Transistor Toggle Flip Flop

Thread Starter


Joined Mar 24, 2008
While I am in this nursing home, I am using my enforced idleness to straighten out several electronic things in my head before I get home. For example I have been trying to recreate this flip flop for several months, I am not sure if this will work but it it is something I will try, as near as I can remember the diodes are for steering the pulse to the low side input. So, will this work:

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Joined Feb 24, 2006
Here is my RTL Flip-flop with Preset and Preclear inputs, which I created from Lancaster's RTL Cookbook. You're on the right track. The JK Flip-flop is more complicated as you might suspect.
@Wendy I can do a simulation of your circuit if it would be helpful. I know your simulator is between the ears, and I respect that.

Thread Starter


Joined Mar 24, 2008
I did. Same result with/without that command. Perhaps the trigger pulse wasn't short enough.
if that were true then C1 would need reduced pretty dramatically. That would have the effect of reducing the pulse width.


Joined Mar 30, 2015
So here is the prototype layout:
Not sure what you want to do with this circuit, but you could make an R#S# flip flop if you add two resistors and another capacitor. You can easily convert it to a T flip flop by connecting Q# to R# and Q to S#.

In my circuit, I put the diodes on the bases of the transistors so I was pulling on the BE junctions directly. I also put speed up caps on the base resistors; and had them connected differently.


Joined Aug 7, 2020
With a 5V supply they both sit with their collectors at just over 1V. LTS doesn't like total symmetry, so a real-world circuit would likely perform differently.
Not on, but in a linear region. Two identical transistors would behave like that - it's a metastable state. Even if one were only very marginally warmer, it would have more gain and would trip the latch into one state or the other.


Joined Mar 14, 2008
By changing some component values and adding a couple speed-up capacitors (C2 and C3), I was able to get the post #11 circuit to toggle on the falling trigger edge (LTspice simulation below):

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