Introduction: Biohazard Toilet Warning

About: I'm a social-worker, working with 12 - 23 year-olds. I used to be a printer. In 2018 I opened a small makerspace ( in my house, where I have lasercutters, 3d-printers, Arduino's, Mindstorms and ot…

We all know the problem when you've been to the toilet for some big business. While washing your hands you think: 'just air-freshner is not enough! There should be a way to warn innocent visitors to stay away from this place for a little while.'

This biohazard warning is the solution for this global problem.

Just set the timer for the hazard you left and in time the warning scale will turn to 'safe' again.

It is an easy and cheap build. The only thing I bought for this project is a cooking timer. I mostly used trash materials and scraps.

Step 1: You Will Need


  • packaging (trash) cups
  • cooking timer
  • something to use as backboard


  • scissors
  • hobby knife
  • sharpy
  • glue
  • color-printer


  • paint
  • hole-punch
  • tools to create the backboard

Step 2: Disassemble the Cooking Timer

This is a really easy step; Just take the timer apart until you are left with just the (still working) inner mechanics.

Step 3: Prepare the Inner Cup

To make the housing I used two desert cups that fit inside each other (for the Dutch: Monatoetje voor de binnenkant en AH-tomaten emmertje voor de buitenkant)

First mark where to cut the bottom from the inner cup. The easy way to do this is holding a sharpy on the inner part of the timer and then move the cup around against it. You will end up with a perfect marking (as long as you didn't move the sharpy)

Use a hobby knife (x-acto or Stanley or something) to cut off the bottom from the cup. Always use sharp tools and be careful not to cut yourself or someone else.

If you use a sharp knife it will be very easy to cut along the line that you have drawn on the cup.

For my timer I needed to make a few holes in the bottom of the cup to mount it on the timer. You can also use just glue or a screw or whatever. This will be different for different cups or timers.

Step 4: Prepare the Outer Cup

The outer cup must fit around the inner cup. You can mark where to cut, the same way. This time you put the sharpy on the (already cut) inner cup. Cutting is also done the same way.

For the turning of the inner cup, we need to make some thumb/finger holes. I used something random and round to make the markings.

Step 5: Paint the Cups

I toughed that it would be nice to paint the cups, but this is totally up to you.

For the window that will show the scale, I need to leave a part transparent, so I taped some tape on both sides to keep that part clean from paint.

I painted the inner cup black and the outer one gold.

After painting I removed the tape from the outer cup.

Step 6: Making the Labels

The easiest is to print out the labels on adhesive material. (I didn't have any so I will need to glue them on the cups)

You might need to change the size of the labels for the cups you are using.

Cut out both labels. One for the scale and one for on the outside.

I needed to punch a hole in the middle of the scale label, but you might not need to.

To open the window where you will see the scale through the label, I used the hole-punch to punch out the corners and my hobby knife to cut in-between.

Step 7: Make a Backboard

I used some scrap pieces of oak to make the backboard, but you can use whatever you have laying around for this, from card stock till metal.

I marked where to drill some holes to mount the timer on with some screws, but you can also use glue to make it easier.

Step 8: Mount the Cups

Mount the inner cup on the turning part of the timer.

Glue (or stick) the scale on top of the inner cup. Make sure to put the green part on the top, so the green will show through the window when the timer is not set.

Glue the label on the outer cup.

Cut off the overlay at the finger holes.

Glue the outer cup on the backplate. (it would be nice to screw this, but I had no easy way for this and the glue turned out nice)

Step 9: Finished

Hang the finished warning timer outside your toilet.

When you turn the inner cup the window will show different colors from green to yellow to red to black. In time it will slowly turn back to green to indicate that it is safe to enter the toilet again.

Glue Contest

Runner Up in the
Glue Contest

Epilog Challenge VI

Participated in the
Epilog Challenge VI

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Participated in the
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