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When I step on a bathroom scale how does the circuit turn on? I've disassembled these types of weight scales looking for obvious hardware switches, but none exist. I suspect the ADC is configured to generate an interrupt and wake the micro controller from a low power mode. If this is the case, how would this spec be translated to a common micro controller such as a MSP430F67 with its built-in 24-bit ADCs?

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2  
"built-in 24-bit ADCs" I wouldn't trust that as far as I could throw it. – KyranF 12 hours ago
1  
Sleep. Wake every x ms, one time adc. If value > n, wake up everything else go to sleep. – Passerby 12 hours ago
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@KyranF Throw distance depends on ballistic coefficient and at such low mass, to get decent rate you'd really want added mass and a long this package with good streamlining. So may be ideal for use in a dart or aerial penetrator. | | The ADCs in the MSP43F067.. are Sigma Delta converters and in ideal conditions could approach that resolution, if not accuracy (as 24 bit reference & stability is harder to come by). As, for a 100kg person, 24 bit resolution = 0.006 gram, the full 24 bits "is unlikely to be needed [tm]. A resolution ... – Russell McMahon 5 hours ago
    
... of say 10g in 100 kg = 1:10,000 ~ 2^14 or 14 bits is probably ample, especially given the linearity and absolute resolution of the overall strain gauge + IA system. Whether you can throw 16 BIT ADC system any further than a 24 bit one is moot, but with a suitable reference it's probably as trustable as it needs to be. To get 0.1 kg FS accuracy at 100 kg FS = 0.1% FS accuracy overall which is 'reasonably challenging' of itself, and suggest the actual reference is several bits better than that, which is unlikely in practice. No ? :-) – Russell McMahon 5 hours ago
    
rate -> range | a long this -> a long slim | – Russell McMahon 5 hours ago
up vote 10 down vote accepted

The scale is never really "off". Instead, it just turns off the display and other peripheral circuits such as the ADC and goes into a low-power mode internally. Every now and then (most likely based on a timer interrupt), it powers up the ADC briefly and checks the sensor to see if the weight has changed, and if so, it turns the rest of the peripherals, including the display, back on.

In low-power "sleep" modes, many microprocessors (and especially the MSP430 series) have a level of current consumption that compares favorably with the self-discharge rate of the battery.

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Re: self-discharge. When you write it compares favorably, does that mean if you use a little bit of current it negates the effect of battery self-discharge? Or, is using the current additive with self-discharge? – Crizzle 12 hours ago
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It's additive, but about the same order of magnitude. – Dave Tweed♦ 12 hours ago
    
Some microprocessors also have comparators that can generate interrupts, but otherwise the same principle applies: the scale is not ever turned off completely. – Simon Richter 5 hours ago

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