Transistor circuit what works when it shouldn't? Why?

I'm looking to build a circuit that will alert me to the location of a
lost model airplane based on modifying a commercially available lost
plane finder. I de-soldered the wimpy 5 VDC piezo alarm off the board
of the commercial unit. I probed the two pads where the piezo alarm was
powered and confirmed I get a steady on-off pulse of 5VDC with a
one-second interval. (I did a test with a small reed relay and got
enough current to throw the relay and close a circuit on a much more
powerful 12VDC piezo siren (this unit has a piezo driver and just needs
12VDC and it starts hollering!)

I would have just stuck with this however my goal was to get all the
goodies back into the black case the loud piezo siren. Space is tight
so I looked to use a switching transistor instead. I hooked up the
commercial lost plane finder so that it had power and as I was
completing the wiring it just started working! My intention was to
connect the emitter to the negative side of the Piezo siren/lost plane
finder (labeled simply as "on/off 5 VDC) in the photo)

Now I'm confused why this is working and am hesitant to start hooking
up more wires when its working already.

How can this circuit work with the emitter not connected to anything?

Here's a link to the photo I took of a schematic I have drawn of the
current working config.

http://www.rcgroups.com/gallery/showphoto.php?photo=21204&sort=1&cat=500&page=1

Any thoughts on this?

Thanks, Lawrence

 

That is one cute circuit.
The transistor is conducting through its base-collector junction, which
is basically a diode.
Your batteries are in series so that you actually have 17 volts
(nominal).

 

+12---------------Piezo------+
c|
330 /
+5------[R1]-------+------b| NPN
| \
-(5)-----+ 10K [R2] e|
| | |
-(12)----+---------+---------+

 

Sorry, I don't understand. I checked the voltage with an analog meter
at the 12 Volt piezo and it's 12 volts so I dont see that the piezo is
"seeing" anything more then just the 12 volts powering it.

Any other ideas anybody?

Lawrence

 

A previous poster is correct, in that you have simply forward-biased a
PN junction (the B-C path), and thus have no switching control. Now,
could you clarify that schematic? What is the 5V on-off thing you
have drawn there? You say it's the piezo alarm, but you have a second
box labeled "pieza siren." What is really going on there?

 

The 5 volt on -off thing is a commercially available lost plane finder.
It works well and also alerts you to low battery voltage etc. It just
isn't all that loud. As I mentioned, I removed the piezo off the
board of the unit and are now using the 5 volt signal that used to
power the little piezo to switch on a much bigger noise maker.

My intention was to use the switching transistor as a relay.

Does this make sense?

 

---
Yes. Your circuit should be hooked up like this: (View in a
non-proportional font like Courier


+-------+
| |
[SIREN] |
| |+
C [12V]
+----[1KR]---B [BAT]
+| E |-
[PLANE ] | |
[FINDER] | |
-| | |
+--------------+-------+

 

A small current at the base of a transistor controls a larger
current through the emitter-collector. In your diagram, a large
current exists at the base, and there is no emitter current.
Your transistor is acting as a diode. That is an undesireable
situation for what you want.

The base current of a transistor should be kept small - too much
current through the base will burn the transistor out. There is
nothing in your existing setup to protect the base from too
much current.

The diagram I drew shows the transistor properly configured
as a switch, with a small base current (that current is limited
by resistor R1) that controls a large emitter-collector current.

I added some insurance in the circuit with R2. It is very
likely not needed, but insures that the base of the transistor
is connected to something (ground in this case) in the event
that your switched 5V circuit leaves it "floating".

Ed

I checked the voltage with an analog meter

 

HI John, I tried again and imported your schematic into word and this
time I could decipher. I got it wired up and it works! Brillant!

Thanks John, and thanks everybody for helping me through this.

Next project will be to build my own lost plane finder from scratch. I
plan to have it start making noise once the transmitter is shut off an
it looses signal.