turtle button
- normally all manually tested
- Still has low rate that the button has no response while pushing
- issue still analyzing but not a big problem, we tested all the buttons before shipping
button types
- 带锁（锁）功能：按下去锁住，再按下松掉。
- 无锁（不带锁）功能：按下去自动回弹。
combinations
- KD2-21 有灯有锁
- KD2-22 有灯无锁
- KD2-23 无灯有锁
- KD2-24 无灯无锁
常开常闭，
- 带灯是8只脚
- 无灯是6只脚。
Buttons Circuits
one analog pin to read multiple buttons
analog-dat feedback multiple buttons 1
voltage divider 1K / 4.7K / 10K == voltage-divider-dat
analog-dat feedback multiple buttons 2
button with ripple capacitor
nice button investigation
small size
