transistor astable multivibrator

[John]

Hi, when i make one of these using say 2n3904's and a couple of 10uf caps
with a cycle time of about 1hz (r=100k approx) i find that the
extinguishing led (the other will be lighting) never quite goes out and
it spoils the alternating light effect. What I'd like is a completely
extinguishing led when the led goes off as the other lights. I could use
a 555 timer and I have but I'd like to do it with transistors.

Would darlingtons do it or perhaps the cycle rate is too short?
Any advice greatly appreciated.

tnx

 

[Rich Grise]

On Wed, 18 Nov 2009 18:04:53 +0000, John wrote:

Hi, when i make one of these using say 2n3904's and a couple of 10uf caps
with a cycle time of about 1hz (r=100k approx) i find that the
extinguishing led (the other will be lighting) never quite goes out and it
spoils the alternating light effect. What I'd like is a completely
extinguishing led when the led goes off as the other lights. I could use a
555 timer and I have but I'd like to do it with transistors.

Would darlingtons do it or perhaps the cycle rate is too short? Any advice
greatly appreciated.

Can you post a schematic? According to your description, there's something

wrong with your design, which we can't diagnose if we don't know what you
have now.

I've never had a problem with multivibrator transistors not cutting off
completely. If you've got a lame circuit from the get-go, then darlingtons
will only make it worse.

Thanks,
Rich

 

[Jon Kirwan]

On Wed, 18 Nov 2009 18:04:53 +0000, John <John@nospam.thanks> wrote:

Hi, when i make one of these using say 2n3904's and a couple of 10uf caps
with a cycle time of about 1hz (r=100k approx) i find that the
extinguishing led (the other will be lighting) never quite goes out and
it spoils the alternating light effect. What I'd like is a completely
extinguishing led when the led goes off as the other lights. I could use
a 555 timer and I have but I'd like to do it with transistors.

Would darlingtons do it or perhaps the cycle rate is too short?
Any advice greatly appreciated.

I'm just guessing, as I don't know what circuit you are using, but do
the two LEDs have resistors in parallel with them? Something like 10k
ohm? If not, add them and see what happens.

Jon

 

[pimpom]

John wrote:

Hi, when i make one of these using say 2n3904's and a couple
of 10uf
caps with a cycle time of about 1hz (r=100k approx) i find that
the
extinguishing led (the other will be lighting) never quite goes
out
and it spoils the alternating light effect. What I'd like is a
completely extinguishing led when the led goes off as the other
lights. I could use a 555 timer and I have but I'd like to do
it with
transistors.

Would darlingtons do it or perhaps the cycle rate is too short?
Any advice greatly appreciated.

As others have said, we're kind of groping in the dark here, not
knowing details about your circuit. If you're using the classic
text-book astable MV, the only reason I can think of right now is
that the 10ľF timing caps are leaky and are pulling some current
through the LEDs even when they are supposed to be off. Any
chance that they are electrolytics and you have them connected
with the polarity reversed?

 

[John]

On Thu, 19 Nov 2009 02:25:15 +0530, pimpom wrote:

John wrote:
Hi, when i make one of these using say 2n3904's and a couple of 10uf
caps with a cycle time of about 1hz (r=100k approx) i find that the
extinguishing led (the other will be lighting) never quite goes out
and it spoils the alternating light effect. What I'd like is a
completely extinguishing led when the led goes off as the other lights.
I could use a 555 timer and I have but I'd like to do it with
transistors.

Would darlingtons do it or perhaps the cycle rate is too short? Any
advice greatly appreciated.


As others have said, we're kind of groping in the dark here, not knowing
details about your circuit. If you're using the classic text-book astable
MV, the only reason I can think of right now is that the 10ľF timing caps
are leaky and are pulling some current through the LEDs even when they are
supposed to be off. Any chance that they are electrolytics and you have
them connected with the polarity reversed?

They are polarized electrolytics but they are not reversed. I could try
tantalum beads.

 

[John]

On Wed, 18 Nov 2009 11:34:03 -0800, Jon Kirwan wrote:

On Wed, 18 Nov 2009 18:04:53 +0000, John <John@nospam.thanks> wrote:

Hi, when i make one of these using say 2n3904's and a couple of 10uf
caps with a cycle time of about 1hz (r=100k approx) i find that the
extinguishing led (the other will be lighting) never quite goes out and
it spoils the alternating light effect. What I'd like is a completely
extinguishing led when the led goes off as the other lights. I could use
a 555 timer and I have but I'd like to do it with transistors.

Would darlingtons do it or perhaps the cycle rate is too short? Any
advice greatly appreciated.

I'm just guessing, as I don't know what circuit you are using, but do the
two LEDs have resistors in parallel with them? Something like 10k ohm?
If not, add them and see what happens.

Jon

No they haven't. Thanks for that info.

