Mechanical Occupancy Counter

This mechanical occupancy counter has been designed for 3D printing. It can be placed in the doorway of a shop or other venue to count the number of people inside. Limiting the number of people inside a space can help to maintain social distancing and reduce transmission of diseases.

The counter is based off of a number of designs, including:

• The Pascaline - https://en.wikipedia.org/wiki/Pascal's_calculator...
• A two-way ratchet mechanism by thang010146 - www.youtube.com/watch?v=4A78AyRVbQ8
• A stable lever mechanism by thang010146 - www.youtube.com/watch?v=Zq5ursxPGPg
• A mechanical counter by brlaw - https://www.thingiverse.com/thing:2167273
• A mechanical counter by burrowsa - https://www.thingiverse.com/thing:4644252

A special mention goes to Matthias Wandel from Woodgears.ca for their excellent documentation and demonstration

• Woodgears website - https://woodgears.ca/counter/
• How mechanical counters work - www.youtube.com/watch?v=rjWfIiaOFR4
• Building a wooden mechanical counter - www.youtube.com/watch?v=lwt8QujQv5s

• 3D printer (minimum build volume 150 x 150 x 70 mm)
• 3D printing filament (PLA is fine, approximately 170g used)
• M6 x 80 mm bolts x 2 (longer is fine)
• M6 x 40 mm bolts x 4 *
• M6 penny washer x 4
• M6 hex nut x 5
• Elastic band (or old elasticated face mask loop) x 2
• A gate arm (foam used here)

* might need 2 x longer bolts if gate arm is especially thick

This design has been through 14 iterations (4 shown here) in Fusion 360 and a couple of prototype 3D prints. With each iteration tolerances were refined and the parts were redesigned for quicker and easier printing. Some notable features that are not immediately obvious include: the hexagonal cut-outs for M6 hex nuts, the split faces to improve bridging direction, and large cut-outs for print volume reduction. Only one part requires support when printing (the guide rail), which has the support built in.

I printed this on an Ender 3 in about twenty hours. Any PLA will be fine, but a colour where you can get good contrast with the numbers is ideal. I opted for white PLA and black acrylic paint.

1. Download the .stl files from this page. You will need to print one of each part, except the 'pawl': print TWO of the 'pawl' files.
2. Arrange on the build plate and slice the files in your preferred slicer: I used Ultimaker's Cura. If you use the 'guide rail with supports' file then everything should print well without additional support materia.
3. Send the GCODE to your printer. I used an Ender 3, but most desktop FDM (Fused Deposition Modelling) printers should work. Remove the supports from the guide rail.
4. Paint or colour in the numbers on the 'tens wheel' and the 'units wheel', then let these dry
5. The gate arm should be made from any available material. I have used a rigid foam 350 x 35 x 10 mm. The two mounting holes on the small bevel gear are designed for M6 bolts 20 mm apart.

The assembly step numbers should correlate with the pictures above. In addition to the 3D printed parts you will need the following:

• M6 x 80 mm bolts x 2
• M6 x 40 mm bolts x 4
• M6 penny washer x 4
• M6 hex nut x 5
• Elastic band (or old elasticated face mask loop) x 2
• The gate arm to attach to the lever (packing foam used here)

Assembly steps:

1. Gather the required parts
2. Place the pawls on the pin, so that the bosses face each other
3. Place the large bevel gear onto the pin as shown to make the pin assembly
4. Place the guide rail onto the Units gear as shown
5. Place the pin assembly onto the units gear as shown to make the units gear assembly
6. Carefully align the units gear assembly with the bracket and partially insert the M6 x 80 mm bolts as shown
7. Align the tens gear with the units gear and holes in the bracket, then fully insert that bolt.
8. Align any two digits on the top face of the counter, then place the eight tooth gear onto the bolt aligned as shown.
9. Fully insert both bolts so that they come through the rear face of the bracket
10. Attach M6 nuts to the bolts as shown^
11. Place penny washers on two of the M6 x 40 mm bolts then push through the gate arm (foam used here)
12. Push these bolts through the small bevel gear and secure with nuts
13. Place the final M6 bolt through the remaining hole in the bracket*, add a penny washer then the small bevel gear, another penny washer, then finally a nut to secure the assembly
14. Attach the elastic bands to the bracket as shown`` and also to one of the bolts on the small bevel gear

^ Do not overtighten, adjust once assembled

*If not using hex head bolts, then place the nut in the hexagonal hole instead

`` On later versions there are also mounting holes on the bracket you can use

Thank you for reading this instructable. If you have any questions or comments please leave them below. This counter should not be treated as 100% reliable and should be regularly checked and recalibrated.

This counter should be accompanied by a poster placed at eye level, which sets the maximum number of people in the shop. You can download and print the attached poster, which is also available here.

This mechanical counter was designed as part of Project Clean Access, with support from the University of Bristol and the Royal Academy of Engineering.