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Haven't used a flip flop since week 2 of uni (well over 20 years ago now) but had a requirement for one in a current project. I need an output that toggles after a button press. Seems like a flip flop is the perfect chip for it.. seems.

My problem is the Q output toggles when the button is pressed, but then toggles back again when the button is released. ie it act as an actual button rather than a flip-flop. Below is an extract of the schematic.

The button is connected via R4/C1 to filter the clock pulse. I've looked on a CRO and it's a nice clean press with no jitter etc. I've changed values of R4/C1 to try both slow and quick ramping, with no difference.

RD is driven high/low by another part of the circuit to enable/reset the flip flop. That seems to work ok.

What am I doing wrong? - Cheers Matt

Flip Flop Extract

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    \$\begingroup\$ there is no button switch in the schematic diagram \$\endgroup\$
    – jsotola
    Commented Apr 1, 2022 at 1:10
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    \$\begingroup\$ You need to provide a complete schematic and a link to the manufacturer's datasheet for the exact flip-flop you are using. \$\endgroup\$
    – Elliot Alderson
    Commented Apr 1, 2022 at 12:27

1 Answer 1

Reset to default
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It may be that your clock transition is too slow. This can be fixed with a Schmitt trigger between the RC circuit and the clock input. https://www.ti.com/lit/wp/slla364a/slla364a.pdf

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  • \$\begingroup\$ I hadn't thought it might be too slow! Just removed the capacitor, no difference, also changed the 1K series to 0R. Still nothing. Have also added a 100K pulldown to GND (button is active high). Then I looked into the schmitt trigger idea. From the datasheet of the 74LVC2G74GS "Schmitt-trigger action at all inputs makes the circuit tolerant of slower input rise and fall times." So that's not it. \$\endgroup\$
    – Mutski
    Commented Apr 1, 2022 at 0:47
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    \$\begingroup\$ @Mutski "Schmitt-trigger action" is not a real Schmitt trigger. Your circuit violates the Δt/ΔV limit in section 9 of the datasheet (and without the capacitor, is bouncing). \$\endgroup\$
    – CL.
    Commented Apr 1, 2022 at 5:28

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