I need help on using a transistor as a switch.

If you've built a project from the magazine, or have something fun and interesting to share, show it off here!
Post Reply
User avatar
cheapNdisgusting
Posts: 79
Joined: Sat Feb 12, 2005 1:01 am
Location: On the bank of, and sometimes in the Castor river
Contact:

I need help on using a transistor as a switch.

Post by cheapNdisgusting » Sun Dec 14, 2014 6:00 pm

[The extension jpg has been deactivated and can no longer be displayed.]

I've made several of these and they are called "fish callers". 9 volt battery operated, Put it in a sealed waterproof container like a pill bottle, add enough weight to cause it to sink, and with a string tied to it - drop it in and I can pull it back up. Works OK (didn't say great - just OK).

It is a pain to use because the container has to be opened to snap the battery in - then sealed back up.

What I want to do is add a transistor (PN2222A) and have it act as a switch. A base wire and a + wire protruding out of the pill bottle about 1/2" apart (waterproofed of course) and when the call is lowered into the water the conductivity of the water would make the circuit and fire the base with enough voltage. Then when it is removed from the water - it automatically shuts off.

I've done this same thing with the water before using the 2 wires as a water detector - works fine.

My question is just where exactly in the circuit to put the transistor - see the red circles in the schematic. A - B - or C?

Or interrupt the wire going to one or the other bases on the transistors that are already in place and not adding one.


Incidentally anyone is welcome to try this - it works best on drawing minnows into an area.

Thanks in advance for any and all comments.
cNd

dyarker
Posts: 1629
Joined: Fri Aug 22, 2003 12:01 am
Location: (somewhere), Afghanistan; from Rochester, NY
Contact:

Re: I need help on using a transistor as a switch.

Post by dyarker » Mon Dec 15, 2014 5:23 am

Location A or B for insertion On/Off switch transistor okay. Location C allows possible leak path through Q2 and Q1.

Let's pick A. Disconnect emitter of Q2 from battery +. Emitter of Q3 (2222) to emitter of Q2. Collector of Q3 to battery + and outside the pill bottle. Base of Q3 outside.

If gain of Q3 is too low, or resistance of water between outside probes to high, then Q3 maybe won't turn on enough to operate the circuit.

Alternate is location B for Q3. That puts the circuit on collector side of Q3, allowing more voltage in base connection.
Disconnect line from battery - that goes to emitter of Q1 and speaker. Connect that to collector of Q3. Connect emitter of Q3 to battery -. Add probe to outside at battery +. A resistor from Q3 base to other outside probe. Value of resistor to allow saturation of Q3 when water between probes.

good luck,
Dale Y

User avatar
cheapNdisgusting
Posts: 79
Joined: Sat Feb 12, 2005 1:01 am
Location: On the bank of, and sometimes in the Castor river
Contact:

Re: I need help on using a transistor as a switch.

Post by cheapNdisgusting » Mon Dec 15, 2014 7:02 am

Thanks Dyarker for the info. I appreciate how in-depth you explained it.
cNd

User avatar
cheapNdisgusting
Posts: 79
Joined: Sat Feb 12, 2005 1:01 am
Location: On the bank of, and sometimes in the Castor river
Contact:

Re: I need help on using a transistor as a switch.

Post by cheapNdisgusting » Mon Dec 15, 2014 1:46 pm

dyarker I tried both of your suguestions and both were correct. Option A - the gain was too low (exactly as you said) and it took touching the probes together to actually turn on Q3.

Then I did option B and it works perfectly.

Thanks again for the help. Much appreciated.

cNd
cNd

User avatar
Lenp
Posts: 1316
Joined: Thu Oct 26, 2006 7:11 pm
Location: Maryland
Contact:

Re: I need help on using a transistor as a switch.

Post by Lenp » Mon Dec 15, 2014 3:58 pm

I produce a 'water finder' for well depth observations and a darlington as the switch. With no water, the leakage current is nil, so no on/off switch was used.
Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
"I must be on the way to success since I already have the junk". (Me)

User avatar
haklesup
Posts: 2828
Joined: Thu Aug 01, 2002 12:01 am
Location: San Jose CA
Contact:

Re: I need help on using a transistor as a switch.

Post by haklesup » Thu Nov 03, 2016 2:13 pm

My first thought is to use an Air switch, similar to what I use to turn my garbage disposal on and off. It would be simple, mechanical and no issues with current leakage (not that safety is a concern with a 9V battery). You might need a bigger pill bottle though. For 4 bucks, its worth a try

http://www.ebay.com/itm/Garbage-Disposa ... SwZ8ZW7i5Q

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest