"John Fields" wrote in message
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On Mon, 13 Jul 2009 02:34:52 +0000 (UTC), chuckcar
wrote:

John Fields wrote in
m:

On Sat, 11 Jul 2009 21:50:55 +0000 (UTC), chuckcar
wrote:

"ian field" wrote in
:


"chuckcar" wrote in message
...
"hhgggff" wrote in
news
My friends dad is building a Tardis for his grandson.

We have rigged up the blue flashing light using a 12 volt car
battery, we now need to sort out the sounds.

I was thinking of using an old cars cassette tape, but I have an old
9 volt portable cassette handy.

Will it burn down the TARDIS if I use that instead ?

The tape motor will run at the wrong speed for one. What you need is
a voltage regulator. A 7809 should do the trick. The 7800 series
convert DC voltages. Granted a transformer does it, but these handle
*much* wider voltage inputs and are tiny in comparison. A 7812 hooked
to the +12v connection with the other connection hooked to a 200
microfarad capacitor should give you a rock solid +9v out. You'll
need a piece of breadboard about 1" square to mount these on.


You should read up on a subject you know little about before giving
advice to others.

Considering the fact that my post is *far* more detailed than yours,
you're hardly a person to make such a judgement = along with your errors
detaied below.

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The fact that your post was more "detailed" than Ian's doesn't mean that
his was wrong.
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Even in older cassette recorders the motors had centrifugal speed
governors and modern ones have an IC speed controller (otherwise
they'd run slower and slower as the battery was used).

And none of this would burn out with 1 1/3 times the voltage input?
dubious.

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Just conjecture without knowing more about the cassette recorder.
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Your mention of transformers is misleading, without a "chopper"
circuit to convert DC into AC a transformer will burn out.

He's *using* a DC power source - a car battery. So a rectifier circuit
or an analogue is completely unncessary

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But, since his application runs on DC, your reference to a transformer
was misleading since a transformer takes an AC input and supplies an AC
output which must then be rectified, filtered, and possibly regulated
before it can be used by the DC input device.

If, by "transformer", you meant an AC to DC converter, then your
terminology was wrong.
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The OP is best advised to ask people who are equipped to give accurate
advice, such as the folk on News:sci.electronics.basics .

One *minor* correction of my post however - that *should* have been a
7809 not a 7812. This I picked up immediately when I read your reply. A
pretty basic error you didn't even notice. a 7812 converts *to* 12v
whereas a 7809 converts to 5v. Hence their names.

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???

A 7809 converts to 9V, not 5V.

Message-ID:

In addition, not knowing the current requirements of the recorder, your
comment to build it on one square inch of breadboard (perfboard?) may
well be inconsistent with the heat-sinking required for the regulator.

And why the 200µF capacitor?

Why not 10?

Too small to absorb the variations

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What variations?
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Why not 1000?

Way too big for the voltage.

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Why?



A 1000µF cap on the output of a 78xx or 79xx regulator will work just fine.
It's just not necessary, but I don't think the power up surge of charging
the cap would hurt the regulator any, and once it's charged the circuit
should work fine.

If you are making a half million gizmos, you would want to use the smallest
cap you can get away with. Use a cap that cost a dollar less, and you just
saved a half million dollars.