Help designing a Freezer Door Open Alarm

[chipstix24]

Hi there to you all,

I am a novice, and could really do with some help with (what I am sure is) a very simple circuit. My apologies if a similar circuit has already been posted I have searched, but the closest I got is:

https://www.electro-tech-online.com/threads/digital-readout-for-load-cell.7101/

with the image fridge.gif

Problem: my freezer door that keeps not quite closing (especially when I was the last person to get something from the freezer!!) = soggy frozen food, and an enraged girlfriend!

Alternatives: Accept that I could go for a mechanical solution such as a sprung hinge, but would rather a buzzer alarm solution. Would also prefer not to use a LDR or similar as there is no light in the freezer, and, in fact I'm not sure any circuit I could build would work at freezer temperatures (<30degC?).

Requirement spec:
- Simple Freezer door ajar/open alarm circuit
- Operated from a 9V battery (flexible on this but aiming to minimise overall size)
- Would quite like to use magnet (on door) and relay/circuit mounted on fridge main body.
- If door is not closed completely, a buzzer will start sounding after a certain time has elapsed e.g. 45 seconds (perhaps adjustable by a potentiometer/variable resistor?).

Bit of a novice, but happy to find components and build once I have a circuit and component list to work to!

Any help would be tremendous!

Best Regards,

Chris, UK.

 

[stevez]

Chris - search on 555 timer IC and you'll find something simple. You might also search for Vellman and other simple kits. Several of the kit manufacturers make/sell low cost timer kits that might work well here. Advantage of a kit is that you don't struggle obtaining parts, circuit board etc - might take some of the fun out but there is still much to be learned with the kit.

 

[Pyroandrew]

You could probably use a thermistor in series with a variable resistor, a pot and a comparitor. If the temperature falls below a certain level, i.e. if the door is ajar for more than 45 seconds warmer air starts to circulate, it will sound a buzzer or something.

quite a novel idea, but it works, highly adjustable. You would only have to adjust a pot until the alarm went off while in the freezer, when twist a little bit, and test for real. hope this helps!

 

[Nigel Goodwin]

quite a novel idea

Not very novel!.

The UK magazine did it as a project a couple of years ago, using a PIC12C508 - extremely long battery life, and very few components required.

 

[Pyroandrew]

Novel means not commonly thought of for the intennded application. Usually thermistors are used in freezer warnings and microswitches for door actions.

I don't think the guy wants to spend a hundred quid on this idea, using a PIC is a bit of a silly thing to suggest unless he already has a PIC programmer. This idea uses 4 componants, 6 if you include the battery connector and the buzzer. It could be made to use a fraction of the power that a PIC uses. Also, last time I checked, you could only run a circuit with a 508 in it for about 15 days off a PP3, a month If you think a lot about the design. A typical device of this application wants to last at least 2-3 months. A better way still, of doing the thermistor thing, could be to use a mosfet and one pot and a thermistor, 5 componants in all and it would last for years, no joke, YEARS, unless you left the freezer door open all day.

 

[chipstix24]

keep 'em coming!

Hi Guys,

<<< By the way, forgot to mention the relay I propose would be at room temperature and so would not freeze, as all of the equipment would be outside of the cold space (mounted on the outside of the freezer body) >>>

Thanks for your ideas. Temperature would be one thing to sense, but how would the battery/circuit cope with the Freezer temperatures (-15C or whatever the temp is?).

Also, if the door is only just open, it could take some time for the temperature to change enough to trigger the sensor by which time I may be out/ asleep etc and also with the door only just ajar, the buzzer sound may have trouble penetrating.

I'm much keener on a circuit that detects whether the door is open or closed (with a magnet attached to the door - can be done easily with no drilling or damage to the freezer).

Consider a simple circuit where you open the door and a buzzer sounds, thats all I need except for the buzzer to not go off until, say, 45 seconds has elapsed (except of course if the door is reclosed properly!)

