Capacitor function in a flip flop circuit

Question

I'm new to electronics and I'm trying to figure out the following circuit. The circuit turns on one of the leds when one button is clicked, if the other is clicked then the current light is turned off and the other is turned on. I understand what most components do and how most of the circuit flows is working, but I don't understand what the purpose/ action of the capacitors is in the circuit.

If I remove the capacitors, even keeping the cable in its place, the circuit stops working.

If I remove the capacitors and the cable, to keep only the path with the resistor, the lights flash on button clicked but they stay on only as long as the button is pressed.

This is the working circuit in Tinkercad.

Can someone help me figure it out, please?

Answer 1

The problem

... is that the base current of the transistors is not enough to turn them on. The reasons are two:

• The LEDs limit the collector voltages.

• The base resistors have not low enough resistance.

The role of capacitors

In this case, they "help" the resistors by shunting them at the first moment, and the circuit manages to switch. Basically, they are not needed in this manually-operated circuit.

The remedy

The problem can be solved in two ways:

• Move the LEDs in the collectors (in series with 510 ohms resistors). It may be necessary to include high-resistance resistors in parallel with the LEDs to divert the transistor residual current.

• Decrease the base resistances (e.g., to 5.1 kohms).

Improvement

With the same success you can connect the buttons between the collectors and ground; this is a more reliable way of control. Note that the rolles of the inputs will be swapped.

See also my old movie.

Answer 2

Other than making the transistors switch more slowly, the capacitors don't perform any useful function.

Once it starts to switch (even if the switch closes for less than a microsecond) it will continue to switch, even if it bounces open and closed once or many times.

Answer 3

Capacitors in this old circuit are intended as speed up caps useful for slow transistors. The very high value of 100 nF may have the opposite effect.

Answer 4

The capacitors are not in the circuit to increase switching speed. They are there because someone forgot to delete them. Also, this is not a monostable circuit. Also, do not relocate the LEDs.

The circuit is intended to be a solid-state version of the original flipflop circuit invented by Eccles and Jordan in 1918. A confusion is that it has coupling capacitors that were in the original astable multivibrator circuit invented by Abraham and Bloch in 1919. Note that there is no clear evidence that one circuit was derived from the other; they might have been developed independently.

The astable circuit uses coupling capacitors, and the flipflop circuit uses coupling resistors. Your circuit has both, which indicates to me that whoever developed that schematic was not quite clear on the intended function of the components.

As others have said, you can delete the capacitors but must keep the resistors. The caps are too big for the speed-up function, and actually are large enough to (maybe) cause damage over time, by very briefly dumping too much current into the transistor bases each time the circuit toggles. On another forum I routinely disagree with a senior member about how large a capacitor it takes to cause this kind of damage over time, but in theory it is a real concern.

Note that if you move the LEDs to the transistor collectors, the holding current that keeps on one transistor is now coming through the LED that is supposed to be off. This is a small current, BUT depending on the efficacy of the LED and the gain of the transistor, you might be able to see the "off" LED glowing dimly.

And - finally - to your question: beyond a wiring error, there are several different pinouts for TO-92 transistors. The ones you are using might differ from the ones in the sim.

https://en.wikipedia.org/wiki/Flip-flop_(electronics)#Implementation

https://en.wikipedia.org/wiki/Multivibrator#Operation