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I need to design a circuit that has this function. There is an input signal whether ON or OFF. I need this: Once the input signal is ON first, my output must be ON also and after that it has to stay ON no matter what!

Like the timing diagram below.

S1 is the input signal and S2 is supposed to be the output,

timing

when the input signal is ON for the first time, I need to output ON and after this, input signal may change but my output should stay ON! How can I implement this in transistor level?

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  • \$\begingroup\$ You can use any FF with asynchronous input to achieve your intent. Just let the synchronous input at a stationary position and connect your switch to the asynchronous SET. But, what do you mean at transistor level? Can you use any TTL circuit? \$\endgroup\$ – Amadeus Jul 3 '16 at 1:05
  • \$\begingroup\$ from your description it sounds like you want an SCR \$\endgroup\$ – Jasen Jul 3 '16 at 1:26
  • \$\begingroup\$ Or a Latch. \$\endgroup\$ – rdtsc Jul 3 '16 at 1:39
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The simplest transistor-based circuit is the RTL flip-flop:

schematic

simulate this circuit – Schematic created using CircuitLab

C1 is added so that the circuit prefers to power up with the output low, with Q1 not conducting and Q2 conducting. It also sets a minimum pulse width on the input signal. Once that threshold is exceeded, Q1 switches on, Q2 switches off, and the output goes high.

If you want a manual reset (other than power-cycling the circuit), connect a switch between Vout and ground.

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from your description it sounds like you want an SCR

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If I understand correctly, you may wanna try connecting a low-voltage-drop diode in series to the signal generator (the output will then be 5V minus something like 0.2 - 0.3 V, if that's acceptable for your application), and a capacitor in series to the diode (something like 0.1uF, for instance). The output voltage is the capacitor voltage.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thanks for the suggestion but I need a more elegant solution. In this configuration the capacitor voltage will slowly fade due to reverse diode currents and capacitor leaks. I sense that I need an active circuit combined of some CMOS structures. \$\endgroup\$ – Alper91 Jul 2 '16 at 22:11

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