Pyroandrew said:
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:
Code:
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.