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?
Electronic – How does a bathroom scale automatically power on when I step on it
load celllow-power
Related Solutions
Don't forget about VTT termination power. Also they have some excel power calculators: micron.com/products/support/power-calc A quick look at their calc showed standby power was around 100mW. The calc seems to have really good info.
Re: VTT Term I believe you're correct when you're not doing anything the power consumed is probably negligible. In DDR you need to provide VTT termination for the signals, basically a resistor tied to 1/2 your VDD voltage. Again it probably doesn't mean anything for power.
Don't forget that you have to refresh the DDR periodically, setting it into self refresh mode should do that for you. That should be your lowest power mode, and you don't have to provide a clock to it at that point.
Oh and if you can, consider LPDDR, the lower power stuff is around 1.35V now I think. I just read an article the other day that low power dram orders have now surpassed regular DDR so it might even be cheaper for you.
OK, problem solved. The bridge is connected like this. Only one resistance in the load cells is variable, the other is fixed.
Why the confusion above? I was measuring resistance of a load cell which came from a different scale. The cells looked pretty similar, therefore I thought they were the same. But they were not! Eureka!
simulate this circuit – Schematic created using CircuitLab
Best Answer
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.