- Difficulty
- Advanced
- Build time
- 120-180 min
- Estimated cost
- $0-$25
- Age range
- 13-18
- Workspace
- A clear table about 90 cm wide
The finish line
What you will build
The ones wheel advances exactly one digit per input for 25 presses, and the tens wheel advances at 10 and 20 without double counting.
Learning goals
- Identify how one controlled lever press produces indexed ones and tens number wheels.
- Construct and explain a reciprocating input-to-intermittent rotary system.
- Measure how the input-pawl travel changes performance.
- Diagnose losses caused by pawl friction and wheel backlash.
Before you build
Materials, tools, and safety
Reuse-material cost: Usually under $5 with an existing kit. Supervision: Adult help recommended for sharp or heated tools.
Tools
- Ruler
- Removable tape for motion marks
Low-cost swaps
- Use equivalent brick-compatible parts from any kit.
- Use cardboard beams and straw bearings for a larger demonstration model.
- Use laminated cardboard number wheels, paper-fastener axles, and bent paper-clip pawls.
Project-specific safety
- Keep fingers, hair, and loose sleeves clear of moving parts.
- Turn the mechanism by hand; do not attach a high-speed motor.
- Use light return bands and rounded carry pins; do not place fingers between counter wheels during a press.
Orient the build
Place the build so one controlled lever press is on your left and indexed ones and tens number wheels is on your right. Call the side facing you the front, the far side the back, the tabletop the bottom, and the opposite face the top.
Build it
Step-by-step instructions
Step 1
Build the counter frame
Support two parallel digit shafts with visible front faces.
Brace the pawl pivots above each ratchet.
Step 2
Assemble the ones wheel
Align a ten-tooth ratchet with digits 0 through 9 on one shaft.
Add a holding pawl at a clear viewing angle.
Step 3
Build the input lever
Place a drive pawl on a returning lever so one full press advances one tooth.
Add stops for press and release.
Builder checkpoint: After build the input lever, the first subassembly should stay aligned when handled gently.
Step 4
Assemble the tens wheel
Build a second ten-position wheel with its own holding and drive pawls.
Set both displays to zero.
Watch for: If this stage binds or drifts, inspect return-band hysteresis before adding more parts.
Step 5
Add the carry pin
Mount one pin on the ones wheel near the 9-to-0 transition.
Place a carry lever that reaches the tens drive pawl.
Step 6
Tune single-step input
Press and release slowly ten times without the carry connected.
Correct skips or double steps first.
Builder checkpoint: After tune single-step input, operate the build slowly and confirm that indexed ones and tens number wheels begins without binding.
Step 7
Tune the carry event
Reconnect the carry and move manually from 9 to 0.
Ensure the tens wheel advances once and the lever resets.
Step 8
Run a 25-count audit
Start at 00 and record displayed value after every press.
Stop at any mismatch and inspect the last event.
Builder checkpoint: At the final checkpoint, The ones wheel advances exactly one digit per input for 25 presses, and the tens wheel advances at 10 and 20 without double counting.
See the engineering
Why it works
- Input
- one controlled lever press
- Output
- indexed ones and tens number wheels
- Motion
- reciprocating input-to-intermittent rotary
- Energy losses
- pawl friction, wheel backlash, carry-pin impact, return-band hysteresis
Why this works
Discrete state and carry
A ratchet advances the ones wheel by one stable state per event. Once per revolution, a carry feature triggers a second ratchet to increment the tens state.
Look for: Press slowly through 9, 10, and 11 while watching the carry pin contact and release the tens pawl.
Where the energy goes
Efficiency and losses
The ideal model leaves out pawl friction, wheel backlash, carry-pin impact, return-band hysteresis. These effects turn some input energy into heat, sound, vibration, or unwanted motion, so measured performance will be lower than an ideal calculation.
Look for: Run the build slowly and locate the first place where pawl friction becomes visible or audible.
Math bite
Connect wheel states to count
Formula: displayed count = 10 × tens digit + ones digit
- Tens digit = 2
- Ones digit = 5
Substitute: count = 10 × 2 + 5 = 25
Result: The two wheels represent 25 input events.
Each wheel has ten stable states in base ten.
Missed or double ratchet steps create counting error.
Make it behave
Test, troubleshoot, and tune
Controlled test
Start here: Press the input five times slowly from 00.
Success looks like: Each complete press-release cycle advances exactly one ones digit and holds between presses.
Measure: Displayed count versus actual press number.
Change: the input-pawl travel
Keep constant: wheel teeth, return force, frame, press speed, and starting zero
- short pawl travel
- one-tooth travel
- slightly long travel with stop
| Symptom | Likely cause | Confirm it | Fix |
|---|---|---|---|
| A press counts twice | Drive pawl travel exceeds one tooth | Watch one slow press from the side | Shorten travel with a firm stop |
| The wheel rolls backward | Holding pawl does not engage | Release input slowly and watch the tooth | Move or preload the holding pawl |
| Carry misses ten | Carry pin or lever is too short | Move manually through 9 to 0 | Adjust overlap and lever pivot |
| Tens advances twice | Carry lever does not reset before next contact | Continue slowly past zero | Add return clearance and a stop |
Choose your tradeoff
Solve single-step indexing before carry. Reliable counters use clear stops, one-tooth pawl travel, and enough dwell for every pawl to reset between states.
Keep experimenting
Try another version
One-digit counter
Build and verify only the ones wheel.
Reset lever
Add a safe manual reset to 00.
Three digits
Add a hundreds wheel and a second carry stage.
Build together
Classroom and access options
Classroom version
Teams can compare the input-pawl travel while keeping wheel teeth, return force, frame, press speed, and starting zero. Assign builder, tester, recorder, and explainer roles; have each team predict the result before collecting three trials.
Access adaptations
- Use high-contrast tape to distinguish input and output parts.
- Replace a small crank with a wider handle for an easier grip.
- Add raised dots for digits and a large input paddle with an audible click.
Reflect on the design
- How did the input-pawl travel change the measured result?
- Where did pawl friction affect the build most strongly?
- What evidence shows that discrete state and carry explains the motion?
- Which change would improve indexed ones and tens number wheels without creating a new problem?
Glossary
- Discrete state and carry
- A ratchet advances the ones wheel by one stable state per event.
- Input
- The action or energy supplied to a system; here it is one controlled lever press.
- Output
- The useful response produced by a system; here it is indexed ones and tens number wheels.
- Efficiency
- The fraction of input energy that becomes useful output instead of friction, sound, heat, or unwanted motion.
Build your dreams
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Explore more guidesSources and build notes
An original BrickLabClips interpretation of a standard mechanical mechanism.
- Mechanism verification: Standard kinematics were checked for motion direction, constraint, clearance, and likely friction points.
Written and edited by BrickLabClips. Published 2026-07-22; updated 2026-07-22.