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There is touch sense circuit that controls a SMPS circuit drives a power LED at output. When disconnect the energy while LED ON, if I reconnect the energy LED must start be as ON state. I need a cheap, logic circuit. Thank you for your help.

schematic

simulate this circuit – Schematic created using CircuitLab

Here is a simple referance design. Touch IC'output state is initially active high, because of them there is a NOT circuit made by NPN. Circuit should save ON state at least 30 minutes.

U2 is IQS127D

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  • \$\begingroup\$ What you need is some form of nonvolatile memory, not merely a logic circuit. This could be anything from an EEPROM or flash to a latching relay. Do you have any preferences? \$\endgroup\$
    – Cristobol Polychronopolis
    Commented Aug 12, 2020 at 19:05
  • \$\begingroup\$ I thought there should be a logic circuit which drives a capacitor somehow. That capacitor should have large discharge time so that it should provide enough voltage to output left high in case of reconnecting. @CristobolPolychronopolis \$\endgroup\$
    – IHK
    Commented Aug 12, 2020 at 19:28
  • \$\begingroup\$ Do you have a requirement to guarantee that the state is preserved for a specific amount of time (undetermined thereafter)? If so, what's the time constraint? \$\endgroup\$
    – Cristobol Polychronopolis
    Commented Aug 12, 2020 at 19:51
  • \$\begingroup\$ @CristobolPolychronopolis It should be min. 5 min. \$\endgroup\$
    – IHK
    Commented Aug 12, 2020 at 19:59
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    \$\begingroup\$ Does this answer your question? How to make 1 bit permanent memory circuit? \$\endgroup\$
    – Dave Tweed
    Commented Aug 12, 2020 at 20:07

2 Answers 2

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Here's a circuit using a ratchet relay.

enter image description here

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  • \$\begingroup\$ This type of circuit is not suitable for my design. \$\endgroup\$
    – IHK
    Commented Aug 13, 2020 at 7:07
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Maxim has an app note on this subject that seems a bit overkill but here ya go:

https://www.maximintegrated.com/en/design/technical-documents/app-notes/4/4624.html

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