 

[John]

On Wed, 18 Nov 2009 11:09:41 -0800, Rich Grise wrote:

On Wed, 18 Nov 2009 18:04:53 +0000, John wrote:

Hi, when i make one of these using say 2n3904's and a couple of 10uf
caps with a cycle time of about 1hz (r=100k approx) i find that the
extinguishing led (the other will be lighting) never quite goes out and
it spoils the alternating light effect. What I'd like is a completely
extinguishing led when the led goes off as the other lights. I could use
a 555 timer and I have but I'd like to do it with transistors.

Would darlingtons do it or perhaps the cycle rate is too short? Any
advice greatly appreciated.

Can you post a schematic? According to your description, there's something
wrong with your design, which we can't diagnose if we don't know what you
have now.

I've never had a problem with multivibrator transistors not cutting off
completely. If you've got a lame circuit from the get-go, then darlingtons
will only make it worse.

Thanks,
Rich

Ok about the darlingtons. I'll post the circuit later - bed time here in
the UK.

 

[petrus bitbyter]

"John" <John@nospam.thanks> schreef in bericht
news:he1ult$m4p$1@news.datemas.de...

On Thu, 19 Nov 2009 02:25:15 +0530, pimpom wrote:

John wrote:
Hi, when i make one of these using say 2n3904's and a couple of 10uf
caps with a cycle time of about 1hz (r=100k approx) i find that the
extinguishing led (the other will be lighting) never quite goes out
and it spoils the alternating light effect. What I'd like is a
completely extinguishing led when the led goes off as the other lights.
I could use a 555 timer and I have but I'd like to do it with
transistors.

Would darlingtons do it or perhaps the cycle rate is too short? Any
advice greatly appreciated.


As others have said, we're kind of groping in the dark here, not knowing
details about your circuit. If you're using the classic text-book astable
MV, the only reason I can think of right now is that the 10ľF timing caps
are leaky and are pulling some current through the LEDs even when they
are
supposed to be off. Any chance that they are electrolytics and you have
them connected with the polarity reversed?

They are polarized electrolytics but they are not reversed. I could try
tantalum beads.

Tantalum will not make a difference except that they are more sensitive for
reverse voltage.
Below you find the classic a-stable. Looking in my crystal ball I see you
may have forgotten the series resistors of the LEDs.


+-------+----+-------+- Vcc 5Vdc
| | | |
.-. .-. .-. .-.
| | | | | | | |
| | | | | | | |
'-' '-' '-' '-'
| | |100k | 270
| | | |
V LED | | V LED
- | | -
| #| | | |
+--#|---)----+ 10uF |
| #| | | |# |
| +----)---|#--+
| | | |# |
| | | |
\| | | |/
|-----+ +-----| 2N3904
<| |>
| |
| |
+--------------------+---GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


petrus bitbyter

 

[George Herold]

On Nov 18, 7:12 pm, "petrus bitbyter"
<pieterkraltlaatdit...@enditookhccnet.nl> wrote:

"John" <J...@nospam.thanks> schreef in berichtnews:he1ult$m4p$1@news.datemas.de...





On Thu, 19 Nov 2009 02:25:15 +0530, pimpom wrote:

John wrote:
Hi, when i make one of these using say  2n3904's and a couple of 10uf
caps with a cycle time of about 1hz (r=100k approx) i find that the
extinguishing led (the other will be lighting) never quite goes out
and it spoils the alternating light effect. What I'd like is a
completely extinguishing led when the led goes off as the other lights.
I could use a 555 timer and I have but I'd like to do it with
transistors.

Would darlingtons do it or perhaps the cycle rate is too short? Any
advice greatly appreciated.

As others have said, we're kind of groping in the dark here, not knowing
details about your circuit. If you're using the classic text-book astable
MV, the only reason I can think of right now is that the 10ľF timing caps
are leaky and are pulling some current through the LEDs even when they
are
supposed to be off. Any chance that they are electrolytics and you have
them connected with the polarity reversed?

They are polarized electrolytics but they are not reversed. I could try
tantalum beads.

Tantalum will not make a difference except that they are more sensitive for
reverse voltage.
Below you find the classic a-stable. Looking in my crystal ball I see you
may have forgotten the series resistors of the LEDs.

   +-------+----+-------+- Vcc 5Vdc
   |       |    |       |
  .-.     .-.  .-.     .-.
  | |     | |  | |     | |
  | |     | |  | |     | |
  '-'     '-'  '-'     '-'
   |       |    |100k   | 270
   |       |    |       |
   V LED   |    |       V LED
   -       |    |       -
   |  #|   |    |       |
   +--#|---)----+  10uF |
   |  #|   |    |   |#  |
   |       +----)---|#--+
   |       |    |   |#  |
   |       |    |       |
    \|     |    |     |/
     |-----+    +-----| 2N3904
    <|                |
   |                    |
   |                    |
   +--------------------+---GND
created by Andy´s ASCII-Circuit v1.24.140803 Betawww.tech-chat.de

petrus bitbyter- Hide quoted text -

- Show quoted text -

Thanks petrus, I was wondering where the LED's were in the circuit.

George H.