Hi! I need to fabricate plant watering watcher circuit for my project. Here's the circuit www.redcircuits.com/Page18.htm .I thought of analyzing the circuit in detail. Can anyone explain how the circuit works in detail ?
And I tried simulating the circuit in multisim 11. But I couldnt get results. I've designed it in multisim in the same way as it appears in circuit except for replacing 3v input votage for IC with 5v IC. Does that change the result?

 

If you scroll down the page, there is a detailed circuit description (in the link you posted)

For the simulation, what resistance did you use for the probes?

 

I tried short circuit, 1k ohm and 500k ohm. LED didnt blink for any of those values

 

The circuit is designed to use a CD4093 Cmos IC that has a minimum supply of 3.0V. The 3V battery will drop to 2.0V over its life then the circuit will not work for most of the time.
When I re-designed the circuit a few years ago I used a 74HC132 high speed Cmos IC that works when its supply is as low as 2.0V.

You replaced the IC with WHAT??

 

Multisim does some strange things sometimes. E.g. put 5M between the probes (equals no-water condition) and put the 47k potentiometer to 5%.
You may have to play around a bit with the resistor divider values R2/R3. It doesn't work initially because the voltage on the input of gate IC1B doesn't get over the threshold.

Also, in my case the simulation is so slow, that I can't see the second oscillator output change, so I decreased C3 to 10n, just to see if it works. If you verify the output of the last gate with an oscilloscope you'll see that it works.

 

Maybe he replaced the 3V to 18V Cmos IC with an old 74xx TTL IC that needs a 5V supply.

 

In multisim simulation software I use , there is no IC4093 as specific component. Instead schmitt NAND gates with predifined input voltages are present(5v,10v,15v input voltages for the gates is only present). So instead of using 3V input IC4093 i used 4 gates whose input voltage is 5v. Thats what i meant when i mentioned " replacing 3v input votage for IC with 5v IC". But led is not blinking when circuit is simulated. led current is in pA. I cant understand where the problem is?

 

See the attached screenshot. (my multisim is in german, don't bother) You don't have the 4093 under Main Database / CMOS 5V?
What Multisim is it ?

 

Ya i exactly used same IC as in picture. IC 4093_5v. But its 3V input in the circuit( http://www.redcircuits.com/Page18.htm).

 

Ya i exactly used same IC as in picture. IC 4093_5v. But its 3V input in the circuit( http://www.redcircuits.com/Page18.htm).

Yes you're right. The simulation doesn't work with 3V. However, it should in real-life, because the thresholds are not fixed, they depend on the supply voltage.
But, as previously stated, 3V is the lower supply voltage limit, it would be better to use a slightly higher voltage.

 

This is the circuit I simulated using Multisim. Can anyone say where the circuit is going wrong? Is it because of 5V input to IC?
I am just simulating the circuit as in the website http://www.redcircuits.com/Page18.htm . So I'm insisting on keeping the voltage 3v. However changing the voltage is not creating much variation in led current.

 

Multisim does not know that a Cmos Schmitt trigger can oscillate.
U2C oscillates continuously and U2b is a gated oscillator that drives the transistor to blink the LED.

 

Multisim does not know that a Cmos Schmitt trigger can oscillate.

What does that mean?

 

"Originally Posted by Audioguru
Multisim does not know that a Cmos Schmitt trigger can oscillate."
What does that mean?

The article explains how the circuit works with two Cmos Schmitt trigger oscillators. I don't think Multisim knows about this kind of oscillator.
Here are the two oscillators:

 

Multisim does not know that a Cmos Schmitt trigger can oscillate.

Negative. My Multisim does "know" that they oscillate. I tested it several times. It even gives the right frequency.

 

Ok. Multisim seems to expect a power supply that corresponds to the name of the device you chose. So if you chose 4093BT_5V it thinks that the chips power supply is 5V. Therefore it also adjusts the input threshold voltage levels to a certain level which cannot be changed.
In real life the threshold levels are proportional to the supply voltage.

Try using the 4093BT_15V and use a power supply of e.g. 10V. It won't work neither. It's a limitation of MULTISIM. The circuit works if the power supply voltage corresponds to the device name voltage.