On Thu, 26 Mar 2009 17:12:00 -0800, David Nebenzahl
wrote:

On 3/26/2009 9:21 AM PlainBill spake thus:

On Wed, 25 Mar 2009 18:01:12 -0800, David Nebenzahl
wrote:

On 3/25/2009 10:24 AM PlainBill spake thus:

Contrary to what several nay-sayers have expressed, this exercise has
a LOT to do with electronics repair. Any fool can keep replacing
components until he finds the bad one. The successful tech will be
able to analyse a circuit and make meaningful tests to identfy the
failing part.

Thank you. It's more than a little annoying being completely written off
as a clueless idiot simply because I reversed a diode's polarity *in a
schematic* (since corrected).

NOW your next exercise is to identify the values for these components.


I'd love to oblige you; unfortunately, I'm not yet at that level. I know
Ohm's law and some other basic stuff, but not enough about circuit
design to assign values with any confidence. I look forward to others
doing that. And someday, I intend to get a good basic electronics
textbook and seriously study it ...

Several comments on this circuit. First of all, it could work, but
would require significant improvements, some of which have already
been discussed. Rather than using a doorbell or chime, a Sonalert
could be used. In part this will depend on the sound level desired.

So what improvements would you suggest to the current circuit (after I
made the changes recommended earlier)? (Circuit at
http://www.geocities.com/bonezphoto/...e-shotBell.gif)

You must add surge supression if you are using an inductive load.
Without it, the pulse generated when the transistor turns off will
eventually destroy it.

Second, some of the criticisms are valid. If you intend to do this as
a design exercise you can have fun designing it, but I would not
recommend building it. You are dealing with potentially lethal
voltages, and fairly expensive components to withstand those voltages.

To be honest, I think it's more fun designing things like this that will
operate "directly" off line voltage (i.e., without a transformer). One
of the criteria of this whole deal was to avoid the use of a transformer.

And in defense of that, there are tons of things in use every day that
operate just this way. Latest example I found were a bunch of electric
staple guns a neighbor gave me that operated directly off 120 volts,
firing a solenoid through a capacitor.

If I were to actually build this, I'm confident I could do it safely--at
least as safely as those staple guns, which have UL, CSA, etc., compliance.

And what components would be expensive? Seems to me the most expensive
part would be the transistor, or possibly the two power resistors, but
even those aren't terribly pricey.

Cost would depend on the nature of the load. If you are using a
Sonalert the load would be under .04A.

On the other hand, if you are using a chime the load will be higher,
and using a doorbell REALLY opens a can of worms. I think you had
better expect a load of .5A, and one heck of an inductive kick as the
transistor turns off. The typical doorbell is designed to operate on
about 15VAC. I would expect rater short contact life if you run it
off DC.


IMHO, you are using the wrong approach. Over 30 years ago I built
something similar using a few small caps, a few resistors, a diode, a
speaker, and a 74C914 hex schmidt trigger. I used 4 AA cells to power
it, your application should use a 'wall wart' putting out 6VDC at 100
ma. I'd have to look up the specs, but a 555 timer would also do the
job.

Could do, but see above.

PlainBill