LED shines when off.

[Russell]

Hi

I have today completed the installation on my mill of my touch DRO.  I'm very pleased with it, thank you for making it available.

However I have a couple of questions.  I have built it with battery backup as described elsewhere on this site but I noticed when it is turned off and relying on the battery backup there is an LED on the MSP430 glowing green.  This prompted frenzied checking of connections and I established that the battery backup was fine and the diodes were the right way around.  However when I disconnected the data lines the LED went off.  It is not the green LED - I thought it was D2 - but having looked at the board diagram it must be the multicolour LED (because D2 is red).

It is using Shahe scales with V2 firmware and a V2 MSP430.

Has anyone else noticed this?  Does it matter - it seems as though it must shorten battery life.

My other observation is that the display seems to "flash" and change position - too fast to really see what's going on but it might be flashing up an absolute value for a fraction of a second.  Has anyone else noticed that?

Thanks.

Russell

[Yuriy]

Russel,
What you are seeing is not a build issue. Shahe scales, unlike iGaging, provide their own clock, so when they are powered by a battery, they are basically sending the position to the MSP430 microcontroller. Since the microcontroller uses very little current, the current that it gets via the clock lines is enough to power it up. This is not an issue with iGaging scales, because the MCU won't be sending clock signal when powered by the battery.
If you want to power the board from a battery long-term, you can replace the series resistor for the heartbeat LED with a larger value, or just remove it altogether. As it's set up right now, the LED might consume around 10-13 mA when powered via the on-board regulator; I doubt the scales can source that much current, so it's probably significantly less.

Regarding the other issue, it's likely caused by line glitches (i.e. the firmware gets a random pulse and mistakes it as data. I'm working on adding a digital filter that will "ignore" outliner data, but that is still in "work in progress".

Thank you
Yuriy

[Russell]

Thanks Yuriy.

Your comments prompted me to do some research.

The current is very low - somewhere around 1.2mA and by removing the jumper that controls the green element of the multi colour LED it can be reduced to 660uA. Although that doesn't sound much an AA battery apparently has a capacity of about 3000mAh giving a life of about 3 months.

I would like to do better that and I measured the current with the data lines disconnected and it reduced to 36uA which would be much better. I was wondering about automatically disconnecting the data lines and I was thinking of using a 74HC08 to do it.

What do you think.

Thanks.

Russell

[Yuriy]

Russell,
These scales need to have inputs configured in a certain way (I don't recall which one needs to be pulled up and which needs to be pulled down) during startup, or the scales won't send the signal.
You could try putting series resistors of relatively large value to limit the current, but you might be making more headaches for yourself.
Regards
Yuriy

[Russell]

Hi
I thought I'd report on what I achieved.

My initial idea was to use the 74hc08 (an and gate) as a relay. It worked OK but it didn't reduce the current when the display was off.

I then tried using a small 4 pole pcb relay so the data lines are only connected when the MSP430 is powered up. That reduced the battery backup current to about 36uA. At that rate the battery should last long enough for the shelf life of the battery to be an issue, which is good enough for me.

Russell