Electronic Refrigerator Thermostats

[BT Humble]

I've been wanting to try out the "chest-freezer-as-a-refrigerator concept for a while now, and as you might have read here I've made the first steps towards it, and made sure that the old clunker freezer that I've got actually works.

Now that I've constructed a watt-hour meter, I'm ready to go with an actual thermostatically-controlled version.  My combination of tightwadedness and wanting to build something myself led me to this prototype:

It's essentially a 555 timer driving a relay, with the trigger voltage set by a 1K thermistor and a 5K pot in a voltage divider network.  Using the 555 timer with an appropriately sized capacitor and resistor allows you to set the minimum length of time that the fridge should switch on for (or hysteresis, for the technical).  I also happened to have one in my junk box, along with the various other parts.  The relay came from a discarded CRT computer monitor, and dictated the use of 12V for the circuit (it won't trigger on any lower voltage).

Here's the draft PCB layout:

I'll try this out in the next few days, and let you know how/if it works.  Needless to say, it'll be placed in a well-insulated box before it gets connected to the mains.

BTH

[dinges]

Hi,

FYI, the schematic would be much more useful than the PCB layout. It seems like a straightforward design, from the quick glance I took.

You may be interested to know the the CMOS 7555 draws only .1mA of current, while the ordinary one (in my case) draws 13mA. This may be an issue for a RE application, esp. if the thermostat-circuit is working 24/7, as is probably the case.

I suppose you know that the 555 can source up to 200mA? This means you can drive pretty big relays (though myself, I'd add an extra MOSFET stage)

I like that box of aluminium heatsinks, guess I'm not the only one who's saving them

Looking forward on hearing your experiences on converting the freezer to fridge, I'm playing around with the idea myself.

Peter,

The Netherlands.

[BT Humble]

FYI, the schematic would be much more useful than the PCB layout. It seems like a straightforward design, from the quick glance I took.



You can see the schematic on the piece of paper to the lower right... ;-)

You may be interested to know the the CMOS 7555 draws only .1mA of current, while the ordinary one (in my case) draws 13mA. This may be an issue for a RE application, esp. if the thermostat-circuit is working 24/7, as is probably the case.



Yup, and version #2 will be using the CMOS chip and a 10K thermistor to save power (this setup draws 25mA in idle mode, and 50mA with the relay switched on).  I'm using a TTL one in the prototype because I had it on hand.

I suppose you know that the 555 can source up to 200mA? This means you can drive pretty big relays (though myself, I'd add an extra MOSFET stage)



That was the other reason for using the TTL version in the prototype - the CMOS version will only source about 25mA or so, if I recall correctly.

I like that box of aluminium heatsinks, guess I'm not the only one who's saving them



Heh!  You should see the BIG box then! ;-)

Looking forward on hearing your experiences on converting the freezer to fridge, I'm playing around with the idea myself.



No wuckers, I'll keep you informed.

By the way, there was an error in that PCB overlay:  pins 5 and 6 on the 555 shouldn't have been bridged.  

BTH

[newguy]

The Netherlands! Holy cow, I'm going to that part of the world in May. What's it like? I'm going to be spending two years there. Any advice for a dumb American? Thanks.

Adam Egan

[GaryGary]

Hi BTH,

You may have already seen this, but just in case:

http://www.builditsolar.com/Projects/Conservation/chest_fridge2.pdf

It has a circuit for a thermostat to convert a chest freezer to a frig.

If you read this a while back, it has been updated -- the commercial thermostat that he was using failed (spectacularly), and he designed and built a new one.  The document includes schematics etc.  

Gary

(hi Peter)

[BT Humble]

It has a circuit for a thermostat to convert a chest freezer to a frig.

If you read this a while back, it has been updated -- the commercial thermostat that he was using failed (spectacularly), and he designed and built a new one.  The document includes schematics etc.

It's a funny thing - he's done exactly what I planned to do with version #2 (power the thermostat with a 9V rechargeable battery, which gets trickle-charged while the fridge is running).

Great minds think alike, eh? ;-)

BTH

[dinges]

You mean the Netherlands, capital of Belgium ?

Just tell everyone that you're a Canadian and you'll be fine :-p

Hmm, this was meant originally to be a joke, it is to me, but I'm afraid there are some people who don't see it that way. Not wanting to scare you off, but by warning you, you may just feel relieved when you visit and will later accuse me of 'over-warning'; I'd rather do this than let you have the opposite experience, of being surprised of some reactions that you hadn't expected.

Feel free to mail me personally if you like.

Peter,

The Netherlands.

[ghurd]

I like the 9V rechargable idea!  Seems obvious now someone said it.

Except I don't like rechargable 9V batteries.  Most are 7.2 or 8.4V and have a very small capacity, but that should be OK.  I don't like that when one cell fails the battery is shot.  I don't like the cost per watt hour.

Could I suggest a nimh AA/LR6 or AAA/LR03 battery pack?  My experience with the rechargable 9V/6LR61 batteries has been they are less than reliable.

G-

[BT Humble]

Could I suggest a nimh AA/LR6 or AAA/LR03 battery pack?  My experience with the rechargable 9V/6LR61 batteries has been they are less than reliable.

I was thinking of using a couple of those heatshrinked-together packs of AAA batteries such as they use in cordless phones (whichis probably what you're talking about above).  Oatley Electronics has them at 10 for $20, including a charger:

http://www.oatleyelectronics.com/battery.html

I'm going to test the 555 concept using a simple jumpered-to-a-car-battery for a few weeks first though. ;-)

BTH