FET or Electronic Relay for Wave Runner

[Dave R]

I'm pretty fair with DIY stuff but electronics are a little over my head. That being said I am trying to replace an OEM fuel gauge on my old SeaDoo. Gauge is no problem but float switch for oil tank is working the opposite way I need. The switch is open when tank is empty and closed when tank is full. I am mounting a new LED next to the new gauge so I would like the warning light to come on when oil runs out. The old OEM gauge must have had a reversing circuit of some kind because it work fine (The old warning light was internal to the gauge). Below is a sketch of how I would do it if I could use an electro-mechanical relay but I can't do that because of the rough pounding the wave runner takes while under power. Is there a brainiac out there that could sketch up what I need to do this? I tried understanding FET's and MOSFET's on the internet but so many variations and some circuits had resistors of unknown values.

 

[Lightium]

Your relay is draw wrong. You need two lines for power to the coil and two lines for the power your trying to switch.

 

[Diver300]

Your relay is draw wrong. You need two lines for power to the coil and two lines for the power your trying to switch.

He has that. The normally closed contacts are in series with the LED.

 

[Diver300]

An automotive relay will be fine in the conditions you get on wave runner. Even if the relay were to bounce occasionally if you really hit a wave hard, the led might light briefly but you wouldn't notice.

The oil splashing around in the tank is more likely to cause an issue.

 

[Dave R]

I know I can do this with a relay but I would prefer to do it with a solid state device such as FET or MOSFET. I was hoping to find a person that knows electronics and could draw a circuit that replaces the relay.

 

[crutschow]

Below is the LTspice sim of an inverter circuit using one NPN transistor (left part of circuit):

R2 is apparently in your LED assembly.
If so it is not needed.
Otherwise you need to add it.

If you need to add an external LED resistor than the transistor is not needed.
You just connect the float switch across the LED so the LED only carries the resistor current when the switch is open (LED D2, right part of circuit).

Note that the LEDs go on (yellow and red traces) when the float switch is open (0V, green trace).

 

[Dave R]

I appreciate the reply but I am super confused now. I only have 1 float switch with two wires coming out and I only have One LED that probably has an internal resistor because it came ready to panel mount. So sorry to ask but could you dumb this down a little? Not sure why you are showing the GRND with plus and minus on the float switches. The float switch is made of plastic with no GRND.

 

[crutschow]

why you are showing the GRND with plus and minus on the float switches.

The GRND is just the negative battery connection.
The (+) and (-) terminals on the float switch bottom are so the simulation can turn the switch on and off. They are not part of the real circuit.
The terminals on the left and right going to the contacts are the float switch contacts.

So if the LED has a built-in resistor to work directly from 12V, then you would use the circuit to the left of the 12V battery.

 

[rjenkinsgb]

The simplest thing is to have a resistor (eg. 680 Ohms) from ignition (switched) power to the LED then the other side of the LED to ground (battery -).

Connect the oil level switch across the LED !

The resistor will take a small amount of current, roughly 20mA.

While the switch is closed, that current goes straight to ground; if the switch opens it passes through the LED.

 

[crutschow]

The simplest thing is to have a resistor (eg. 680 Ohms) from ignition (switched) power to the LED then the other side of the LED to ground (battery -).

As I showed in my post #6.
But if the LED already has a built-in resistor as the TS thinks, that will make the LED dimmer (possibly too dim for daylight use).

 

[ZipZapOuch]

As I showed in my post #6.
But if the LED already has a built-in resistor as the TS thinks, that will make the LED dimmer (possibly too dim for daylight use).

The alternator will likely be putting out 13.5 to 15 volts and you should be able to see a high brightness LED just fine.

 

[danadak]

Any concern to protect LED from load dump ?

https://www.allaboutcircuits.com/uploads/articles/protecting-and-powering-automotive-electronics-systems_compressed.pdf

 

[ZipZapOuch]

Any concern to protect LED from load dump ?

View attachment 148441

https://www.allaboutcircuits.com/uploads/articles/protecting-and-powering-automotive-electronics-systems_compressed.pdf

A 15V Zener can be added. But, as the article you posted said, "most automakers utilize avalanche diodes, increasing reliability demands are driving some manufacturers to require that ECUs satisfy peak load dump voltages approaching that of the unsuppressed case." So no, no concerns.

Also, Jet Ski manufacturers are so concerned about ruining someone's weekend (and the bad publicity in the face of stiff competition) that they go to extremes about specifying materials, electronics, and anything else that may fail under harsh uses.