Happy to go down the 555 route, but will still need guidance on what to buy and what to connect to what!

Thanks,

Chris

 

[Nigel Goodwin]

Novel means not commonly thought of for the intennded application. Usually thermistors are used in freezer warnings and microswitches for door actions.

I'm quite aware of the meaning of 'novel', which was why I was pointing out that it wasn't - the idea being used in an article published in EPE April 2002 (and I doubt it was very novel, even then!).

I don't think the guy wants to spend a hundred quid on this idea, using a PIC is a bit of a silly thing to suggest unless he already has a PIC programmer. This idea uses 4 componants, 6 if you include the battery connector and the buzzer. It could be made to use a fraction of the power that a PIC uses. Also, last time I checked, you could only run a circuit with a 508 in it for about 15 days off a PP3, a month If you think a lot about the design. A typical device of this application wants to last at least 2-3 months. A better way still, of doing the thermistor thing, could be to use a mosfet and one pot and a thermistor, 5 componants in all and it would last for years, no joke, YEARS, unless you left the freezer door open all day.

Here's a clip of the text file off the EPE website:

PIC Freezer Alarm by Humphrey Berridge
======================================
EPE Magazine May 2002

A simple design to monitor the temperature of freezers, giving
an audio warning  of low temperature caused by e.g. leaving
the door open.

based on the PIC 12C508, extremely easy to build, using
only five components. Battery powered.

As you see, it only uses five components.

It also has a long battery life, the last time you checked you obviously didn't check very well :lol:

Have you never heard of 'sleep mode'?, this reduces power consumption to very low levels. It also has a rather clever method of monitoring the temperature trigger level, which makes it independent of battery voltage changes. It doesn't compare the thermistor to an absolute level, but to a preset resistor - both are measured using exactly the same capacitor charging technique (and the same capacitor), so as battery voltage falls it doesn't affect the validity of the comparison.

Also, PIC programmers are fairly cheap, nothing like a hundred pounds! (unless you try REALLY hard) - you can also buy the PIC ready programmed, or ask someone to program one for you!.

As for chipstix24's concern over the temperature for the electronics, the EPE project overcame this VERY simply - and could be done however you happened to construct it. Simply have the electronics OUTSIDE the freezer, with a pair of thin wires passed through the seal to the thermistor INSIDE the freezer.

This also has the added bonus of the beeper being OUTSIDE the freezer, if it was inside it wouldn't make much of a warning noise!.

If anyone is interested in the EPE PIC version, I have some improved software I wrote for it (which they published on the letters page) - a simple improvement really, using one of the spare pins I connected the piezo speaker across two I/O pins - and fed it in bridged mode. This doubles the voltage swing available, and makes it much more audible.

 

[MrMikey83]

My suggestion is that you mount your circuit on a weak magnet to hold it in place and stick it to the top of the fridge at the corner opposite the hinge. Use a reed switch on this circuit and place a magnet on top of the door opposite the hinge as well. Use sticky tape on the circuit side if the weak magnet used to attach it to the fridge affect the reed switch too much.
Make the magnet adjustable so you can move it around and adjust the sensativity.

This way, its out of the way and is not very visible unless you build the circuit tall or you are 6-7 feet tall.
~Mike

 

[jrz126]

I have the same problem on an old fridge in the basement. I've got a bungie cord on each door. I also put a piece of rope on them near the front so that you can let go of the bunge cord and it wont go back behind the fridge.

 

[Halogrunt1234]

OH COME ON!!!
think with a simple, more lazy mind.
you dont need to mess with detecting temperature or magnetic fields,
all you need to do is get two metal contacts, place one on the fridge, one on the door, have a timer circuit connected with it so when the circuit is broken at the door, a 45 second timer is engaged (via resistor-capacitor part of a 555 timer circuit) and when the 45 seconds are up, a buzzer is triggered until current passes beck through the contacts.

ths method can be used with any door, as it is not specialized to only a freezer (which is good, by the way).

