TL;DR — I replaced a fried board in a super-cheap scribble pad with a tiny PCB that boosts a coin cell up to 27V to clear the bistable LCD. It now works great, and the power draw is coin-cell friendly.

You might remember this little device from a previous post — I took it apart, and (oops) broke the original PCB. It’s now back to life: scribble away, press the button, and the display clears cleanly.

I designed a drop-in replacement board that generates 27V, which is enough to fully reset the bistable LCD. The fit isn’t perfect yet — the battery and PCB need a touch of case “surgery” — but functionally it’s solid.

How the Circuit Works

If you missed the earlier exploration, I used a simple Joule Thief circuit to step a coin cell up to ~27V. This version adds output regulation, so once we hit the target voltage it sips power.

BOM & Key Bits

Input side

• 120 Ω inrush limiter
• 10 µF input capacitor
• Tactile “blister” button (salvaged from the old board after a new one turned out slightly oversized)

Output side

• 10 µF output capacitor
• 1 MΩ bleed resistor

Boost core

• Two 220 µH inductors (magnetically coupled by proximity)
• Main switching transistor + feedback transistor
• 27 V Zener for regulation (pulls the main transistor base low when the target is reached)

⚠️ Crucial detail: the two inductors must be soldered in opposite orientations. Get this wrong and the oscillation won’t start.

Power Profiling

I profiled the board using a Nordic Power Profiler. At 3.2 V input:

• Button press (charge-up):
  • Peak current ≈ 19 mA
  • Average during ramp ≈ 17 mA
• Regulating at 27 V:
  • Average ≈ 7–7.6 mA (saw peaks ≈ 10 mA)

This is very reasonable for a coin cell, especially since the high current is short-lived during the charge pulse.

All in all pretty neat - this will last for quite some time on the single coin cell. Just need to do a bit of work to make the repair a bit more seemless…