SensingRebuild shelfHand tools, works as-is

Float switch

Also called level switch, sump float.

A float with a magnet and a reed, or a simple buoy that pushes a microswitch. Dishwashers, humidifiers, dehumidifiers, aquarium gear, bilge pumps.

If you are new to this category

A sensor reports a condition such as weight, light, temperature, or position. Many sensors do not switch fully on or off. A kitchen scale bar, for example, changes by thousandths of a volt. Keep the small board that read the sensor if you still have it.

Keep the connector. Write down the wire colors before you cut them. If you do not know which wire is which, the sensor is hard to reuse.

Four wires on a scale sensor

Red and black feed the sensor a steady few volts. Green and white are the tiny signal, measured between them, not from either one to ground. Pressing the bar changes that signal by millivolts. A normal meter on DC volts can show that it responds. Turning it into a number on a screen wants a small amplifier board, often an HX711.

Mains voltage. Line voltage, and the DC bus just after the rectifier, both kill. A disconnected cord is not a discharged supply.

Specification. The right-hand column is what is typical. The label on the part in your hand wins if they disagree.
What to look for	Usual figure
Reed float	Often reversible by flipping the float, so you can choose normally open or closed for rising water
Current	Reed types are signal-level. The ones on a mains bilge pump are not; read the stamp.

Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.

What people actually use it for

Tank level
Overflow cutout
A rain alarm

Leave this one when

The float is cracked and full of water. It will sink, which is the unsafe direction.

Buying a new one is the better idea when

A polypropylene float rated for the liquid you are actually using.

How to tell if this one still works

Beep while you lift the float. Confirm it fails the way you want if the wire breaks.

Tools you will want: Meter.

Words on this page

Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.

mains: Ordinary household wall power. It can kill. Unplug before you open anything that used it, and do not experiment on live wiring.

Where people actually find it

2 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.

Harvest at a glance
From	Call	How many	How often	Where it sits
Dishwasher	Worth it	1	usually	In the tub.
Ultrasonic humidifier	Worth it	1	usually	Reservoir.

Worth pullingDishwasher1 · usually
In this particular machine
A float and a switch.
Where to look
In the tub.
How to get it out
Check that the float still floats.
The condition to expect
Sticks with grease.
Worth pullingUltrasonic humidifier1 · usually
In this particular machine
A float that tells the board it is dry.
Where to look
Reservoir.
How to get it out
Clean it.
The condition to expect
Sticks.