Because NPN triode, PNP triode and unidirectional thyristor are similar in shape, they can't be distinguished because they can't see the text marks clearly.
This is a schematic diagram of a touch-sensitive switch: the touch PNP transistor is turned on, the touch NPN transistor is turned off, the single-phase SCR is the switch actuator, the LED is the status indicator, and the 100 ohm resistor is the load, providing 5v DC power supply.
The principle of this circuit is feasible, but we should pay attention to the device parameters: according to different models and specifications, the on-voltage drop of SCR is about 0.3- 1.2v, and that of LED is about 1.6-3.2v, so the voltage drop across 100 ohm resistor is 0.6-3.1v. Therefore, the maximum current of LED needs to be greater than 3 1ma (or increase the load resistance value), and the rated current of in-line ultra-bright LED is generally 20mA；; The trigger power of the unidirectional thyristor should be small, and the holding current should be less than 6 ma; The power of 100 ohm resistor needs to be greater than 1/8w (inclusive); The magnification of the two triodes should be as large as possible.
If the sensitivity is not enough, Darlington triode can be used instead of two triodes, or the positive pole of power supply can surround the base (B pole) of NPN triode and the negative pole can surround the base (B pole) of PNP triode.