SSR controllable by independent touch button (TTP223) and I/O for Micro-controller
Before delving into an answer, I want to suggest that you carefully consider why you feel you can only use 1 microcontroller GPIO pin. The assumption that you must use only one GPIO pin makes the circuitry external to the microcontroller much more complex. It may be so complex that a simpler solution might be to use a larger microcontroller, or to use an I/O extender, or a second microcontroller. Similarly you should consider why you want the relay to be controlled either by a microcontroller or by a manual switch. Many solutions to control problems involve a switch providing input to a microcontroller, which uC is the only point of control for the relay. The uC can make its decision to turn the relay on or off depending upon input from the switch. Or it can make a decision based upon other inputs. But the uC remains in control of the relay, and always knows whether the relay is active or not, because it controls whether the relay is active or not.
Having explained why another solution may be better, I will turn to the problem of using a single gpio pin to both modify and monitor the state of a circuit node external to the uC.
I think the basis of such a solution must involve something akin to the following circuit.



simulate this circuit &ndash; Schematic created using CircuitLab
The idea here is that the feedback loop of inverters (&quot;not&quot; gates) form a latch or memory element. When neither the GPIO nor the alternate input is acting upon the feedback loop because the both GPIO and alternate input are in a &quot;high impedance&quot; state, then the inverter loop will maintain it's state. That state can be read by the gpio pin, and the loop can drive something at the output.
However, if the GPIO pin of the uC becomes an output and drives the GPIO node, it may bring that node to ground or Vcc. When the GPIO pin becomes hi-impedance again, the circuit will maintain it's state.
The resistors in the loop are necessary. They must be of a sufficiently high value that the gpio pin can both a) &quot;win&quot; in the contest between the gpio pin and the output of the inverter which is also trying to control that node, and b) so that the gpio pin is not over-driven in its effort to control the node. They must also be sufficiently larger that the outputs of the inverters are not over-driven. The value of the resistors will depend upon 1) the specific microcontroller, the specific inverters (which should have cmos inputs and outputs and not TTL), and possibly the voltage.
An unavoidable side effect of using those resistors is that if the gpio pin is changing the state of the inverter loop, it must hold that state for some time because the inverter input capacitances will take some time to charge. The larger the resistors, the larger the required hold time. Similarly, some set-up time is required before the gpio pin can read the state of the inverter loop.
Everything said above applies to the &quot;alternate input&quot; as well.
If the alternate input were fed by another microcontroller, the solution would be near complete. the capacitative switch module could be an input to the microcontroller, and a gpio pin could read from or drive the alternate input (which is inverting obviously).
Something less than another microcontroller that might be a solution involves a combination of an edge triggered D-flip-flop with a tri-state buffered output.
This flip-flop would read the state of the inverter loop somewhere, and upon a logic one coming from the capacitative switch would a) lock in that state, and b) enable the tri-state output so that the alternate-input node gets fed with a state opposite to the one it was in.
It is vital for correct functioning, that the locking of state happen sufficiently prior to the enabling of the tri-state output that there is not hazard or glitch. This could be achieved in a number of ways, such as a delay, or more complex circuitry.
Anyway, that is the basis for a solution that involves only one gpio pin. As can be seen, it is not trivial, and a larger uC, an I/O extender, or a second uC might seem more attractive once the difficulties of the single GPIO pin approach are appreciated.
Would be happy to explain or consider this or other solutions further.