ELECTRONIC WATER LEVEL INDICATOR & CONTROLLER With LM3914

Thread Starter

hhhunzai

Joined Nov 14, 2011
72
Hello Every one!
I have made a water level indicator with an LM3914 IC, with five LED's output for five water levels.
I want to make this circuit controller & indicator. For doing this I need to know the output current of LM3914 on its output terminals where we connect LEDs, so that I will be able to connect two relays to control the water level of a tank by stopping water pump, I have read the data sheet but I couldn't understand from it.
I want to control the pump like this.
when the level goes down below the last level sensor of water a relay will on and the pump will start running, after successive four levels when it will reach the last level sensor then a relay with last LED will be ON. Each LED will lit with a 1K resistor in series with it.
But the difficulty is connecting relays, either SSR (15mA) or an electromechanical relay.
For doing this I need to Know the output current of LM3914 output,please suggest me a solution, should I connect SSR directly on output parallel to LED? or connect it with a transistor? If with a transistor, how should it be connected and how It will control relay either SSR or electromechanical?
Thank you :).
 

SgtWookie

Joined Jul 17, 2007
22,220
You really should post your schematic of exactly how your circuit is wired now.

The datasheet is here: http://www.national.com/ds/LM/LM3914.pdf

The LM3914 outputs can only sink current.

If you try to sink too much current with the LM3914 outputs, you can burn it up.
For a given set current, the higher the voltage is on the anodes of the LEDs, the more power will be dissipated in the LM3914.

Setting the output current is done via a resistor (R1) between REF_OUT (pin 7) and REF_ADJ (pin 8); the individual pin current is roughly 12.5 Amperes / R1. I suggest that you do not attempt to exceed 20mA sink current if you are operating in bar mode, which means a minimum value of 625 Ohms for R1 (12.5A / 0.02 A = 625 Ohms). If you are operating in dot mode, then you can go to 30mA current, using 420 Ohms or more from pin 7 to pin 8.

You really should not try to operate a relay coil directly from the output of the LM3914. You will need to use a transistor as a driver. A pair of resistors per output will need to be used to divide the current between the LED and the relay driver transistor.

But, we will wait for more details until you have posted your schematic drawing.
 

Thread Starter

hhhunzai

Joined Nov 14, 2011
72
This is the circuit diagram
http://www.siliconchip.com.au/cms/gallery/article.html?slideshow=0&a=30607&i=8

Thank you very much for reply.
I have made this circuit and it is working properly, but this isssue is it only show the level but couldn't control motor.
Therefore I want to make a slight modification in it by adding two SSR relys.
Dear sir please let me know how may I make this Indicator a controller adding transistors. If there will be a diagram it will be easier for me.

Regards.
 
Last edited:

KMoffett

Joined Dec 19, 2007
2,902
One problem you are probably going to have with that detection circuit is that the electrodes will eventually corrode due to electrolysis. Passing a direct current between two electrodes will cause one to erode and the other to be plated.

Ken
 

Thread Starter

hhhunzai

Joined Nov 14, 2011
72
All right Is there any other solution do u have To prevent this problem sir.
Actually I hardly manage to make this circuit and facing controlling problem.
May you suggest me how it could be accomplished.
Thank you.
 

SgtWookie

Joined Jul 17, 2007
22,220
You could place the low-voltage side of the SSR in series with the LED and the 1K resistor of the channel where you want the pump to turn on.

Since we don't know what the Vf and current rating of your LEDs are, and are not certain if you are actually using a 12v power supply, and don't know what SSR you may be considering using (or can get), we will need more information before we can supply a solution for your existing circuit.

As far as the electrolysis - that is a real problem.

Only way I can think of handling that at the moment might be to use a RRIO (rail-to-rail input-output) quad opamp, having 1/2 of it being an astable multivibrator with a 50% duty cycle and an inverter to drive the "sensor network", the other half being a precision rectifier to obtain an equivalent of the original signal.
 

KMoffett

Joined Dec 19, 2007
2,902
I've used versions of this circuit: http://www.simplecircuitdiagram.com/2010/10/19/cd4093-water-level-sensor-detector/ With 316 stainless steel TIG welding rods for electrodes it is virtually corrosion free. The CD4093s are quad NAND gates. You can use one gate for the oscillator and the other three for detectors...or with another CD4093, you can have up to seven levels detectors. With summing resistors on the outputs and one common resistor to ground you can create a stepped analog voltage to your bar graph.

