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Testing the Display

A project log for Salvaged bathroom scales rewiring

Re-implementing the electronics in a digital bathroom scale, for. low power consumption and cost.

fractalFractal 07/29/2016 at 23:470 Comments

The display arrived today - at £3.80 it wasn't too bad.

Now I have it, it's time for the testing. For now, I've just used the ebayer's provided library, and a simple routine for measuring the AVR's internal voltage at 10Hz. No optimisations have been done on the ATtiny85, asides from making it run at 1 MHz.

A quick test was done to check all the segments and features worked, at the rated 5V. Then it was moved to solderless breadboard and attached to meters.

The next step was of course to check out the display over the operational voltage range - using a power supply. The current consumption of the display alone was also measured at a few points.

Overall, not bad: the voltage measured matched the input fairly well over the 5v-2v range

Which matches up plenty well enough to use for threshold testing of a battery! Once it's on a final style board (so minimal resistive losses) I can retest this relationship to end up with a better calibration. For now, simply changing the Vref value in code to 1100*1.0279=1130 should provide a more accurate system. The 0.1 v offset is likely be somewhat due to resistive losses in all the wiring from the power supply.

Top left: The display, showing the uncorrected voltage on the ATtiny85, as measured by the AVR. Top Right: Proposed lithium ion batteries (4.06v). Bottom right: Breadboard with ATtiny85 and wires. Bottom left: Multimeter in current mode, reading 1.70 mA total draw.

Typical LCD current draw was 270 µA at 4.0V, dropping to 136 µA at 3.0V. The datasheet value for the controller at 3.0V is 150 µA, so this is great!

Additionally, the LCD contrast varied quite a lot over the range , and in fact was best between 4 and 3 volts. This may need to be a topic of future change - there is a 20k resistor to VLCD on the LCD board, and the controller can drive at 1/3 and 1/2 bias - which may change the characteristics.

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