Simple circuit, stupid problem

[Eriswerks]

I'm trying to build a two transistor LED flasher, as shown at
http://www.reprise.com/host/circuits/transistor_flasher.asp. It should
be perfectly simple, but I must be making some mistake that I'm just
not seeing yet.

When I build this circuit, both LEDs light continuously rather than
flashing. I've tried varying the values of the resistors and caps that
should control the timing, but it doesn't make a difference. I can
actually pluck the capacitors right out of the circuit and both LEDs
are still lit. I've built this thing a dozen times now from several
different people's instructions, and I get the same result every time.

The transistors I have on hand are a little mixed... I've tried
2N3904s, 3906s, and 4424, any of which ought to work as they're all
NPN.

Does anyone have any suggestions as to what stupid mistake I'm making
here? I am starting to get sick of looking at this thing. Thanks in
advance..!

 

[Andrew Holme]

I'm trying to build a two transistor LED flasher, as shown at
http://www.reprise.com/host/circuits/transistor_flasher.asp. It should
be perfectly simple, but I must be making some mistake that I'm just
not seeing yet.

When I build this circuit, both LEDs light continuously rather than
flashing. I've tried varying the values of the resistors and caps that
should control the timing, but it doesn't make a difference. I can
actually pluck the capacitors right out of the circuit and both LEDs
are still lit. I've built this thing a dozen times now from several
different people's instructions, and I get the same result every time.

The transistors I have on hand are a little mixed... I've tried
2N3904s, 3906s, and 4424, any of which ought to work as they're all
NPN.

Does anyone have any suggestions as to what stupid mistake I'm making
here? I am starting to get sick of looking at this thing. Thanks in
advance..!

Have you reversed the base and collector leads? That would light both LEDs.

Failing that, with the capacitors temporarily removed -
1. Try grounding one of the bases - LED should go out
2. Try removing a 39k resistor - LED should go out
3. Try placing wet fingers across where the 39k was - LED should light

 

[John Larkin]

I'm trying to build a two transistor LED flasher, as shown at
http://www.reprise.com/host/circuits/transistor_flasher.asp. It should
be perfectly simple, but I must be making some mistake that I'm just
not seeing yet.

When I build this circuit, both LEDs light continuously rather than
flashing. I've tried varying the values of the resistors and caps that
should control the timing, but it doesn't make a difference. I can
actually pluck the capacitors right out of the circuit and both LEDs
are still lit. I've built this thing a dozen times now from several
different people's instructions, and I get the same result every time.

The transistors I have on hand are a little mixed... I've tried
2N3904s, 3906s, and 4424, any of which ought to work as they're all
NPN.

Does anyone have any suggestions as to what stupid mistake I'm making
here? I am starting to get sick of looking at this thing. Thanks in
advance..!

This circuit has two stable states: "oscillating" is one, and "both
transistors saturated" is the other.

If the transistors have high beta, saturation is more likely. In this
example, if betas are above about 80, the thing can lock up, and lots
of transistors are hotter than that. Bigger base resistors might help,
but that has other problems.

If you return the tops of the 39Ks to their own transistor's
collectors, rather than to V+, that will prevent saturation lockup.

Or maybe something else is wrong. If you briefly short one base to
ground, then turn it loose, it should start oscillating. If not, look
elsewhere.

John

 

[john jardine]

I'm trying to build a two transistor LED flasher, as shown at
http://www.reprise.com/host/circuits/transistor_flasher.asp. It should
be perfectly simple, but I must be making some mistake that I'm just
not seeing yet.

When I build this circuit, both LEDs light continuously rather than
flashing. I've tried varying the values of the resistors and caps that
should control the timing, but it doesn't make a difference. I can
actually pluck the capacitors right out of the circuit and both LEDs
are still lit. I've built this thing a dozen times now from several
different people's instructions, and I get the same result every time.

The transistors I have on hand are a little mixed... I've tried
2N3904s, 3906s, and 4424, any of which ought to work as they're all
NPN.

Does anyone have any suggestions as to what stupid mistake I'm making
here? I am starting to get sick of looking at this thing. Thanks in
advance..!

