djb

I am looking for a sensor that can measure oil level in a lawnmower engine. I am familiar with the reed switch type sensors, but am more interested in capacitive sensors. Does anyone have a simple design for a capacitive sensor that doesn't use a function generator ? Also, any other types of sensor to measure liquid level (ie: chemical sensors to measure molecules of oil)?

Optikon

why is function generator out of the question? You need power source anyway, whats the big deal if you need to generate a square wave or something to get the best design idea to work?

djb

The only reason I want to avoid a function generator is because you can't very easily attach a function generator to a lawnmower engine. It needs to run off a battery.

Oznog

A 555 can be used with the sensor as the timing capacitor. Then you need a frequency-to-voltage converter.

__________________
I thought what I'd do was I'd pretend I was one of those deaf-mutes.

Russlk

The system should prevent the motor from starting when the oil is low, other wise it is pointless because the dummy that can't remember to pull the dip stick won't look at a readout either. The dielectric constant of oil is about 2.2, I believe so an oscillator could vary nearly 2:1 between full and empty. A frequency discriminator could switch the motor off.

The plates of the capacitor cannot be too close together or surface tension will keep the oil between the plates. The formula for capacitance of parallel plates is: C = 0.225*Dr((n-1)*A/t) where
Dr = dielectric constant = 1 or air
n = number of plates
A = area of plate in square inches
t = thickness of dielectric (plate spacing)
C = picofarads

For a two plate capacitor, plate 1/2" X 1", spacing 1/8", the capacitance is about 2pF in oil, 1pF in air. Those values will be difficult to work with.

__________________
see my website: www.geocities.com/russlk

Optikon

:-D

not all "function generators" come in shoe boxes. whats wrong with an 8mm x 8mm lil ol chip function generator for your frequency source?

Klaus

Why not investigate an oil pressure safety cut out? With oil level sensing you will have to assume the mower is always used on level ground, which is a bit unrealistic:wink:.
The diesel engine in my truck has a pressure sensor that allows the engine to start but will cut it out after a preset time ( ~10sec) if the oil pressure fails to build up, as would be the case if insufficient oil was in there.

Russlk

Lawn mowers don't have oil pressure, they operate on the splash system. For that reason, oil level sensing won't work when the motor is running.

__________________
see my website: www.geocities.com/russlk