Astable multivibrator

[greenacarina]

So, I'm intrigued by this fairly simple circuit and think it could provide some interesting possibilities for a pedal.
I am trying to stick this together from parts out of my "junk drawer" and playing with it all on a breadboard.

Here's what I'm working with- http://www.sentex.ca/~mec1995/circ/flash2.htm
Both of my resistor values are a little lower than what is listed (330 ohm, and 33k ohm)
Caps are same as what's listed.
Transistors are mystery NPN's.

I'm running 5 vdc to this circuit but nothing is happening. Voltage and continuity seem fine. I can trigger the LED's if I manually ground the Base of the transistor, so I'm thinking there's something I'm not understanding about the capacitors. (not charging/discharging)
I feel like I know how this circuit is SUPPOSED to work, but I am missing something here.
I'm sure someone here can educate and enlighten me!

Thanks!
Chris

[CynicalMan]

Are the capacitors in the right direction? Have you tried other transistors? Are you sure about the transistor pinout? Other than that, all I can suggest is to double and triple check that you have the circuit on the breadboard correctly.

This site has more info on this multivibrator as well as a breadboard guide:
www.play-hookey.com/digital/experiments/rtl_astable.html

[greenacarina]

Yep. I had double checked the orientation of my capacitors and even tried them backwards just for the heck of it. Nothing.
I have tried a few different transistors (double checking the pinout with a multimeter)....still the same result.
I will read the info provided in the link and try to get a better understanding of what's going on here. Will probably try the circuit again with different values.

Chris

[EATyourGuitar]

you might have to inject a signal to start the oscillation. I dont have experience but I have done the reading on that same circuit as an audio oscillator. that schematic is more of an example than a ready to build schematic. there is some stuff you would need to add to make the injector without compromising the circuit. once it starts oscillting though, you dont need the injector. another thing is that values out of range will make it impossible to oscillate. like balancing on a knife edge. the values have to work together. its not really used as an audio oscillator too much these days cause of all the problems.

[maarten]

shouldn't you need 9 volts here for a 20mA current tot he leds? With 5 V it probably wont start...
Maarten

[johngreene]

-snip-
Transistors are mystery NPN's.

-snip-
I can trigger the LED's if I manually ground the Base of the transistor,
-snip-
Thanks!
Chris

The LED lights if you ground the base? Grounding the base should turn the transistor off. You may have PNPs, not NPNs or you have them wired incorrectly.

--john

[CynicalMan]

Not necessarily, because grounding the base of one transistor would normally light the LED connected to the other transistor.

I can trigger the LED's if I manually ground the Base of the transistor.

What do you mean by that? Which LEDs light when you ground the base of which transistor?

Also, posting voltages from the transistor could help us figure out what's wrong.

[johngreene]

Maybe it will help if you think about how the circuit operates. If we start with everything at 0V and then apply the power the following sequence of events should happen:
1. The collectors come up to a high level quickly because of the 330ohm (in your case) resistors. The bases charge up more slowly because the cap has to charge through the 33K resistor. At this point it is a race
2. The first base to charge up above the Vbe threshold turns that transistor on. This immediately pulls the collector side of the capacitor down to almost ground.
3. The voltage on both sides of a capacitor cannot change instantaneously so when the collector drops, it takes the base side (of the other transistor) down with it. To below ground.
4. This turns that transistor off hard while the other remains on.
5. The base of the off transistor then slowly charges back up until it crosses its Vbe threshold.
6. It turns on which yanks the other transistor's base to below ground, turning it off.
7. The process then repeats for the other transistor.

--john

[Manny]

If you don't mind swapping a few parts an NE555 chip in astable mode would do what you're after. You'll also be able to change the frequency of the flashing which is great for use with an LDR to control trem/filter/wah effects.

Take a look at the Tiny Tremolo thread to see how if you're interested.  

[earthtonesaudio]

shouldn't you need 9 volts here for a 20mA current tot he leds? With 5 V it probably wont start...
Maarten

+1

[PRR]

van Roon's plan does not give supply voltage.

> "Obviously, the 470 ohm resistor ...limits the current flow to about 20mA"

20mA in 470 is 9.4V. Take 0.1V for saturated transistor, 1.7V for LED, is apparently 11.2V supply. van Roon may have used 9V to 12V.

This type astable "CAN" work on 5V or lower supply, EXcept here we have ~~1.7V added loss in LEDs. Yes, the basic astable can work below 2V (or 4V with LED losses), but it gets VERY critical.

van Roon's bias seems a bit scanty for 5V operation and "mystery" transistors. If it does not quickly start flipping, it gets stuck with both transistors ON.

The ratio 39K:330 suggests we need transistors with hFE over 118. The full analysis is NOT that simple. However it may be that van Roon found and tested a condition that works for higher voltages and better transistors, not for lower voltages and mystery parts.

Try 9V supply.

Try changing 330 ohms to 1K. (We don't need 20mA to see an LED!)

Try changing 39K to 22K.

> orientation of my capacitors and even tried them backwards

That's bad for electrolytics. If BIG power is available, they boil and burst. In this case the 330 limits max power to 0.02 Watts and they won't burst, but may become leaky.

If still not flipping: take those caps out. Both LEDs should stay ON. Both collectors should fall under 0.2V. Shorting a base to ground should darken that LED.

If the collectors won't fall very-low, raise the 330 or lower the 33K.

If transistor won't go OFF with base shorted to emitter, there's something not wired right.

Once optimized as a "stable", try fresh caps for astable.