Offhand it looks like the Leds should be flashing about 4 times a second.
Ensure the 10uF capacitors have their "+" terminals connected to the
transistor collectors. If not the caps will 'leak' and keep both transistors
and Leds on.
regards
john

 

[Bob Eldred]

I'm trying to build a two transistor LED flasher, as shown at
http://www.reprise.com/host/circuits/transistor_flasher.asp. It should
be perfectly simple, but I must be making some mistake that I'm just
not seeing yet.

When I build this circuit, both LEDs light continuously rather than
flashing. I've tried varying the values of the resistors and caps that
should control the timing, but it doesn't make a difference. I can
actually pluck the capacitors right out of the circuit and both LEDs
are still lit. I've built this thing a dozen times now from several
different people's instructions, and I get the same result every time.

The transistors I have on hand are a little mixed... I've tried
2N3904s, 3906s, and 4424, any of which ought to work as they're all
NPN.

Does anyone have any suggestions as to what stupid mistake I'm making
here? I am starting to get sick of looking at this thing. Thanks in
advance..!

No Mistake, you have found one of the stable states of this circuit, both
trnasitors on. This can happen when things are closely balanced. This
circuit can be improved by adding two diodes that stop the posibility of
both transistors coming on together.

Disconnect the ends of the 39K resistors connected to +V on the NPN circuit.
Connect these together. Connect the anode of a signal diode, 1N4148 or
equiv, on the collector of each transistor. Connect the cathodes together
and to the common point of the two 39K base resistors.

This circuit provides base bias to the transitors by rectifying the square
wave on the collectors of the transistors. They can't latch on because a
both on state will rob the bias. The circuit starts because a linear
amplification region is traversed as the bias developes. This allows the
switching action to increase over several cycles until full operation is
attained.

The circuit will work equally well on both the NPN and the PNP versions of
the circuit but the diode polarity will change. All astable multivibrators
should have this or a similar circuit to insure operation. Check the
literature.

Be sure you are using the right polartiy transistors. A 3906 is a PNP part.
Bob

 

[John Fields]

I'm trying to build a two transistor LED flasher, as shown at
http://www.reprise.com/host/circuits/transistor_flasher.asp. It should
be perfectly simple, but I must be making some mistake that I'm just
not seeing yet.

When I build this circuit, both LEDs light continuously rather than
flashing. I've tried varying the values of the resistors and caps that
should control the timing, but it doesn't make a difference. I can
actually pluck the capacitors right out of the circuit and both LEDs
are still lit. I've built this thing a dozen times now from several
different people's instructions, and I get the same result every time.

The transistors I have on hand are a little mixed... I've tried
2N3904s, 3906s, and 4424, any of which ought to work as they're all
NPN.

Does anyone have any suggestions as to what stupid mistake I'm making
here? I am starting to get sick of looking at this thing. Thanks in
advance..!

---
You're not making any mistakes, it's a stupid circuit.

Try this:


+V>----+------------------------+
| |
[470] [470]
|A |A
[LED] [LED]
| |
+-[+10µF]--+ +-[10µF+]-+
| \ / |
| X |
| / \ |
C / \ C
NPN B-------+ +------B NPN
E | | E
| [39K] [39K] |
| | | |
GND>---+---------+-----+--------+

If it won't start, change one of the 39k's to 47k

 

[John Larkin]

I'm trying to build a two transistor LED flasher, as shown at
http://www.reprise.com/host/circuits/transistor_flasher.asp. It should
be perfectly simple, but I must be making some mistake that I'm just
not seeing yet.

When I build this circuit, both LEDs light continuously rather than
flashing. I've tried varying the values of the resistors and caps that
should control the timing, but it doesn't make a difference. I can
actually pluck the capacitors right out of the circuit and both LEDs
are still lit. I've built this thing a dozen times now from several
different people's instructions, and I get the same result every time.

The transistors I have on hand are a little mixed... I've tried
2N3904s, 3906s, and 4424, any of which ought to work as they're all
NPN.

Does anyone have any suggestions as to what stupid mistake I'm making
here? I am starting to get sick of looking at this thing. Thanks in
advance..!

---
You're not making any mistakes, it's a stupid circuit.

