More push button bistables

Following my trials and tribulations with seemingly-easy problem, Mike M contacted me with some simpler ideas to the same end.


The application demands that two selectable leds to replace one, with any added circuit drawing power from across the feed to the led.

Mike said:

No need for fancy ICs – the attached flip flop circuit using humble CMOS 4069 inverters has served me well since the 1970s working from 3-15V supplies.

I think I may originally have discovered it in Don Lancaster‘s CMOS Cookbook.

more-push-by-mike-web.jpgAny high input impedance inverters will do so why not use the mosfet led drivers themselves (Circuit 2) thus eliminating the IC ?

Thanks Mike

I am suitably humbled, because circuit 2 is exactly the sort of thing that I tried, and failed, to come up with before I tried the D-type circuits.

Mike went on to point out that the diode clamps in my original circuit would stop the circuit 2 scheme from working unless low gate voltage mosfets could be found.

However, my clamps were there to prevent both leds from being off simultaneously should the CMOS hiccough – allowing the supply voltage to rise and blow up the power supply.

Mike’s circuit 2 looks a lot more robust and is going to hold at least one fet on all the time, so I think my diodes could go and a resistor could be added in parallel with each of the leds to increase gate drive.

There are a whole bunch of mono, bi and n-stables on this blog and they were mostly hyperlinked until some IT changes at EW disconnected them all. As I can’t face putting all the links back in the right places, here is a list instead. 


A 74HC74 hysteresis mystery
Hysteresis mystery solved, probably
Two leds for one
And now the pcb
Simple circuit selects LEDs? – a tri-stable
More push button bistables
A 74HC4060 monostable?
Thoughts on another EEVblog – a power on-off bistable


One comment

  1. I would describe the flip flop circuit that I recently posted as MML or Mickey Mouse Logic – a term perhaps not familiar to younger engineers ?
    Many years ago I read the truly wonderful ‘ Art of Electronics’ by Horowitz & Hill
    Within it describes how simple logic gates can be used to create flip flops, monostables and the like with the addition of just a few resistors, capacitors or diodes. This was often a life saver before the days of low cost microcontrollers often eliminating the need for an extra (costly) IC to carry out a particular function.
    For a brilliant description of these techniques see:
    The site name is more than worthy of its name and I can personally recommend building/buying their low cost Weird Sound Generator Synth (WSG) built with just a 4069 Inverter a few transistors and a 741 Op amp !
    Amazing fun and just what electronics was meant for !

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