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I have this simple circuit

enter image description here

My concern is that it has of course a kind of memory as the relay is mechanical.

I would like OUT1 selected at power-on, whatever the state before the previous power off.

Will this do the job?

enter image description here

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  • \$\begingroup\$ Some kind of relay has a "mechanical" memory ... \$\endgroup\$
    – Antonio51
    Commented Dec 6, 2022 at 19:11

3 Answers 3

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That circuit could damage the transistor since there is no series base resistor. If that doesn't kill it immediately, then the lack of a diode across the relay coil might also do the job.

If you add the base resistor (or use a MOSFET which doesn't need it) and the coil suppression diode then it will work if you always get a clean power-up after a long time off.

If you really, really need it to always reset, I strongly recommend using a supervisory chip that will give you a bulletproof 200ms (or so) output pulse which can drive a transistor.

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    \$\begingroup\$ Forgot to mention that the relay has an integrated diode. Thanks a lot for your answer ! \$\endgroup\$
    – Jibeji
    Commented Dec 6, 2022 at 21:38
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That will kill the transistor. You need an additional resistor, either in series with the base, or in series with the capacitor. Then it will work. Or, you can replace the BJT with a P-channel MOSFET.

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A standard electromagnetic relay (K0) may be used for 'power on' reset.

enter image description here

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  • \$\begingroup\$ No, that won't work: K0 is always closed and therefore K1 cannot be used. \$\endgroup\$ Commented Dec 7, 2022 at 13:22
  • \$\begingroup\$ @Davide Andrea - Hi, At power-up, K1 is reset before the 'NC' contact of K0 opens and stays open as long as the power is on. K1 may be set or reset as desired when the power is on. \$\endgroup\$
    – vu2nan
    Commented Dec 7, 2022 at 15:44

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