On The Air


December 16, 2021

Live streaming from home for work, school, and social interaction is becoming more common (thanks 2020). Also more common is the family member walking in on the broadcast creating a plethora of YouTube live stream fails. This idea is intended to aggressively remind family members before they enter that you are broadcasting.

Invention idea #03043.1 pulled from my notebooks to share.

Build it for fun & profit.

Explore the interactive blueprint.

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Overview of the concept:


Yes, you could lock the door or scatter broken glass in front of it; however, this doesn't stop the persistent knocking on the door and keeping everything open for ease of access for children. On the subject of children, I'd think a version taking the form of a scary clown of doom with an evil laugh might be effective:

Making the display plate swappable allows you to sell these separately. It also opens up all sorts of niche-marketed ideas using simple text as well as images/logos/icons:

  • Eating cookies
  • Watching movie
  • You don't want to know...
  • Slaying dragons
  • Under Bulbasaur protection
  • So on...

You can take it up a level for creative customers and sell a diy kit to make the gels.

  • Blank acrylic display plate.
  • Letters/numbers/symbols sticker sheet (removable white matte).
  • Large blank stickers on a laser printable 8.5x11" sheet (perhaps like this).

Then they can print an image on the sticker, cut it out with a hobby knife, and apply to the display plate. 

If you want to go bananas and plan to 3D print, then you could design the faceplate to be swappable allowing you to create 3D geometry with light bleed effects. This would be great for licensable characters like Pokemon, Star Wars, etc:


  • This is a great tutorial on using the QT1010 and they sell a handy breakout board to learn to work with it.
  • The green flashing LEDs may not sync perfectly. If this is annoying you'll need a separate blinking circuit or reduce to a single flashing LED.
  • For user sensitivity adjustment, consider adding a trimmer capacitor (Cv) on the touch sensitivity and a volume control (Rv) on the buzzer. Should add access holes and/or knobs on side of case for these.
  • The buzzer has oscillator built into unit so only need to provide +3V to make sound. If using simple speaker you will need to supply your own oscillator circuit.
  • You can get custom-cut LED acrylic sheet here. Amazing stuff!
  • When designing the size of the case, consider that it has to clear the door jam. The door jam is fairly close to the knob, so the simple design I drew may not work with most doors. Most likely, your design will need to be asymmetrical, using the batteries (or an iron slug) as counter weight so it hangs nicely on the knob.


Use a tilt switch (or here), batteries, inverter, LED(s), and buzzer. When the unit is rotated it shorts the battery to the LED(s) and buzzer.

  1. Clamp the device to the knob and switch it on. Turns green or red depending on switch position.
  2. Adjust the level of the device until it turns green. Close door.
  3. When knob is rotated, the device will go off. Turns red with buzzer noise.

Tricky part is the mechanism that grips the door knob. It should be user-adjustable and universal for most common knobs on the market. But there are a lot of design differences making this tricky. On top of that you should not interfere with the knob itself, so you'll need to attach to the neck of the knob. An interesting problem to solve.


Another undocumented thought I had was to clamp a lever to the neck of the knob, and mount the sensor on the door with removable adhesive. An exposed microswitch is tripped when the knob rotates:

It has similar issues as the “clamp” version I mentioned above, but probably the simplest electronically. The lever would need to “kiss-touch” the switch so it can freely rotate both directions so as not to impede normal knob operation.

Note: I provide Amazon and misc company links to some parts so you can prototype quickly. After proto, you'll want to source in bulk from places like Digikey, Mouser, Alibaba, Ebay, etc. to get the best pricing.

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