Hey guys can anyone help me with a water detector included is the wiring diagram, the only thing is I am not sure I have R2 hooked up correctly.

Is R2 supposed to be a potentiometer? I couldn't find any 1M pots, so to test I just put in a regular 1M Ohm resistor

other than that everything else is wired up per diagram... I am not getting any closure on the relay with the sensor probes in water...

what am I doing wrong? I do have a couple of 100k pots, maybe I can switch out?

I rebuilt this circuit without pcb and without the relay, it seems to function correctly, but when I add in the relay, things go south...

thanks,

-Jason

 

What are the specifications for the relay you are using?

The circuit you are building is taken from "Getting Started in Electronics" by Forrest Mims.

The relay that he suggests to use has a 500 ohm coil resistance.


hgmjr

 

The transistor does not have enough gain.

Try using a Darlington configuration.

Like this:

If that isn't enough gain, another stage could be added; collector resistor would need to go to +5v though.

I've been a fan of Forrest Mims III's books since they first came out. They are not completely error-free, however nearly all of the circuits will work if assembled properly.

He did a couple of generations of electronics enthusiasts a big favor by writing all those books.

 

Yes this is from his book.

here are the specs for the relay I am using:
-Maximum switching power of 10 (watt/VA)
-Maximum switching voltage of 60 VDC, 120VAC
-Maximum switching current of 0.5 A
-Maximum initial contact resistance of 150m ohms

and it looks like its a 250 Ohm (thats whats written on it anyway)

http://www.radioshack.com/product/index.jsp?productId=2062478#tabsetBasic

What are the specifications for the relay you are using?

The circuit you are building is taken from "Getting Started in Electronics" by Forrest Mims.

The relay that he suggests to use has a 500 ohm coil resistance.


hgmjr

 

You need to get the 1 Meg potentiometer to give you the range of adjustment that will be needed to set the sensitivity of the detector.

hgmjr

 

Yes, his books gave me hours of enjoyment (read:frustration) as a youngster... I havent picked up a soldering iron in 10 years... I have forgotten most of what little I learned back then.

He lives just up the road from San Antonio if I remember.

I will give the darlington a try.

thanks!

-jason

The transistor does not have enough gain.

Try using a Darlington configuration.

Like this:

If that isn't enough gain, another stage could be added; collector resistor would need to go to +5v though.

I've been a fan of Forrest Mims III's books since they first came out. They are not completely error-free, however nearly all of the circuits will work if assembled properly.

He did a couple of generations of electronics enthusiasts a big favor by writing all those books.

 

Wookie, I was not able to find a 1M Potentiometer I have 1K, 10K, &100K pots can I substitute the 100K where the 1M is and up the fixed resistor value?

-jason

 

Well crap! radio shack didn't have any 1M pots back to the drawing board...


Thanks for all the help!

-Jason

You need to get the 1 Meg potentiometer to give you the range of adjustment that will be needed to set the sensitivity of the detector.

hgmjr

 

There is always Digikey if you don't mind ordering online.

hgmjr

 

Wait a minute.

Go ahead and use your 100k pot.

If the relay still doesn't close, you need still more gain in the transistor stage.

Pure distilled water is a very good insulator. You may very well need more gain.

Also, you may have mis-identified the leads to the transistor(s). Please triple-check them.

 

You could check the Radioshack for a 500K potentiometer. Then you could use a 470K resistor in series with it to get you close enough to the 1M. This would give you a reduction of adjustment range.

hgmjr

 

Will do thanks for all the help!


I'm using 2N2222 Transistors, pin 1(bottom pin IF flat side is on left IF viewed from top) is emitter. that is what should be going to ground right?

What's the difference in a 2N2222 and a 2N3904 I see the 3904 has a 350mW Dissipation and the 2222 has 1.8W Dissipation... Wait a minute

As I am looking on the packaging I see the small print that says "Viewed from bottom" Hmm, OK maybe I have the Collector and the Emitter Swapped.



-J

Wait a minute.

Go ahead and use your 100k pot.

If the relay still doesn't close, you need still more gain in the transistor stage.

Pure distilled water is a very good insulator. You may very well need more gain.

Also, you may have mis-identified the leads to the transistor(s). Please triple-check them.

 

That will definitely keep things from working.

hgmjr

 

I'm using 2N2222 Transistors, pin 1(bottom pin IF flat side is on left IF viewed from top) is emitter. that is what should be going to ground right?

You have to view them from the bottom.

What's the difference in a 2N2222 and a 2N3904 I see the 3904 has a 350mW Dissipation and the 2222 has 1.8W Dissipation...

Actually, TO-92 packaged devices all have a power dissipation limit of 625mW. It's a function of the package, not the individual transistor.

2N3904/PN3904 transistors are rated for 200mA maximum collector current, but the realistic maximum is 100mA.

2N2222/PN2222 transistors are rated for 800mA maximum collector current, but the realistic maximum is 400mA.

... Wait a minute

As I am looking on the packaging I see the small print that says "Viewed from bottom" Hmm, OK maybe I have the Collector and the Emitter Swapped.

There you go - that's why I suggested "triple checking".

It's very easy to mis-read datasheets.

 

I agree that it is very easy to misread a datasheet.

It never hurts to review your circuit hookup whenever the circuit does not work.

hgmjr

 

The purpose of the diode is to protect the transistor from the flyback voltage that can occur when the transistor is switched off.

Look closely at the orientation of the diode. It is reverse-biased when the relay is powered.

hgmjr

 

well, I am really glad the fate of the world doesn't rest on my ability to put together simple circuits.. I have been trying this silly thing for several hours... I have been able to get my transistors hot... but thats about it.

I am going to bed, and shall try again tomorrow hopefully some rest will help...

ahhh I am beginning to remember why I stopped doing this so many years ago. even the small victories are hard to win.

thanks everyone for the help...

-Jason

 

It's OK - if you find yourself getting frustrated, give it a rest and do something else for a bit.

It'll be there when you get back, patiently waiting for you.

 

Don't get too frustrated. Sometimes it helps to set a problem aside and come back to it later. You can hit it fresh tomorrow.

hgmjr

 

Good luck with with your water detection circuit troubleshooting today.

hgmjr