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The Float Switch: A Liquid Level Turned Into a Contact
George J. Papashvili, Los Angeles · Public domain (expired patent)
Volt

Créé par

Volt

27. septembre 2026SE
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The Float Switch: A Liquid Level Turned Into a Contact

The float valve shuts off a cistern by itself. A float switch does the same job for an electric circuit: a float rides the liquid, and at a set level it opens or closes a contact — to start a sump pump, stop a tank overfilling, or warn that something has run dry. George J. Papashvili of Los Angeles applied it to the most expensive thing to run dry. US 1,719,078, *Float Switch*, filed 25 May 1926 and granted 2 July 1929, connects a cylinder to the engine's crankcase so the oil stands at the same level in both. A float on a spring-held rod drops as the oil falls; first it lights a lamp behind a glass marked "No oil", and then, if the driver drives on, it shorts the ignition and stops the engine. This rung works out a float's lift and why most float switches do not rely on it directly, sizes a sump pump's switching band, and builds a two-level float switch.
Débutant
About 3 hours

Consignes

1

Buoyancy, and the starts a band allows

Chargement du notebook Jupyter…
2

Read Papashvili's cut-out

Fig. 2 of US 1,719,078 shows the indicator in section: cylindrical container 8, cover 10, a rod 12 through the cover with a disc head 13, and compression spring 14 between cover and head holding the rod up. Float 16 slides on the rod above an adjustable collar 15 with a set screw. When the oil is low, "the float descends on the rod and engages the collar 15, the weight of the float compressing the spring 14" and lowering the head onto plate 20 — grounding the ignition lead to the distributor and stopping the engine. An insulating ring 31 on the head carries a conducting strip 32 that bridges contacts 28 to a lamp 26. The contacts sit higher than the ignition contact, "so that in the descent of the rod … the electric circuit through the signal lamp is established before the ignition circuit is short-circuited": warning first, shutdown second.
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Warning before shutdown

Espace de travail Blockly

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Build a two-level sump switch

Cut the top off a 2 L bottle to make the sump. Glue a table-tennis ball to a thin dowel that slides freely in a guide bored through a plywood bridge across the top. Fix two collars (a cable tie pulled tight) on the dowel above the bridge, 60 mm apart. Mount a lever microswitch on the bridge so the upper collar lifts its lever as the ball rises, and the lower collar pulls it back down as the ball falls — the gap between the collars is the band. Because the ball's lift is small, use the lightest lever switch and a lever long enough to need little force. Wire the microswitch so it switches the coil of a 12 V relay, and let the relay drive a small submersible pump emptying the bottle into a bucket. Pour water in steadily and count the starts a minute; move the collars closer and count again. Then fit a commercial float switch in its place and compare.

Matériaux pour cette étape :

Bouteille PET (boisson gazeuse)Bouteille PET (boisson gazeuse)1 pièce
Balles de tennis de tableBalles de tennis de table1 pièce
Goujons de bois durGoujons de bois dur1 pièce
Panneau de contreplaquéPanneau de contreplaqué1 pièce
Microrupteur (à levier)Microrupteur (à levier)1 pièce
Relais SPDTRelais SPDT1 pièce
Pompe à eau immergéePompe à eau immergée1 pièce
Interrupteur à flotteurInterrupteur à flotteur1 pièce
EauEau5 litres

Outils nécessaires :

Alimentation en courant continuAlimentation en courant continu
Perceuse sans filPerceuse sans fil
Jeu de foretsJeu de forets
Pistolet à colle chaudePistolet à colle chaude
ChronomètreChronomètre
Verre doseurVerre doseur
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A float switch not switching when it should

Float switch faults, from the float up.

Flow

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History and honest limits

**George J. Papashvili** of Los Angeles filed **US 1,719,078** on 25 May 1926; granted 2 July 1929. Float-operated contacts had been used before for level indication; his applied the float to protect an engine, with a warning stage before shutdown. Low-oil shutdown switches are still fitted to small engines and generators. **Honest limits.** A float switch reads the level where the float is: foam, waves and sludge fool it. Its lift is small, so it must move something light. Where a level cut-out protects against harm, a second independent switch is used.

Matériaux

9

Outils requis

6

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