Flip-Flop Circuit Using BJT's, OP Amps, and Cap Timer

[theDUDE]

Hi all, first time post.
I have a pretty thorough understanding of electrical theory and electronics applications (2nd Year EE student).
This project came about because using a 555 timer was getting real frustrating.

Conceptual Process
1: The battery charges a 100uF capacitor through a 10k resistor (Tau = 1s).
2: Capacitor is fully charged. LED 1 (left side) Turns OFF. (Charged Cap Voltage = [9V - V(LED) - V(BJT) = ~ 7v] )
3: OP AMP 1 (left side) has equal voltage across inputs - Output is low - BJT 1 (left side) is fully unsaturated.
4: OP AMP 2 (right side) has higher +'ve input. Output it high.
5: BJT 2 is fully saturated by OP-AMP 2 Output.
6: LED 2 is ON. Capacitor Discharging.
Duration of discharge is dependent on Variable Resistor, which is acting as a voltage divider for OP-AMP 2 -'ve input.
7: As approaches 0V, BJT 2 is unsaturated. BJT 1 is fully saturated. Capacitor begins charging and LED 1 is ON
I built this circuit on a breadboard, using very close to 16KΩ on the Variable Resistor. Neither LED lights up, the capacitor charges, and still neither LED is lit.
I feel like im over-looking something really silly.

If anyone can point out my mistake(s), It'd be really helpful!

 

[AnalogKid]

First, are you trying to make a flipflop or an oscillator? These are not the same thing.

Second, 3 M is way too large a resistor for base current limiting. You don't say what any of your components are, but with a normal opamp you're looking at less than 2 uA base current.

Third, the 555 is the largest selling IC of all time, and one of the best understood parts ever. What about it was "getting real frustrating"?

ak

 

[Arouse1973]

What do you want the circuit to do. You explained what you think should happen. But Mother nature has other ideas.
Adam

 

[theDUDE]

First, are you trying to make a flipflop or an oscillator? These are not the same thing.

Second, 3 M is way too large a resistor for base current limiting. You don't say what any of your components are, but with a normal opamp you're looking at less than 2 uA base current.

Third, the 555 is the largest selling IC of all time, and one of the best understood parts ever. What about it was "getting real frustrating"?

ak

I thought a flip-flop and an oscillator performed essentially the same function, but one is manually switching and the other automatic.
What I want this circuit to do is start with LED 1 On/LED 2 Off, some time passes, LED 1 Off/LED 2 On, repeating indefinitely.

All Transistors: 2n3904 NPN
All OpAmps: LM38ON
LEDs are standard 1.7v red.

Source is a 9V battery hijacked from an unfortunate DMM.

As far as the 555 i spent an hour or two fiddling with it after reading a short tutorial, and decided to walk away. Other/More basic projects with the 555 will have to come first.

After percolating for a while, I think the problem is reliance on precise synchronization between the switching of the 2 BJT's.
Ill keep this updated as I go along.

 

[AnalogKid]

A ff and an osc do not perform essentially the same function. One is designed specifically NOT to oscillate on its own, and the other is an oscillator. While a 2-transistor multivibrator and a 2-transistor flipflop look similar in a schematic, the nature of the cross-coupling is completely different.

The LM380 is not a conventional opamp. It is an audio power amplifier. Compared to a normal opamp it has non-standard inputs and output, and is not stable at unity gain.

The 555 is designed to be an oscillator with only 1 resistor and 1 capacitor for timing, plus 1 cap for decoupling. Doesn't get any easier than that.

The function you describe can be done with the two transistors only, and no opamps. Search the web for multivibrator schematic or multivibrator circuit and you'll get a ton of schematics. There probably are a few with the LEDs already designed in.

ak

 

[Martaine2005]

What I want this circuit to do is start with LED 1 On/LED 2 Off, some time passes, LED 1 Off/LED 2 On, repeating indefinitely.

Sounds like a simple multivibrator circuit would eliminate the 555.'

EDIT: AK already mentioned a multivibrator.

Martin