)
a 4.8v or a 5.1v zener will do us fine http://uk.farnell.com/vishay-semiconductor/1n5231b/diode-zener-5-1v-0-5w/dp/1612371 with vat call it £0.02

the part we need to make some assumptions............
ok lets assume we are building a basic unit no LCD screen, but we want to incorporate the ability to adjust the tempreture, without an LCD screen this is harder to do than it appears as we are using digital sensors and a digital chip to controll, so using a variable resistor (pot) isnt realy an option as you wouldnt know what you had set it at. sooo lets go down the 4 option preset route................... i better explain this

probaly the easiest way to do this would be via jumpers on the board like you get on some pc cards for those that have no idea what i am on about

they are the little pins sticking up that are in the green circle. you could have the presets at 99f 99.5f 100f 101f
this way you could preprogram the chip to set the tempreture depending on wich jumper you used, to do this we need 8 header pins and four pull down resistors call this £0.30

before we forget we need to add a couple of decoupling capacitors lets call it £0.04

we also want some screw connectors for wireing in the lamp/s and for the board power theese are probaly about £0.40

we also want so means of knowing when the board has power and when the relay is switched on etc, without a LCD then our options realy are limited to the good old trusty LED 3mm LED's are fine. theese are about £0.10 each so.............
we want 1 to show the board is powered lets make it green and just to be clever we can make it flash so we know the chip is working

making it flash dosnt cost as its just a matter of programing the chip to tell it to flash.
we want 1 to show when the heat is sposed to be on (so if the bulb isnt lit and the led is we know the bulb is toast) we will make this one red
lets also add 4 more so we know what temp we have set the jumpers at, and also lets add a bicolour led so we know the temp is ok, i would sugest a red/green bicolour led that is green when the temp is in the set range and constant red when the temp is below but ok (i.e its heating) and flashing red when its too low (like say a blown bulb) a bicolour led is about £1.30
so for LED's allow £1.90
we also need 7 resistors for our LED's and several resistors generaly so call it £0.40 for them
we also need a few bits like wire and solder costs so add another £1 to be on the safe side

so far thats £17.74 (approx $26.88) i havnt added the cost of the case as this would be a matter for people to decide and vote on but would probaly start at around £5 rising to whatever you wanted,
so thats £22.74 ($34.45) for a digital thermostat without alot of bells and whistles, obviously you could add the capability to drive a pc fan according to temp (this is why i chose the chip i did

) with the chip we chose you could also automaticaly have the fan speed altered according to tempreture etc by using pwm, the cost of adding the fan function would be small if you suplied your own fan (say from a pc) all you would need is a connector and small signal diode so again add approx £0.60
you could add a LCD to the set up but prices then start to climb a little as you would need to add about £8 for the lcd, if you wanted to go the lcd route you might as well add some switches so you could set the temp to any value you wanted.
so i guess you could make a realy good thermostat for under $50

 

[crossedwires]

i like the look of the herpstat that redcatcher posted above. for that kind of money you could build a realy good thermostat with several sensors, LCD readouts and probaly add in humidity control, its at the high end of the spectrum like the herpstat where you make the real savings by building yourself, i have had a quick look at there products and once you start getting to the realy fancy ones you can save huge amounts self building them, its harder to build cheaper at the lower end