.....or just put an LED/resistor (~5mA max) on each gate's output and forget the LM3914.

Ken
 

Thread Starter

hhhunzai

Joined Nov 14, 2011
72
Dear Sirs,
Thank you very much for valuable advices and assistance.
I am going to use this SSR please check.
http://www.crydom.com/en/Products/Catalog/a_so.pdf

And I am using an 800mA transformer directly this with the circuit given of water level indicator, in my previous reply.
I am not using a particular power supply for it, Should I use a dedicated supply for this circuit or I am ok with this 800mA transformer?

Mr. KMoffett I will try this circuit too, thanks for such a good idea. But I will need complete circuit so that I will be able make this circuit. Where should I connect its power supply and what will be its voltage.

Regards.
 
Last edited:

KMoffett

Joined Dec 19, 2007
2,902
.....
Mr. KMoffett I will try this circuit too, thanks for such a good idea. But I will need complete circuit so that I will be able make this circuit. Where should I connect its power supply and what will be its voltage.....
I think the attached should work. It's not bench tested (or simulated).

The concept:

1. The circuit will power-up with the pump OFF...no matter what the tank level.
2. The five LEDs will indicate the current level of the water.
3. The MANUAL START button will start the pump, but only if the water level is below the High level detector.
4. Once started, the pump will remain ON until the water level is above the HIGH level detector electrode, or the MANUAL STOP button in pushed.
5. Once stopped, the pump will remain OFF until the water level is below the LOW level detector electrode, or the MANUAL START button in pushed.
6. The pump will automatically cycle ON and OFF as water is used and re-supplied.

Ken

Corrected schematic in Post#40
 

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Thread Starter

hhhunzai

Joined Nov 14, 2011
72
Dear Sir,
Thank you very much for the circuit.
I have made almost complete the circuit but there is a problem.
That is I could find the BAT85 schottky diode. Instead I have find 1N5819.
Is this suitable or should I do some thing with it.
Regards,
 

KMoffett

Joined Dec 19, 2007
2,902
1N5819 is physically huge compared to the BAT85. The idea of using Schottky diodes was to get as low a Vf (forward voltage drop) as possible for best signal detection. Though they are bothy Schottky, the Vf for the Bat85 is ~0.24V, for the 1N5819 is ~0.55V. To what major electronics vendors do you have access?

Ken
 

Thread Starter

hhhunzai

Joined Nov 14, 2011
72
Dear sir,
Here in Pakistan we have not as such vendors who represent a major company, but they import different kinds of components from different companies.
If there are any particular vendor of a company, I don't have any idea about it.
Sir please tell me If I use this 1N5819 diode will it work in circuit or not?

Thank you.
 

KMoffett

Joined Dec 19, 2007
2,902

Thread Starter

hhhunzai

Joined Nov 14, 2011
72
Hello Sir,
I actually have made this circuit with 1N4148 diodes, due to non availability of BAT85s, previously I had made this circuit with 1N5819 diodes it is not giving me any feedback either with 1N4148 or 1N5819, what may be the reason.
I am using a power supply with a 800mA transformer and 78L12 regulator should I increase the amperage capacity of power supply or it is ok ?
Or Should I use an AC power supply or DC I am confused sir.
Please advice me in this regards.
 

KMoffett

Joined Dec 19, 2007
2,902
Hello Sir,
I actually have made this circuit with 1N4148 diodes, due to non availability of BAT85s, previously I had made this circuit with 1N5819 diodes it is not giving me any feedback either with 1N4148 or 1N5819, what may be the reason.
Since it has never worked for you, I have no idea. Perhaps you can post pictures of what you have done. I would breadboard the oscillator and one detector, to verify that you have something working.

I am using a power supply with a 800mA transformer and 78L12 regulator should I increase the amperage capacity of power supply or it is ok ?
Or Should I use an AC power supply or DC I am confused sir.
Please advice me in this regards.
You need DC. A 12VDC/800mA supply should be sufficient.


Ken
 

Thread Starter

hhhunzai

Joined Nov 14, 2011
72
OK sir I will provide the pictures after two days as i have to find a proper digital cam. when I will get it I will send it to you.
 
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