Try this:


+V>----+------------------------+
| |
[470] [470]
|A |A
[LED] [LED]
| |
+-[+10µF]--+ +-[10µF+]-+
| \ / |
| X |
| / \ |
C / \ C
NPN B-------+ +------B NPN
E | | E
| [39K] [39K] |
| | | |
GND>---+---------+-----+--------+

If it won't start, change one of the 39k's to 47k

But that circuit has a stable non-oscillating state: both transistors
off.

John

 

[John Fields]

On Sun, 21 Aug 2005 16:50:56 -0500, John Fields

You're not making any mistakes, it's a stupid circuit.

Try this:


+V>----+------------------------+
| |
[470] [470]
|A |A
[LED] [LED]
| |
+-[+10µF]--+ +-[10µF+]-+
| \ / |
| X |
| / \ |
C / \ C
NPN B-------+ +------B NPN
E | | E
| [39K] [39K] |
| | | |
GND>---+---------+-----+--------+

If it won't start, change one of the 39k's to 47k

But that circuit has a stable non-oscillating state: both transistors
off.

---
I think the 47k will take away that stability, but the circuit's got
more problems than that, since I forgot the cross-coupling
resistors.

Here's one from GE on abse:

 

[Jasen Betts]

http://www.reprise.com/host/circuits/transistor_flasher.asp

Does anyone have any suggestions as to what stupid mistake I'm making
here? I am starting to get sick of looking at this thing. Thanks in
advance..!

try running it off closer to 12v than 5V

you could also try putting 1K resistors in parallel with the LEDs.


here's my LED flasher circuit, drawn sideway's because it's easier to draw
diodes -|>|-- and resistors --[XXX]- running that way.

//
+3v--+--|>|---.
| | OV
|--[1K]--+----+--C E----+--
| | |
|--[10K]-+----|----B |
| | | |
| 47uF_L_ _L_+ |
| +~T~ ~T~47uF |
| | | |
|--[10K]-)----+----B |
| | |
|--[1K]--+-------C E-----+
| |
| // | for the transistors practically
+--|>|---' any small signal NPN should work

last time I used some scrounged from an old VCR

if the leds get stuck on try increasing the 10K resistor to 20K

use a fixed pitch font like courier if it looks garbled.

Bye.
Jasen

 

[John Fields]

http://www.reprise.com/host/circuits/transistor_flasher.asp

Does anyone have any suggestions as to what stupid mistake I'm making
here? I am starting to get sick of looking at this thing. Thanks in
advance..!

try running it off closer to 12v than 5V

you could also try putting 1K resistors in parallel with the LEDs.


here's my LED flasher circuit, drawn sideway's because it's easier to draw
diodes -|>|-- and resistors --[XXX]- running that way.

//
+3v--+--|>|---.
| | OV
|--[1K]--+----+--C E----+--
| | |
|--[10K]-+----|----B |
| | | |
| 47uF_L_ _L_+ |
| +~T~ ~T~47uF |
| | | |
|--[10K]-)----+----B |
| | |
|--[1K]--+-------C E-----+
| |
| // | for the transistors practically
+--|>|---' any small signal NPN should work

last time I used some scrounged from an old VCR

if the leds get stuck on try increasing the 10K resistor to 20K

 

[Jonathan Kirwan]

use a fixed pitch font like courier if it looks garbled.

The problem is that you used tabs in your text (0x09) and these are
not necessarily translated the same on various readers, regardless of
the fixed spacing of the font. When relying upon fixed spaced fonts,
I'd recommend avoiding tabs entirely.

Jon

 

[ehsjr]

http://www.reprise.com/host/circuits/transistor_flasher.asp

Does anyone have any suggestions as to what stupid mistake I'm making
here? I am starting to get sick of looking at this thing. Thanks in
advance..!

try running it off closer to 12v than 5V

you could also try putting 1K resistors in parallel with the LEDs.


here's my LED flasher circuit, drawn sideway's because it's easier to draw
diodes -|>|-- and resistors --[XXX]- running that way.

