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PNP as switch with different emitter voltage

--- That, however, doesn't diminish its excellence.

Here's Larkins's suggestion:

. +18 . | . [PIEZO] . | . +----+ D . | Q|----G 2N7002 . | _| E . | Q| | . +----+ GND

And here's Kirwan's, arguably the best 2 transistor submission:

. +5 +18 +18 . +----+ | | | . | Q| | [22k] | . | _| B | E . | Q|O---[12k]--E C----+----B PNP . +----+ NPN C . | . [PIEZO] . | . GND

In both examples the piezo works at the proper time, and in view of the OP's statement that he had PCB real estate issues and wanted to do the least possible amount of modification to the PCB, which would you recommend: two transistors and three resistors or a single transistor?

--- JF

Reply to
John Fields
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Kirwan's is actually mine with "student-calculated" resistor values ;-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson 

--
I know, and an excellent design it is!

Larkin's design is the same as mine, but posted about 12 hours later.
;)
Reply to
John Fields

What? He had to wait for it to appear in A of E ?:-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

Well. Two resistors (unless the +5 isn't readily available and a resistor divider is to be used to develop that base voltage -- then 4.) Regardless, the OP also said the reason for the /Q choice is that there is a pad just sitting there open and unused on /Q. Since the OP said "no" to two BJTs, too, neither of the above appears to be "a solution."

So, no choice is possible given the above two choices and the OP's boundary criteria.

By the way, Jim suggested the idea although I've used that exact method many times, myself. So I immediately recognized it. I actually like it and have discussed it more than a few times in sci.electronics.*, though sometimes pulling current via the emitter isn't workable if the heavy load itself is on the collector and the driver has to sink all of it.

I also took a look at the OP's own idea about using an NPN to bypass the piezo. It _might_ work, but cannot be guaranteed to do so without more information. Testing would be required if he's dead-set on one BJT and the /Q pad. Hope it's not powered by a battery, though -- it always draws current. (I don't remember if the OP mentioned how it was powered, but

18V might be a pair of 9V batteries.)

Jon

Reply to
Jon Kirwan 

--
Brain fart error on my part, I looked at the piezo and counted it as a
resistor. :-(
Reply to
John Fields

not much!

Jamie

Reply to
Jamie

Hey, thats a good idea, dual package logic transistor..

Reply to
Jamie

Aye, thanks Jon. I think even if I did get it all dialed in it probably wouldn't be very stable, if the temperature change.

Jon

Reply to
Jon Danniken

Aye, indeed you are correct. I got the circuit breadboarded earlier today, and although I was seeing 0.7mA with a 22k base resistor, with a 16v zener on the base, it only drew 0.1mA. I subbed in a 5.6k resistor, and I'm just a hair below 0.5mA.

The peizo beeper is getting it's full voltage though, so I'll stick with this combination. Since the transistor has an Hfe of 170, running it at a fifth of that should give me adequate saturation.

Thanks again for the suggestion to use the zener, it's working perfectly, and I only had to bugger piggyback the existing circuitry at one point (to pull off the 18V before the regulator).

Jon

Reply to
Jon Danniken

Thanks Spephro. I ended up going with 0.5mA Ib to control my 15mA load, so I'm going with 30. The transistor is measured at 170, so I think that should be good.

Jon

Reply to
Jon Danniken

Thanks to everyone who contributed to this thread. I ended up going with Pimpom's suggestion of a zener on the base, which is working well for me.

I ended up learned a lot more than I had set out to do, which, of course, is half the fun of doing a project like this, and I printed a few notes to stick in my Engineer's Notebook for the next time I have a funny project.

Thanks again for all of the suggestions, they are all very much appreciated.

Jon

Reply to
Jon Danniken

Pimpom a écrit : > John Fields wrote: >> On Sun, 16 Jan 2011 16:03:00 -0800, John Larkin >> wrote: >>

So use the FET common gate tied to Vcc, and hook the FET source to the logic output.

Just one transistor...

--
Thanks,
Fred.
Reply to
Fred Bartoli

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