<mrdarrett@[EMAIL PROTECTED]
> wrote in message
news:7edf1122-2064-4799-9661-4828c556a5ce@[EMAIL PROTECTED]
>I built an astable multivibrator with blinking lights (much simpler
> than using a 556!), as a test for a circuit that will use a power
> transistor (or MOSFET) to pulse a transformer primary for future
> experiments.
>
> http://mrdarrett.googlepages.com/blinkenlights002.pdf
>
> Strangely, when I replaced the two 2N3904 transistors with TIP31As,
> the circuit would not oscillate unless I briefly disconnected then re-
> connected R2 or R3. (I bumped the voltage up from 3V to 6V during
> these tests.)
>
> As a work-around, I'm considering just putting the 2N3904s back in,
> and connecting the positive end of C2 to the base of a TIP31A.
> Inelegant, but I think it will work.
>
> I'm trying to figure out why TIP31As won't work, but it also doesn't
> help me any that the TIP31A data sheet does not specify a minimum
> V_BE_on.
>
> http://www.st.com/stonline/products/literature/ds/12292/tip31a.pdf
>
> Input?
>
> Michael
One of the nice things about 555s and 556s is that they always start if
you
keep within the specifications in the data sheet. The main issue folks
have
is that they forget to bypass the power supply, which often causes
retriggering problems.
If you have a cmos 555 laying about, you can do this in a way that is
pretty
much guaranteed to start.
As a side note, the easy way to build a square wave oscillator from a cmos
555 is to connect the output pin to a resistor, then connect the other
side
of the resistor to ground through a capacitor. Now, connect both the
trigger
and threshold inputs to the junction of the resistor and capacitor.
Using this configuration, you can also use the discharge pin as an 'open
collector' output, and use it to drive arbitrary bits of circuitry. For
your
application, you might use the discharge to directly drive one LED, and
have
the other LED driven by a power PMOS or PNP transistor.
If the resistor you use is a pot, you can then adjust the frequency
without
affecting the duty cycle by varying the resistance. You can work out the
limits of the frequencies you can obtain, given your parts.
Note that the TTL outputs of NE555 variants don't easily lend themselves
to
this approach, due to the asymmetric current draw of the output for high
and
low output signals. CMOS versions offer nice rail-to-rail square wave
output.
This is a nice way to do it, which I learned here from a post by John
Fields
a couple of years ago. (Yet another example of what a nice resource this
group is). I'm not sure if it was his idea, or if he learned it from
somebody else. I haven't seen this idea in any of the data sheets I've
looked at.
Regards,
Bob Monsen
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