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Simple/ and or cheap variable thermostat? Answered

Well I have a friend who is looking into the purchase of a snake, and meanwhile picking out the snake and the heating element isn't the hard part for him (he's looking at a 23 watt 120vac heating pad for it). He was also looking at a thermostat designed for snake tanks, however they want close to $150 for it, and it doesn't even have many features, it's basically a black box with an 3 digit lcd and a programable timer. I know it wouldn't be hard to put something similar together from a pic and a few different parts, however it's been a long few years since I've designed a full schematic of something, let alone look at one. So I was wondering if someone so kind might aid me in making of a schematic so my friend can go order the parts he needs and I can build it for him (don't have a problem assembying circuits from raw schematics, I find it fun). However, there is a couple things he wants out of it. Number one most importantly, he wants it to be able to adjust the output voltage so he doesn't fry his snake (most simple thermostats simply turn on until temperature is reached, then off), no he wants something more dynamic than that. Second, he needs to have an external temperature probe so that he can keep the thermostat outside of the tank whilst keeping a handy eye on the temp and also obviously so that the thermostat knows when to come on (is there a limit to the length of wire you can put on a thermocouple?). Third and last, we need a simple timer circuit that keeps track of the time (what's that involve? a 13mhz crystal and a few other parts?) so that at night the thermostat can change it's "heating schedual", I assume so that it tries to maintain a temperature say 3-4C above the programmed temp. (night mode anyone?) However the thermostat he was looking at buying wanted you to buy a second cord, which connected to an external appliance timer (yes you heard me, a $9 wall timer) so that it knows what time it was. My friend really needs this as he's looking into getting this snake within the month. I appreciate anything anyone has to offer. Thanks guys! -Punkguyta

Discussions

How about the thermostat out of a heating system? They can have timers, pretty good temp controls and lots of other gizmos...

This is what me and my friend were thinking, however a simple thermostat (digital or not) simply turns a signal wire to the furnace on or off, which I think runs at a lower voltage than 120V, I could be wrong. He wanted to be able to adjust the voltage also along with timer features, in order to not be just turning the heating pad on or off, but rather lower the temperature so that it's more acceptable to the snake.

Well you could find an acceptable temperature and either use PWM or resistors to calm the heating pad down...

ooo I have always wanted to hold a giant resistor in my hand (they do exist mate), but I think maybe something like an adjustable wall dimmer?

my toaster is a giant resistor some cheap back massagers have heating in them with controllers mine is for the car and runs off 12 vdc

Could do the job, many heating pads are resistance based anyway, though a thermostat wouldn't ever have the too hot pad problem, give the cage a day or two to find it's balance then the thermostat will be turning the pad on for very little time...

. You are correct - most t'stats use 12-24V. You would need to use an interposing relay to handle the power to the heating element. And a transformer to drive the t'stat.
. HVAC thermostat outputs are on/off. I think you're looking for a temp controller (eg, kj's PWM). Probably going to be much more expensive than a t'stat, but most of them use external sensors, so you can drop your RTD/tc into the tank and mount the controller where ever you like.
. If the temp in the room is stable, you could use a rheostat or variable transformer to adjust the voltage to the heater.

>You would need to use an interposing relay to handle the power to the heating element. And a transformer to drive the t'stat. . Really, are relays that hard to wire together? I think it's more a problem of finding relays appropriate for the job. The transformer end, pff that'd be a chinch, usually they're just battery operated (well depending on the relay circuit, could stay as battery operated?) but otherwise I suppose a 9V power wart would handle the task. (I've only seen 9v batteries in electronic thermostats) > HVAC thermostat outputs are on/off. I think you're looking for a temp controller (eg, kj's PWM). Probably going to be much more expensive than a t'stat. .Well you'll see in my other comment to you, we'll scrap the variable temp control idea. As far as rheostat or variable trans. to have a more fixed voltage control, VARIAC comes to mind, remember the days.. Regardless, I'm sure a 500W wall dimmer would do him justice as he was planning on a 23W heating pad. -Punk

The transformer end, pff that'd be a chinch, usually they're just battery operated (well depending on the relay circuit, could stay as battery operated?) but otherwise I suppose a 9V power wart would handle the task. (I've only seen 9v batteries in electronic thermostats)
How it works: The 24V transformer in the controller on the furnace supplies voltage to the switch in the thermostat, and that voltage then operates the relay(s) that turn on the circulation (pump for water, fan for air). That transformer also operates the relays that keep the furnace at a usable temp.

The thermostat itself is just a switch. If the thermostat uses a battery, it's because it's some sort of programmable one and needs it for the clock mechanism and similar, although more advanced ones could have a relay-like action, probably solid state with a MOSFET or similar that uses practically no current.

There are line voltage thermostats used for heat pumps, but you'd have to see what the amperage ratings are, maybe by themselves they can switch a 23W resistive load, 0.19A ain't much. But don't try it with a normal low voltage thermostat. The mechanical thermostats, like the old trusty round ones, often have a small heating coil for the anticipator, when the current is flowing it slightly heats the main bimetal coil so the circulation shuts off before the set temperature is actually reached, since things like heated radiators will continue to heat some more after the circulation stops. Pump 120VAC thru that normally 24V coil, and if you still have a thermostat it might not work right. Likewise, if the voltage is too low there may also be problems.