//
+3v--+--|>|---.
| | OV
|--[1K]--+----+--C E----+--
| | |
|--[10K]-+----|----B |
| | | |
| 47uF_L_ _L_+ |
| +~T~ ~T~47uF |
| | | |
|--[10K]-)----+----B |
| | |
|--[1K]--+-------C E-----+
| |
| // | for the transistors practically
+--|>|---' any small signal NPN should work

last time I used some scrounged from an old VCR

if the leds get stuck on try increasing the 10K resistor to 20K

use a fixed pitch font like courier if it looks garbled.

Bye.
Jasen

What limits the current through the LEDs?
Ed

 

[Kitchen Man]

The problem is that you used tabs in your text (0x09) and these are
not necessarily translated the same on various readers, regardless of
the fixed spacing of the font. When relying upon fixed spaced fonts,
I'd recommend avoiding tabs entirely.

Similarly, I like to avoid tabs when writing code. That way,
indentations always line up neatly, regardless of the user's source
code interface program.

 

[Jonathan Kirwan]

Similarly, I like to avoid tabs when writing code. That way,
indentations always line up neatly, regardless of the user's source
code interface program.

I insist on doing the same thing, as well, with what I write. There
was a time in my earlier days when the tab actually saved enough space
in source files that it really made a difference in one's life. That
day is long since gone. I use a little detab facility I wrote many
years ago when I'm faced with code written under some fixed tab stop
mentality. I can list out the tab stops or let it default, but it
neatly strips the tabs from the source.

Jon

 

[Dan Akers]

Eriswerks wrote;
"I'm trying to build a two transistor LED flasher, as shown at
http://www.reprise.com/host/circuits/transistor_flasher.asp. It should
be perfectly simple, but I must be making some mistake that I'm just not
seeing yet.
When I build this circuit, both LEDs light continuously rather than
flashing. I've tried varying the values of the resistors and caps that
should control the timing, but it doesn't make a difference."
______________________________________
Re;
This is one of the first circuits I ever built back when I was kid in
the early 70's and have used it many times sense. Your inquiry prompted
me to put this one together on a breadboard. I built it exactly per the
NPN schematic with a 10V supply. It worked great with a running freq or
about 4.2 Hz. I tried several methods of reproducing your
non-oscillating result and I couldn't get it to NOT oscillate. I tried
varying the supply voltage from as little to 3V to 12V with no effect
except for an increased freq and dimmer LEDs with lower voltage; it does
stop oscillating at about 3V. I tried very slowly increasing the supply
voltage from 0V in an attempt to "gently" charge the caps and maybe
"fool" the circuit into the non-oscillating state; no dice. I tried
switching the supply voltage off and on in an attempt to achieve your
stable state; again no luck. I tried swapping transistors out to no
avail. I tried two different LEDs and it still kept going. I tried
varying values of caps and got differing freqs and with different cap
values, I got a proportional duty cycle of the transistors.
So maybe you've got a wiring problem? Did you pay particular attention
to the tie and NON tie points on the schematic? Make sure that the base
of each NPN transistor is tied to V+ via the 39K resistor and that each
transistor's base is tied to the opposite transistors collector via a
10uF cap. The schematic may be confusing; THERE IS NO DIRECT TIE
BETWEEN THE BASES OF THE TRANSISTORS NOR IS THERE A TIE BETWEEN A
TRANSISTOR'S COLLECTOR AND IT'S BASE VIA ONE OF THE CAPS. The tie
points are schematically represented by a heavy "dot" (see where 39K
resistors tie to bases of transistors); lines that simply cross are not
connected (see "cross over" of the bases in the middle of the schematic;
no connection there). Good luck. I hope that helps...

-Dan Akers

 

[Dan Akers]

John wrote;
"If you return the tops of the 39Ks to their own transistor's
collectors, rather than to V+, that will prevent saturation lockup.
_____________________________________
Re;
The circuit still works in this configuration, however, the LED's do not
fully extinguish during their respective off cycles.

-Dan Akers

 

[Rikard Bosnjakovic]

If the transistors have high beta, saturation is more likely.

What does "high beta" mean?

 

[John Fields]

What does "high beta" mean?

 

[Jasen Betts]

What does "high beta" mean?

AIUI "beta" is another (archaic?) name for "Hfe",

So, high gain.


Bye.
Jasen