1. Home
  2. Electronics Design
  3. Temperature Relay Circuit Required

Temperature Relay Circuit Required

=_NextPart_000_0000_01C4D4AE.4D9A1C80 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit

Hi I have a fish tank that if it gets around 28 degrees Celsius I need to turn on some fans. Can anyone help me with a simple circuit that will.

  1. Maybe use a thermistor to measure the temp of water. I will obviously need to water tight the test probe. That's easy!

  1. Have a variable resistor so I can calibrate. I already have a digital thermometer that measures the tanks temp.

  2. Have the circuit switch a 12v relay when it reaches > 28 degrees (28 or more). I will need to calibrate via VR.

  1. Turn off when temp gets below 28.

Something simple with maybe an over ride "On" switch?

Thanks Heaps. I am familiar with electronics, just not design.

Reply to
Daren Hawes
Loading thread data ...

Here's a simple circuit to do that (in two formats for flexibility):

formatting link
formatting link 

--
Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

Thanks, I will let you know how I go.

Daren

-----Original Message----- From: Terry Pinnell [mailto: snipped-for-privacy@THESEdial.pipex.com] Posted At: Saturday, 27 November 2004 7:43 PM Posted To: sci.electronics.design Conversation: Temperature Relay Circuit Required Subject: Re: Temperature Relay Circuit Required

"Daren Hawes" wrote:

Here's a simple circuit to do that (in two formats for flexibility):

formatting link
formatting link

-- Terry Pinnell Hobbyist, West Sussex, UK

Reply to
Daren Hawes

Cutthroat business.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Not for $75. Maybe for $150 or $199 unit price (much, much less for OEMs and less again for distributors). Cutthroat business.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

"Daren Hawes" wrote

Never re-invent the wheel, there are so many more interesting (and profitable) things to do.

A little Google goes a long way:

formatting link

I am sure there are lots more to choose from. Try asking in the *.aquaria (ium?) groups.

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer:  Electronics; Informatics; Photonics.
Remove spaces etc. to reply: n o lindan at net com dot com
psst.. want to buy an f-stop timer? nolindan.com/da/fstop/
Reply to
Nicholas O. Lindan

[snip]

Hysteresis would speed up the on/off transitions and prevent relay chatter near the threshold point. Consider adding a large resistor (1M) from the op-amp output to the +ve input thereby providing a small amount of positive feedback.

Reply to
Andrew Holme

Now that product packs a lot of punch for the price.

Reply to
Fred Bloggs

They do need to get with it and include an option for either network connection or web appliance.

Reply to
Fred Bloggs

That takes all the fun out of it!

I used to work for an amusement company and it was in their best interest not to repair circuit boards and instead just swap them. So I was an electronics tech turned into a board swapper! That's when I left the industry...

I like a challenge and if it costs me $20 then that's better than $75 + Postage (To Australia!)

Thanks anyway maybe when my probe needs an upgrade I will be over the "Challenge" part and just buy the "Real Thing"

Thx Again.

-----Original Message----- From: Nicholas O. Lindan [mailto: snipped-for-privacy@sig.com] Posted At: Saturday, 27 November 2004 10:29 PM Posted To: sci.electronics.design Conversation: Temperature Relay Circuit Required Subject: Re: Temperature Relay Circuit Required

"Daren Hawes" wrote

Never re-invent the wheel, there are so many more interesting (and profitable) things to do.

A little Google goes a long way:

formatting link

I am sure there are lots more to choose from. Try asking in the *.aquaria (ium?) groups.

-- Nicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. Remove spaces etc. to reply: n o lindan at net com dot com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/

Reply to
Daren Hawes

Darren: Sorry, I briefly saw your email but it was then accidentally zapped with the spam before I could reply! And I'm not sure if I have your valid email address, so I'll post here too. Please either resend, or just post it here in sed. That way you stand to get wider help.

From what I did see, you were asking whether a 1M feedback resistor was necessary. Try it, maybe on a breadboard, and see which you prefer. From your spec, IMO hysteresis wasn't required. IOW, I thought you were not asking for the relay to close as the aquarium temperature rose through 28 deg only to open again if the temperature dropped to say 27 or 26. So I reckon you'll be happier with that simple, sensitive circuit I suggested. I've used it myself and it works well.

