A balance trigger is an object poised so that a very small push or pull tips it over and makes a noise, and its sensitivity is set by a lever ratio you can calculate: a 0.5 kg can sitting 5 cm from a board’s pivot, held by a line 20 cm on the other side, trips at about 1.2 N. Keep the fall short and the object light, so the alarm drops a can on the ground and not on a head.
Part of Early Warning Systems
A balance trigger is an alarm in which a noise-making object is held near its tipping point by a light restraint, so that a touch to a line, a door or a latch releases it with almost no force.
Balance Gives Sensitivity Without A Taut Line
A trigger that must hold a heavy weight needs a strong line, and a strong line trips people. A balanced object needs only a tiny holding force, so the restraint can be a thread. That is the sense in which these triggers are gentle on the household and sensitive to a touch. The parent article lists the end effects: bottles crashing off a tipped board, bells, stacked objects that tumble. The thing to control is the fall.
Typical uses: a bell or can set to ring when a gate or store door is opened, a board that tips cans onto a stone when a line is pulled, an object balanced on a post top so a bumped line drops it. The main use is at openings: the gate, a store door, a window you want to hear.
What You Need
- A pivot: a post with a notch, a nail or a peg through a board.
- A board or arm with a stop so it cannot swing far.
- The noise maker: a can of stones or a bell, light.
- A fine line from the trigger end to the thing being watched.
- A hard surface to fall onto so the sound is loud without needing a big drop.
Method: Choose The Ratio, Then Keep The Fall Short
Step 1: Build the lever. A board pivots on a peg. On the short side, at distance a from the pivot, sits the can. On the long side, at distance b, the line holds the board level.
Step 2: Compute the holding force, F = W × a ÷ b, where W is the weight of the can.
Step 3: Tune. A bigger b ÷ a means a lighter pull trips it. Choose the ratio so a hand’s touch on the line works it, and wind does not.
Step 4: Set the fall. The board drops the can a short way onto a flat stone or metal plate. Keep the can below waist height.
Step 5: Test from the bed, and test with a child’s weight: if a child or a household member walks into it, what happens?
Worked Example: Trip Force And Drop Energy
Trip force. A can with stones has mass 0.5 kg. Its weight is 0.5 kg × 9.81 m/s² = 4.9 N. At a = 5 cm and b = 20 cm the restraint needs F = 4.9 N × 5 ÷ 20 = 1.2 N, roughly the weight of 125 g (1.2 ÷ 9.81 = 0.125 kg). At b = 10 cm, F = 4.9 × 5 ÷ 10 = 2.5 N: the same alarm needs twice the pull and is less sensitive.
Drop energy. A falling can has energy m × g × h. From h = 0.3 m: 0.5 × 9.81 × 0.3 = 1.5 J. From h = 2 m: 0.5 × 9.81 × 2 = 9.8 J, about 6.7 times as much, landing on whoever is below. The alarm sounds the same from either height, because the noise comes from striking the stone. So keep the drop low and put nothing balanced overhead.
Common Mistakes
- A trigger over a doorway. The first one through is the household member who forgot it was there.
- A fine setting with an exposed line. Wind, frost and small animals will set it off every night.
- No stop on the arm. The board swings round and returns.
- A heavy object. A larger can does not give a better alarm, only a worse fall.
Safety
Never balance a weight above anyone's head
A falling weight cannot tell a child from an intruder. Keep the object light, like the 0.5 kg can above, the drop below waist height, and the position where the weight lands away from where people stand to open the door. No sharp, pointed or glass objects as noise makers. If the household cannot move through the opening safely with the alarm set, the alarm is in the wrong place.
What’s Next
- Learn next: Animal Sentinels — the living alarms that need no setting
- Combine with: Noise Makers — what the falling object should strike