I have no schematic, but a basic understanding of the 555 timer and transistor applications will allow you to build this simple circuit WITHOUT any kind of PIC crap or programming.

JUST THINK ON A MORE SIMPLE LEVEL

 

[Nigel Goodwin]

JUST THINK ON A MORE SIMPLE LEVEL

But it needs modifications making to the freezer, adding the contacts - a temperature sensing device needs no changes at all - and is completely portable.

If he was wanting a simple 'non electronic' solution he probably wouldn't have asked here?.

 

[chipstix24]

Update

Thanks to you all for your input.

Its true, I had wanted specifically a simple electronic solution. To use 555s, I would have needed 3, 1 for timing, 1 to drive piezo buzzer, and 1 to pulse alarm tone. As it happens for a low cost I have managed to locate a company that can custom design and build a circuit (using PIC microcontrollers - which whilst complicated for me are the most cost effective way for a professional to achieve the design and also, as discussed in a previous thread, they consume minimal power in sleep mode thus prolonging the battery life).

I let you know how I get it on when the circuit has been completed.

Cheers,

Chris

 

[Oznog]

As for chipstix24's concern over the temperature for the electronics, the EPE project overcame this VERY simply - and could be done however you happened to construct it. Simply have the electronics OUTSIDE the freezer, with a pair of thin wires passed through the seal to the thermistor INSIDE the freezer.

Putting wires past the fridge seal is always going to be the questionable part. It may cause slight air leakage and distort the seal over time.

How about putting the thermistor on the outside and detecting the cold air leaking out?

 

[Nigel Goodwin]

Putting wires past the fridge seal is always going to be the questionable part. It may cause slight air leakage and distort the seal over time.

Two thin enameled copper wires wouldn't upset the seal at all, it's not a problem!.

Another advantage of the thermistor approach is that it will also let you know if the freezer is losing temperature for any other reason as well, not just the door open!.

 

[eblc1388]

Two thin enameled copper wires wouldn't upset the seal at all, it's not a problem!.

There are magnetic strips inside the door seal so that by magnetic force, it will press against the seal and form a tight and complete seal around the door edge when the door is closed.

Using wires across the seal is a EXTREMELY BAD idea. You are worse than having no alarm in the first place.

 

[Nigel Goodwin]

Two thin enameled copper wires wouldn't upset the seal at all, it's not a problem!.

There are magnetic strips inside the door seal so that by magnetic force, it will press against the seal and form a tight and complete seal around the door edge when the door is closed.

Using wires across the seal is a EXTREMELY BAD idea. You are worse than having no alarm in the first place.

The magnetic strips are rubber, and seal perfectly around a pair of thin wires - not a problem at all!.

 

[Phasor]

Two thin enameled copper wires wouldn't upset the seal at all, it's not a problem!.

Using wires across the seal is a EXTREMELY BAD idea. You are worse than having no alarm in the first place.

Umm... we are talking about wires of 0.1-0.2mm here - I'm with Nigel on this one - I wouldn't imagine any significant disruption with wires so small.

 

[David Bridgen]

The thread you pointed to had a simple circuit using a reed switch/relay, a magnet and a buzzer.
All you need to add to that is a delay, which can be done with an R and a C, but is better if you use a transistor as well.

One of these circuits will do the job. Choose one according to whether your reed switch is normally open or normally closed.
Without the zener the transistor would come on once the voltage across the C reaches about half a volt. Use a zener of around half, or a bit more than half, the supply voltage. Choose the R and C to taste.

**broken link removed**

 

[eblc1388]

If the time delay required is over a minute, C value and its size becomes large.

For 6V or higher power supply, maybe one can also consider using a MOSFET to drive the relay or buzzer, because:

R can be much higher in the Mohm ranges and C value can be smaller. The zener diode is not needed too. If the second configuration is use, where the reed switch closes when door closes, the battery will last longer if R is large.

 

[David Bridgen]

Yes, a f.e.t. would be a better choice.