Water level inidicator using 74LS00 need help

Hi,

I am v.new to electronics and helping my kid science school project.
We are trying to build a v.simple water level indicator. We have to use one of the 74xxx series I.C. So we decided to use NAND gate I.C (74LS00).

Circuit diagram is attached. It works fine in the breadboard when input of one gate is connected to (-ve) of the circuit the LED lights on. The problem is when we dip the wires in the water the light doesn't turn on.

I searched internet and it seems that in those cases one should put pull-up or pull-down resistors. Is that correct?

Can anyone please help us in this. His project is due next week and we can't figure out why it doesn't work in the water. Water supposed to be good conductor of electricity.


Much appreciated.

thanks,

Actually, no. Water is a poor conductor of electricity. Deionized water is an insulator.
You need ions such as common salt for water to conduct electricity.

You can try changing the 7400 to 74C00 or 74HC00 which require lower input current.
You may need a transistor driver to produce sufficient current to turn on the LED.

Thanks for your quick reply.

I used 74LS00. Is there any way to avoid the transistor or capacitors. The problem is my kid only knows about gates and how resistors work. the transistor would be too complicated for him.

I can provide you with an answer but it will have to wait for about 10 hrs when I get a chance to try it out on a breadboard. Perhaps someone else can help.

In the meantime, I do not know why you have a battery symbol connected to pin 7. Remove the battery. Connect both pin 7 and the lead to the water probe to the power supply ground.

If you want to increase the conductivity of the water, add a pinch of table salt to the water.

What are you using for +5V supply?

Pin 7 is just ground. I wanted to show that as ground but you are right it looks like battery symbol. I am using 5V power adapter.

I can wait till tomorrow its 3am here . I really appreciate your help.

Thanks again,

would you consider float switches which are cheap to buy and easy to make workable.

This is a solution of how you could do it, but I would each connect it with a 5V in the input and output, of each a 150ohm resistor, with an LED grounded. But since what you need to use a 74HCxx ic, you need to attach the output from the float switch to the input of the logic.
http://cmcontrols.com/images/level_control_floats.png

Type of hardware which suites you I feel are these two types:
http://www.profimess.eu/gifs/ls-15.gif
http://media5.rsdelivers.cataloguesolutions.com/LargeProductImages/R0519242-01.jpg

Hope this helps as it worked for me this way as I did a similar project which worked ones but was more complicated as it had to be used with micro.

The distance between, and surface area of the probes will have a great effect on it's operation. Probes made out of copper strip for example, will work better than a copper wire.
But as has been stated before , water is not a good conductor.

Thanks Fiatuno. The project is for school Grade 4 so i wanted it to be simple. The float switches won't fit in the small tube plus it is more for commercial.

Thank Gerty. I tried to keep the input ground wire and the input gates wire close together even milimeters apart still it doesn't work. May be electrical copper wire will help. But the wires I am using are the standard breadboarding wires. Do they have too much resistance?

IMO you are better off with a chip that has schmitt trigger inputs like the CD4093. There are several others.

In order to bring the output of the gate into a defined state the inputs also need to be in a defined state.

If you take the probes out of the water the inputs are floating, no defined input voltage.

IMO you should use a pull-up resistor at the gate inputs and connect the other probe directly to GND. (was said before)

The pullup resistor value depends on the conductivity of the water (quite high resistance). You can start with a 100k to 1M resistor . You can also add salt to your water solution which will increase it's conductivity.

Thanks praondevou. I am very new to the electronics. Would you be able to show me where to add the pull-up resistors. I had attached the circuit diagram in my original post.

You should be using a 4000 series CMOS 4011B or 4093 quad NAND gate instead of a 74LS00.

The transistorized versions of the 74xx series ICs are not suitable for this project, as their input impedance is far too low; much lower than the impedance of the water.

4000 series CMOS ICs have extremely high input impedance. Additionally, 74sxx series ICs have a limited Vcc range of 4.5v to 5.5v; 4000 series CMOS can operate from about 3v to about 16v (some go to 18v) and require far less current to operate.

You could use a 74C00, but not 74HC00, nor 74HCT00, etc.

You should ground the inputs of unused gates.

Your water will conduct but it is not a good conductor. What you are essentially asking your circuit to do is for the water to short the two wires together to produce the ground or near ground at the input of your NAND gate. This is not going to happen. The tiny change made by putting the two wires in the water would have to be greatly amplified with something like an operational amplifier (op amp).
You can get more information about electronics at www.MoreDat.com

Try this circuit. You can try with pin-1 not connected.

Thanks. I'll try tonight and let you know.

Really appreciate your help guys.

But the wires I am using are the standard breadboarding wires. Do they have too much resistance?

Click to expand...

It's not that they have too much resistance, it's the fact that the bars would have more surface area than the wire. More area touching water means more conductivity .

MrChips,
Your circuit won't work due to the low input impedance of to the 74LS00 IC. They need to use CMOS.

Yes, thanks wookie, that was the first thing I suggested in post #2.
That is why it took me 10 hours to respond. I wanted to test it on a breadboard.
The High-level Input current for 74LS00 is 20uA.
Low-level input current is 400uA.
I tested this on a breadboard with 1cm of bare wire in tap water and it works.
If the experimenters want more reliable operation, they can use bare copper wire, spiral wound but not shorting on a piece of tubing or doweling. Better yet, add a pinch of table salt into the water.

Can someone give me a quick answer why no pullup / pulldown resistor is required? Does the 74LS series have something built-in to define the input state of an unconnected pin?
With no water in the glass the corresponding input is floating, so how can I be sure about the output state of the gate?

That is a very good question praondevou. CMOS gates have extremely low leakage currents and hence pullups and pulldowns are very critical.

The 74LS00 has a 20K-ohm internal pullup. In this experiment we are taking advantage of this pullup. In proper design, inputs should never be left unconnected.

I think its not a recommended practice, but the old TTL used to read a floating input as a "High"