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I would like to activate a portion of my circuit electronically. A single pulse should activate a portion of the circuit, and this portion should remain on until the device is disconnected from its source. A transistor wouldn't work because I would need the circuit to remain ON even when the initial pulse has faded.

I know using a 555 timer, it's possible to do this. A single "trigger" input will leave the 555 timer in its ON state until power is cut. However the 555 timer is sort of a noob tool? I'm wondering if there's a similar alternative using an op-amp or other cheaper device that could be used.

Alternatively, is a relay switch the cheapest, smallest, and best option here? What if the initial current is extremely small, like 1 mA?

Additionally... are there such things as relay switches that shut off after a certain amount of time without an activating pulse?

Kind of a long question but thanks

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    \$\begingroup\$ What's wrong with a D flip-flop? \$\endgroup\$ – John D Mar 10 '17 at 1:43
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    \$\begingroup\$ What's wrong with an SCR -- or two discrete transistors wired to behave like an SCR? \$\endgroup\$ – Dave Tweed Mar 10 '17 at 2:36
  • \$\begingroup\$ I will probably go with the SCR. I tried to make one out of two transistors but it's was finnicky and required a number of extra resistors. Accepting the op amp answer since i asked my question without knowing what i wanted \$\endgroup\$ – nick carraway Mar 11 '17 at 18:09
  • \$\begingroup\$ The SCR has too big of a voltage drop for my applications. I have a 5V supply and would like to maintain that voltage with minimum current loss. Looking into the D flip flop and op amp solutions now. \$\endgroup\$ – nick carraway Apr 4 '17 at 22:43
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Here is positive-feedback opamp method; the 2 diodes are LOW_ACTIVE; you may want a rail-rail vin-vout opamp; scale up the resistors equally; the resistors may set your standby or latched-on current draw, so make them of high value.

schematic

simulate this circuit – Schematic created using CircuitLab

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A latching relay is often used for this sort of task - one contact applies power to the relay coil when the relay is operated.

A relay would be somewhat wasteful in a low-current circuit, as the relay coil would require a few tens of mA - if the switched load only draws 1 mA, that's a big waste of power.

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