Excercise your brain cells with this problem

metron9

I have a small battery operated thermometer display that runs from 2 1.5 batteries. 110ma hours

I am using a tiny13 that uses only 0.1uA in power down mode

The only way to wake up the chip from that mode is a rising or falling edge on its int0 pin

If I just use idle mode it takes 300uA so I have to use power down mode

I could use a big resistance value and a small capacitor to hold the pin low until the cap charges to pin threshold so the chip would wake every 5 seconds or so, but that is not as dynamic as I would like it to be because when the temperature changes it can take up too five seconds to see the adjustment.

I would like to make a circuit that could trigger the interrupt on a change in the thermistor resistance, rising or falling edge that way the chip could sleep for many minutes or hours or days and only wake on a change in temperature.

The interrupt trigger circuit should only take perhaps 20uA that would allow 6 months of battery life.

Any ideas?

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Post Edited (metron9) : 11/29/2006 4:21:00 PM GMT

Bean

Usually you just use the watchdog timer to wakeup the processor at whatever internal you need. Check what needs checking, then go back to sleep. That will probably use less current than a circuit that generates an edge with a change in temperature.

Bean.

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metron9

I know Bean, that's one thing i plan to try I have not used the watchdog timer as of yet, I am using the timer overflow now in idle mode and that is just too much power waste.

I think what I am really want it to do after thinking about it a bit more

Is use a tiny motion sensor that would trigger the interrupt from power down mode.

Using a simple pendulum and a + and - contact I could use a pin change interrupt as the pendulum would bounce between the contacts when the device was moved. I would then monitor on a more frequent basis until the temperature stabilized, and put the chip to sleep with the pin change interrupt.

I am just trying as an exercise to find the longest battery life possible and still have a dynamic response within a second of moving the object. that would mean if the object is on a shelf it would take zero power until it was picked up. I don't care to display the temperature when it is not in use.

Yes the change in the thermistors resistance would be more difficult to do with low power than a movement sensor.

It needs to operate down to 1.8V as well.

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Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!

metron9

I found what I was looking for a tiny motion detector.

I can put the chip to sleep, and never wake up until someone moves the product.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Think Inside the box first and if that doesn't work..
Re-arrange what's inside the box then...
Think outside the BOX!