555 Flip-Flop - Opinions wanted

[edvard]

Sorry for my (possibly) heretical opinion, but the DOD and Boss-style flip-flop always seemed a bit too complex, so I've been searching for a simpler flip-flop.
I almost found what I was looking for with a Dual J-K setup, but then I ran across this 555-based circuit, which I re-drew and simulated in Falstad's circuit simulator:



The only changes I made to the original circuit are the 10nf cap (original was 2uf, I think; way too slow) and the 10k voltage divider at the output to get 4.5V for driving JFET switching schemes.
One drawback is it only has one output, but if I used P-channels for the alternate side, I think I can get away with it.
Falstad's Circuit complains about JFETs being reverse-biased, so I can't simulate it there.

I'm breadboarding now to give it a go, anybody here note any potential problems before I blow up something?

Here's the import, if you'd like to try it for yourself in Falstad's Circuit:

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[Ice-9]

A 555 flipflop has been around for a long long time, the 555 is a very versitile chip that can do a number of things. As a flipflop that is used in place of the boss style switching circuit it will function just the same.

Instead of using p-channels for the other side you could just use an inverter and use the output of the inverter for the opposite "state"

[edvard]

That'd be a nice way to use up a spare inverter in 4049 designs.
I'm wondering if a simple transistor circuit would work just as well...

[earthtonesaudio]

If you hold the switch closed it will oscillate at low frequency, and the output state on power-up is sort of vague.  But overall a good circuit, and a good value for the parts count/parts cost.

[edvard]

If you hold the switch closed it will oscillate at low frequency

Awesome!
A built-in tremolo!! 

Seriously though, I can't name an instance where my foot is stuck to the switch, so I think I'll be good there; maybe a bigger cap from the switch?
I'm also not too worried about where the thing is on power up.
Sure, it's a nice 'extra' to have it always one way or the other on boot-up but that's the LEAST of my worries when I power up.
I usually turn everything down before plugging in anyways...

I just did a bunch of sims in LTSpice and it seems the output voltage needs to be taken from the full 9V, I had that mixed up with 4.5V biasing to the FET, sorry about that.
Just remove the 10k divider and take the control voltage from 'out'

Also based on the sim, P-channel fets live with N-channels just fine and the signal switches proper; no inverter needed.

[edvard]

If you hold the switch closed it will oscillate at low frequency, and the output state on power-up is sort of vague.  But overall a good circuit, and a good value for the parts count/parts cost.

After breadboarding, I can confirm that the power-up state is 'ON'.
I disconnected power a hundred times, with the switch OFF and ON and either way it always powers up in ON state.

Also, I've tried everything to get it to oscillate when holding the switch down but it doesn't, so no bonus tremolo but no stuck switch worries either.

[earthtonesaudio]

Well that's good news.  If I had thought a little harder I would have noticed that it can't oscillate when the switch is held down, at least not with the resistor values you've used.  The only time the cap can charge past the trigger threshold is when the switch is open.  

I think it would oscillate if you used all equal-value resistors, or if the feedback resistor is smaller than the bias resistors.


As for the reliable power-up state, that has to do with the internal structure of the 555 and I'm not that familiar with it.  In this case the non-ideal aspects of the part work to your advantage.  Score.  





As a side note, you can construct an inverting schmitt trigger with an op-amp also.  So if you have a spare op-amp section and want an electronic switch...

[edvard]

As a side note, you can construct an inverting schmitt trigger with an op-amp also.  So if you have a spare op-amp section and want an electronic switch...

Naw, I'm just trying to do FET switching with the lowest parts count possible.
P-channels work very well in simulation, but there's not much variety in the catalogs...