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Narrowboat (Waste) Holding Tanks - LED level indicator

SiFi

Feb 28, 2015
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Hi, I have a small application but have close to zero circuit design experience. I want to fit two float switches to my boat's holding tanks, one for when it is about 3/4 full and when it is full. I had hoped that the switches I have bought would be normally open and closed when the levels are reached and power a couple of LEDs. However the switches I want to use don't do this, they start closed and will become open as the level rises!

$_12.JPG

Can anyone help me with this?

Ideally I would like to have the following, please note voltage is 12V,

When tank is empty that a green LED is lit (both switches closed).
When the first switch is opened the LED/second LED becomes orange.
When second switch is opened the LED/third LED becomes red.

It is important that the current required is as low possible.

Thanks, Simon
 

duke37

Jan 9, 2011
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You could use a CMOS 4093. The power will be negligibleleds.jpg only the led currents at about 5mA will be significant. I apologise for the poor quality of the diagram.

Some filtration of the 12V may be necessary.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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It looks like you need to connect the two inputs of the third NAND gate to the outputs of the first and second NAND gates to light the GREEN LED when BOTH switches are CLOSED. As drawn, the GREEN LED lights only when both switches are OPEN.
 

SiFi

Feb 28, 2015
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Feb 28, 2015
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Thank you for this, will it switch on the red LED when both switches are open? I have little experience but I will try to put your circuit together and let you know how it goes, Simon
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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The RED and ORANGE LEDs operate independently. When the corresponding switch is OPEN, its LED will be ON. If both switches are OPEN, both the RED and the ORANGE LEDs will be ON.
 

KMoffett

Jan 21, 2009
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Maybe a little more simple. +Supply > resistor> LED>ground. N.C. switches across the LEDs. One arrangement for Red and one for Yellow. When the switch opens the LED lights.

Ken
 
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SiFi

Feb 28, 2015
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I know there are alternatives out there, I'm just reluctant to use plastic alternatives because they are to go into a toilet holding tank and I want a solution that will last for a long time. I don't relish having to service them! If I can't get a circuit to work as above then I'll just have the LEDs wired in so that they are normally on and extinguished when the float switches are opened.
 

KTW

Feb 22, 2015
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Feb 22, 2015
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sje_double_float_pump_switch.jpg
This is probably what you should be using, it's for that purpose.
 

SiFi

Feb 28, 2015
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Thanks everybody for your help and suggestions, I've just discovered that if I turn the float upside down on its shaft the switch starts closed rather than open!! Mixed blessing though because part of me was keen to 'do some electronics'. The CMOS 4903 arrived today!!
 

KMoffett

Jan 21, 2009
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Revision of my earler circuit. +Supply >N.C. switche> resistor> LED>ground. One arrangement for Red and one for Yellow. When the switch closes the LED lights.

Ken
 

KTW

Feb 22, 2015
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Don't you want it to start open and end up closed?
You can buy a cheap alarm circuit kit that works on an open or a closed circuit.
 
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