Regardless, I'm sure a 500W wall dimmer would do him justice as he was planning on a 23W heating pad.
Well both a heating element and an incandescent bulb are resistive loads, an old cheap rotary one should work. However, as NachoMahma was pointing out, a newer electronic one might have issues.

So simplest construction, if it works, is line from wall plug to line voltage thermostat, thermostat to dimmer, dimmer to socket (outlet), outlet back to wall plug. Then plug the heater into the assembled unit. You can test it with a table lamp with a small appliance bulb if you want.

. Yikes! I forgot all about the anticipator. Be kinda fun to see what happens with 120V. heehee . . I like your idea of combining a 'stat and dimmer. . I didn't know they made a home t'stat with 120V contacts - much more simple than an interposing relay. ;)

I didn't know they made a home t'stat with 120V contacts...
I think that's some sort of oddity as offhand I've only seen them listed for heat pumps, although you can get them at Wal-Mart. The odd part is they'd need a proper electrical installation with boxes and romex etc, and I would think that normally a heat pump would draw so much current they'd need a relay anyway. Although, come to think about it (with my HVAC knowledge limited to around hot water baseboard systems) maybe that's just for the circulator, would be just a squirrel cage fan for air, a separate system maintains the temp for circulation. Still, regular 24V thermostat line is some of the cheapest looking wire you'll see, needing nothing more than holes to run it thru and some staples to hold it in place. Going from installation cost, it'll be much cheaper to toss in a transformer and relay for the heat pump and use thermostat line. Safer too, for example that nail you just missed the stud with won't make a hot spark at that 24V, if it even hits that tiny wire.

> Really, are relays that hard to wire together?
. Nothing hard about it. Use the contacts (probably NOT powered internally) on the t'stat to energize the coil of the interposing relay. Use the contacts of the relay to drive your load.
                  t'stat       relay                contacts   coil wallwart ----||---------( )--- back to wallwart               relay             contacts        load mains ---||------------/\/\/\/\//---- back to mains
.
> a 500W wall dimmer would do him justice
. Actually, there's a slight chance that it won't work properly with such a light load. It may be very hard to adjust. A dimmer rated for 50-100W would probably work better. May not be a problem - try it and see.

. I think killerjackalope has a good idea. . Humidity may be a problem, but you can find cheap thermostats at garage sales and replace it every year or two. . IIRC, snakes don't require a real stable temp, so you probably want a thermostat that is designed more for reliability (don't want to have toasted snake) than accuracy.

Well like I said before, the thermostat wasn't to be placed INside the tank but rather OUTside the tank with a long enough wire that he could say have the controls on his computer desk and be able to keep and eye on the temp of his snake cage. Are you sure that snakes don't mind fluctuating temps? Is that not why they curl up on hot rocks in the sun, because they get cold easily?

. Most "home" thermostats have the temperature sensor mounted inside the unit. I don't think you will get an accurate reading through the glass of the tank.
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. My point was that you want a controller that will turn off reliably, so you don't cook your pet. I'm not a herpetologist, but I don't think a 3-4oF temp swing will bother snakes.

>Most "home" thermostats have the temperature sensor mounted inside the unit. I don't think you will get an accurate reading through the glass of the tank. . My bad, I see where I lost you.. The retail model my friend was looking at had a temperature probe with a 12ft. cable on it in order to have as least as possible inside the tank with the snake (what snake would want to hear the clanging of a mercury thermostat in their snake pit?). So the question is, can said temperature sensor in a digital thermostat be removed and placed on a cable? ...some how I have a feeling that one of your first words is going to be 'calibration', and I don't even know much about thermocouples. >My point was that you want a controller that will turn off reliably, so you don't cook your pet. I'm not a herpetologist, but I don't think a 3-4oF temp swing will bother snakes. . Okay okay, I didn't think it would either, he's not a herpetologist either but he's just rather careful with animals. I'll point it out to him that it probably won't be a problem. He'd just hate to spend $500 on a beautiful snake to find it crispy in the morning.

> So the question is, can said temperature sensor in a digital thermostat be removed and placed on a cable?
. If it uses an RTD sensor (probably does), you can add a few feet of wire without affecting the accuracy too much. You want to keep the resistance of the added wire/connections to a minimum - use fairly large wire and solder your connections.
. If it uses a t/c, you _may_ be able to get away with using Cu wire. You'll just have to experiment.
.
> 'calibration'
. Checking the calibration is pretty easy. Ice water is very close to 0oC and boiling water is about 100oC. Use a known good thermometer to compare at room temp. If the reading is close at room temp, then don't worry if it's off 2-5oC at the extremes.
.
. BTW, don't take my word for anything when it comes to $500 snakes! Or $500 anything else.

If you got a digital thermostat, I have one made by hunter, all I did was remove the temp sensor from the board and soldered some wires to it and sunk it a plastic tube with silicone (( old bic pen )) and I have a remote sensor and I use it to control the temp on plant roots when I doing cutting, with a low wattage mat. And most household digital thermostats are not that expensive nowadays either ((I think it was $30 )) and about the low voltage, Nacho is right most are 24 volt, I got around this by using one made for electric baseboard heaters (( it's good to 2000 Watts but it doesn't do the multiple time of day things but the plants don't seem to care about that )) Now I don't know about snakes but my plant roots are with in a deg C of the set temp when ever I check them...

Oh yeah and it's good to see ya still kicking around Nacho -Cheers