If there *is* some 'chatter' (oscillation of the relay contacts), first just try a 10 nF cap from the NPN base to ground.

There will be inherent 'inertia' in the relatively large volume of water in an aquarium, so the temperature is likely to be smoothly continuous.

If you decide you *do* want some hysteresis, follow Andrew's suggestion. The larger the feedback resistor, the smaller the hysteresis. So 10M or 4.7M would give you very little, while adding some useful 'snap action' to the relay.

Actually, for a circuit with hysteresis, I might drop the op-amp approach and instead use a circuit based on 1/2 a CMOS Quad NOR 4001 (or NAND 4011), like this:

formatting link

The downside to any circuit with hysteresis is that it takes a fair bit of trial and error to get set up the way you want. It's all too easy to get more hysteresis ('slack') than you want. Too much, and your guppies or goldfish could see hypothermia before your relay sees its contacts open again!

--
Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

Now I am a little confused.

The circuit simply needs to...

  1. turn on fans when the temp goes over 28 and
  2. turn off when the temp drops below 28 again.

So whenever the tank is hot fans are on and whenever it is not fans are off.

Is this what the first circuit will do?

Thx Daren

-----Original Message----- From: Terry Pinnell [mailto: snipped-for-privacy@THESEdial.pipex.com] Posted At: Sunday, 28 November 2004 10:35 AM Posted To: sci.electronics.design Conversation: Temperature Relay Circuit Required Subject: Re: Temperature Relay Circuit Required

Darren: Sorry, I briefly saw your email but it was then accidentally zapped with the spam before I could reply! And I'm not sure if I have your valid email address, so I'll post here too. Please either resend, or just post it here in sed. That way you stand to get wider help.

From what I did see, you were asking whether a 1M feedback resistor was necessary. Try it, maybe on a breadboard, and see which you prefer. From your spec, IMO hysteresis wasn't required. IOW, I thought you were not asking for the relay to close as the aquarium temperature rose through

28 deg only to open again if the temperature dropped to say 27 or 26. So I reckon you'll be happier with that simple, sensitive circuit I suggested. I've used it myself and it works well.

If there *is* some 'chatter' (oscillation of the relay contacts), first just try a 10 nF cap from the NPN base to ground.

There will be inherent 'inertia' in the relatively large volume of water in an aquarium, so the temperature is likely to be smoothly continuous.

If you decide you *do* want some hysteresis, follow Andrew's suggestion. The larger the feedback resistor, the smaller the hysteresis. So 10M or

4.7M would give you very little, while adding some useful 'snap action' to the relay.

Actually, for a circuit with hysteresis, I might drop the op-amp approach and instead use a circuit based on 1/2 a CMOS Quad NOR 4001 (or NAND 4011), like this:

formatting link

The downside to any circuit with hysteresis is that it takes a fair bit of trial and error to get set up the way you want. It's all too easy to get more hysteresis ('slack') than you want. Too much, and your guppies or goldfish could see hypothermia before your relay sees its contacts open again!

-- Terry Pinnell Hobbyist, West Sussex, UK

Reply to
Daren Hawes

It depends on your thermistor. If it's the most common type, an 'NTC; thermistor (negative temperature coefficient), both circuits switch the fan *off* as the temp rises. But it's trivial to reverse it, in any of four ways:

  1. Swap the positions of the thermistor and the preset.
  2. Swap the connections to pins 2 and 3
  3. Change the relay contacts from Normally Open to Normally Closed
  4. Use a PNP instead of NPN transistor, (with R6 to 12V of course).

The way it works as presently drawn is straightforward:

  1. Assuming it's adjusted so that bridge is close to balance at 28 deg, as temp drops below that the thermistor's resistance increases.
  2. So voltage on pin 3 rises above that on pin 2
  3. So op-amp output swings quickly positive
  4. Therefore NPN goes on and relay is activated
  5. Its normally open contacts therefore close, powering fan.

In practice, it's quite easy to set up.

The CMOS circuit works in a very similar manner, but with a small amount of slack as explained earlier.

--
Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

Thermostats do not operate that way because you have no control over the cycling of the fan- they will go ON/OFF-ON/OFF-ON/OFF- etc in some indeterminate way. If that's what you want, it would be better to use speed control of the fans so as to maintain the temperature at constant

28oC. Are your fans 12VDC "brushless"?
Reply to
Fred Bloggs

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.