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Original question:

SN74AUP2G80: IC that converts momentary push button switch to maintain switch

SN74AUP2G80: Logic forum

Leslie Valmadre
Prodigy 70 points
Part Number: SN74AUP2G80
Other Parts Discussed in Thread: TLV62569, SN74HCS74, SN74HCS72, SN74HC14

Hello Emrys, I have just found your post response to Daniel dated about 4 years ago in relation to converting a Momentary switch to a maintained switch and am hoping you can help me with my question.

I have found some similar circuits using a TI 4093B IC that seem to make fairly simple work of the conversion when used in conjunction with a Power PFET and a Ntype 2N2222 Transistor.

I am wondering if TI might have a circuit diagram suitable for the conversion and using that IC or similar.  I have asked a similar question of Stefano Panaro elswhere but just found your 4 yo post.  In my case I will have a LIPO battery supplying 3.7v and will use the switch to Enable/Disable the TLV62569 IC for a Buck Converter I am building using a schematic I gathered from WEBench.  The current though the swith will be approx 0.001mA.

Hope you can advise.

  • 0
    Guru 272770 points

    NAND gates could be used to build a flip-flop.

    But nowadays, you can simply use flip-flops with integrated schmitt-trigger inputs, e.g., SN74HCS72/SN74HCS74.

  • +1
    TI__Guru* 96712 points

    I completely agree with Clemens here -- the SN74HCS72 or SN74HCS74 would be the best bet for this circuit. You can eliminate the two Schmitt-trigger inverters because the device incorporates those.

    For the debounce side of things, you can select if this is rising edge triggered (ie when button is released) or falling edge triggered (ie when the button is pressed) by selecting either the rising edge (74 function) or falling edge (72 function) triggered flip-flop.

    Here's a complete circuit with the values I would recommend starting with:

    I've found that many switches bounce for less than 20ms, so this is a good place to start. Things to keep in mind:

    The 100k and 1uF cap should have a larger time constant than your debounce circuit - this will prevent any false triggers at startup. Currently this has a delay of about 220ms, which is fairly long, but should be unnoticable in most applications.

    The debounce circuit produces a faster falling edge (~2.2ms) than the rising edge, (~24ms). This isn't necessarily required -- but it does give some flexibility to the design. If you want to have a more balanced rising/falling time, then you can swap the 10k and 1k resistors.

  • 0 in reply to Emrys Maier
    Prodigy 70 points

    Thank you for your help and the schematic. I think this has answered my question so will mark it accordingly.  Very much appreciated.

    Regards

  • 0 in reply to Leslie Valmadre
    Prodigy 70 points

    hello Emrys, hope it is ok to ask a follow up question.
    Both the SN74HCS72 and SN74HCS74 are 14 pin ICs but your recommended schematic appears to use 12 pins and with my very limited knowledge i am unable to determine which pins are used and even what the used Pin numbers are.

    Would you mind providing the schematic with a little more detail of the IC pin connections so i can better analyse the circuit and learn from that analysis?

    I am in Perth, Australia and neither the SN74HCS72 or SN74HCS74 are locally available but other 74HC ic are available. My problem is identifying which of those available will best match the SN74HCS72 or SN74HCS74. I believe i will need a chip with AND gate and Schmidt Triggers incorporated. Is that correct? I would like to breadboard the design so am hoping to use an IC from a local source called Altronics (altronics.com.au). 
    i do hope you can advise as i am on a very steep learning curve here as i am entirely self taught in this electronic journey i am so enjoying. 
    regards

  • 0 in reply to Leslie Valmadre
    Prodigy 70 points

    Hello again Emrys,  Following on from my previous question I attempted to create a schematic that accurately reflected your recommendation and have developed the following.  I am hoping you may be able to check it for me and advise my errors.  I will use this design to bread board the design using a locally available IC https://www.nexperia.com/product/74HC14D. 
     I realise I am asking a lot from you but do hope you could advise on the schematic. 

    Thank you for any advice or help you can provide

    Regards

  • 0 in reply to Leslie Valmadre
    Guru 272770 points

    The 74xx14 is a completely different device.

    And this circuit needs a flip-flop with inverting output; Altronics does not sell this. Use a distributor like DigiKey or Mouser, or the TI web store.

  • 0 in reply to Leslie Valmadre
    TI__Guru* 96712 points

    Jumping into electronics with no formal training can be very challenging. Fortunately, there are a lot of great resources out there today that you can use to learn what you need to know.

    Your schematic for the HCS72 looks good to me.

    The only D-Type Flip-Flops on the market with Schmitt-trigger inputs come from the HCS logic family from TI. These are pretty new, so our competitors haven't copied them yet.

    You can use a separate Schmitt-trigger device to 'clean up' the signals such as the SN74HC14, which is a six channel Schmitt-trigger inverter. For any channels you don't use, just connect the inputs to ground (or VCC, whichever is more convenient).

    The two red triangles indicate the locations of the Schmitt-trigger buffers (or inverters). The idea is to keep the slow rising edges from the RC circuits from directly interacting with the inputs of the 'HC74 device -- which requires fast input transitions.

    You may want to check out the EEVblog forum - since Dave is based out of Australia, I would think that there would be some people there that might be able to help you better with local sourcing.

  • 0 in reply to Emrys Maier
    Prodigy 70 points

    Thank you Emrys,  That information helps me a lot.  Just to clarify.  The circuit I drew will work probably with the SN74HCS72 IC I used as that chip is a Dtype Flip Flop with Schmidt Triggers built in? but if I use a Dtype Flip Flop with no Schmidt Triggers I will need to introduce them as you have shown in the diagram?  Am I understanding that correctly?  Or will I still need the Schmidt Triggers even in the diagram I have drawn and as has been said by   in his post?

    Thank you for the advice on the EEV blog.  I have looked at the information from there on many occassions as the presenter Dave Jones provides great information in a very helpful way (and in my patois!!) but didnt think to use the Chat until now.  I have used All About Circuits and received a great deal of help.  Starting from my position is challenging but what  I have found is the amazing amount of help available and offered from people in the electronics professions.  Truly a generous and helpful group of talented people.

    Cheers

  • 0 in reply to Leslie Valmadre
    Guru 272770 points

    You always need Schmitt triggers. These can be either integrated (in the HCS device in your schematic), or separate (as shown in Emrys' schematics).