Scales, again…

Enjoying a couple of days off at the moment so getting plenty of time to play with stuff. The scales have been progressing although I couldn’t get anywhere with the 3 transistor soft on/off switch. I tried the 555 timer based switch too and couldn’t get anywhere with that either. At this point, doubting my competence even more than usual, I got the multi-meter out. Voltages on the breadboard were all over the place. Tried a different breadboard and the 555 timer circuit worked perfectly!

At last!

At last!

The circuit is from Bowden’s Hobby Circuits and it works really well.

Once that was done, it was pretty clear that I didn’t want one side of the power switch pulled to ground. That just needed the ground plane scraping off the circuit board around the on/off switch:

Not pretty, but you can't see it anyway

Not pretty, but you can’t see it anyway

By this point, I was pretty happy that everything was going to work out. Time to fit the display to start figuring out what’ll go where:

It'll do...

It’ll do…

Inside view

Inside view

That’s the display and buttons sorted out. I had a play with de-bouncing the buttons – on a clock I made last Xmas I used software de-bouncing and wasn’t too happy with that. It’s a fairly standard hardware de-bounce, one side of the switch goes to ground, the other side has a 10K pull-up resistor and then connects via a 1K resistor to the +ve pin of a 10uF capacitor. That’s then sampled by an Arduino using interrupts.

To be honest, that’s still too “bouncy” so I made room for a hex Schmitt trigger and it all seems happy now. Here’s the half of a 1/4 size Adafruit perma-proto board I put it all on:

3 button de-bounce

3 button de-bounce

That was a bit of a squeeze, I obviously had to cut tracks to isolate each half of the Schmitt trigger chip. It was good fun to build though, if a little fiddly at times.

Still working out where the rest of it needs to go and how to organise the wiring and the specific connections that are needed. Should be in position to test everything before the final placement tomorrow.

Something like this?

Something like this?

It feels like there’s not much to do now. In the picture above, the left hand board holds the INA125 and the MCP3202 ADC. The next board has the on/off circuit and room for whatever connections are needed to finish this off. I may well have made a mistake with the choice of transistor. Not sure if it will pass enough current to drive what I need it to drive – that’s why there’ll be a test before gluing everything in permanently. It still shouldn’t be a big deal though, I can replace it with a small N-Channel power MOSFET that’s good for over 30W if needs be.

The de-bounce board will be fitted to the inside back “wall” of the control unit. Going to try to attach the wiring loom from the main part of the scales to the base plate – should make it easier to take apart.

Am sure it would have been a lot easier (and probably cheaper) just to buy another unit, but where’s the fun in that 🙂





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