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
