Bulbdial Mod Revisit

The new copper tape in place

I’ve been getting great use out of my modded Evil Mad Scientist Laboratories Bulbdial clock (you might remember, I modded it with a touch sensitive strip to turn the display on and off), but it’s been having some problems lately. Because I couldn’t solder to the aluminum tape I previously used, the contacts taped in place were getting loose, causing the clock to flicker on and off. The loose contacts also made functionality very sporadic. The solution was to swap the aluminum strips out with the copper foil I bought a while back from Sparkfun for a whopping $3. 

While I was working, I decided to revisit the mod. Here is the module I had previously made:

Top view of circuitry

And from the bottom:

Bottom view of circuitry

And finally, everything put together:

Assembled

I also decided to actually add the schematic into the original, just for clarity. Before:

Original switch schematic

And the modded version, as it stands now:

Schematic with mod

However, I’m curious as to what would happen if I scrapped the center pin on the header I added to the switch, which is jumpered to VDD elsewhere on the board. I could instead connect the touch strips to the pin of the µC itself, in effect creating a diode-connected transistor. The main problem I can see is that this would only allow for the transistor to pull the input pin (which must be internally pulled up) to GND+Vf of the transistor’s base-emitter diode. I haven’t checked what the ‘low’ voltage of the pin is now with the current design, but I’d bet that the input is unstable at ‘low’ = .7V. Still, it’d be cool to eliminate the one part and the one extra assembly step. (Update: I did check what the current ‘low’ voltage is, around 0.07-0.08V, quite stably ‘low’ indeed).

Perhaps some more experimentation next time. But in the mean time, the problem has been fixed.

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