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Given are two signals A and B. I want to control the connection between them by a signal C. However, signal C is a logic signal with a voltage of 3.3V for high and 0V for low whereas signals A and B are at a level of 4V.

How can this be done? Using a Mosfet with C as gate and A and B as Drain or source, respectively will not work as C would have a lower voltage than the source. Therefore, the difference G - S would always be negative.

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    \$\begingroup\$ What is the impedance of these signals and can some current (even a tiny current) be drawn from them? Are they always 4V or varying? AC or DC? Frequency? What is the desired resistance of the switch? Switching time/frequency? \$\endgroup\$
    – bobflux
    Commented Sep 19, 2019 at 18:54
  • \$\begingroup\$ What do you mean by "control the connection between them"? \$\endgroup\$
    – Justin
    Commented Sep 19, 2019 at 18:55
  • \$\begingroup\$ @peufeu: Yes, some current can be drawn from them. They're varying around 4V DC. A is a battery and B the charging voltage. So, resistance should be as low as possible. Switching time/frequency should be less than 1 second. \$\endgroup\$
    – Hermetica
    Commented Sep 19, 2019 at 20:32
  • \$\begingroup\$ @Justin: I mean that A and B are connected when C is high and disconnected when C is low. \$\endgroup\$
    – Hermetica
    Commented Sep 19, 2019 at 20:33

2 Answers 2

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Is this what you are trying to think of.

enter image description here

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  • \$\begingroup\$ Nice solution, but where does that 5V come from? I'd suggest ORing Sig_A and Sig_B to the net which now is called +5V. \$\endgroup\$
    – Huisman
    Commented Sep 19, 2019 at 19:08
  • \$\begingroup\$ Huisman - Hermetica gave very little information about the rest of the circuit. If there is a signal there is a source of power for the signal and 5V is very common. If more information is given then a better solution can be provided. \$\endgroup\$
    – Rob B.
    Commented Sep 19, 2019 at 19:19
  • \$\begingroup\$ Thanks! That's what I was looking for. In my circuit, A is a battery and B the charging voltage. I want to be able to cutoff the charging by signal C. \$\endgroup\$
    – Hermetica
    Commented Sep 19, 2019 at 20:29
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Assuming signal C originates from at least a 3.3V power supply, you could use a relay that accepts a 3V3 coil voltage and drive that relay indirectly with signal C.

Example: JQC-3F-03VDC

Another solution could be using a (mosfet) solid-state relay, but check the maximum load current does not exceed the specs. Maybe signal C can drive the LED directly (i.e. with a resistor in series).

Example: VO1400AEF

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