SCR circuit answering circuit

See Figure 1 for the circuit. Turn on the power switch K and divide the voltage between the resistors 3kΩ and 1.8kΩ. The potential at point A is about 2.2V and the potential at point B is 0V. After the host announces the start of answering questions, if a certain button (assuming K1) is pressed, the corresponding one-way thyristor SCR1 will be turned on and L1 will light up. After SCR1 is turned on, two 1N4007 diodes clamp the potential of point B at about 1.4V, and the cathode potential of all thyristors is 1.4V. When other groups press the button again, due to the voltage drop of 0.7V in the diode connected to the trigger electrode of the thyristor, the voltage between the trigger electrode and the cathode is only about 0.1V, which cannot make the corresponding thyristor If it is connected, the corresponding indicator light will not light up. After the first round of answering, the host disconnects K to turn off the thyristor, and then closes K to start the next round of answering. The LED in the picture is the power indicator, and the number of answering groups in this circuit can be expanded arbitrarily. If you want to make a sound while the light is on, you can use the 1.4V voltage between point B and the negative pole of the power supply as the trigger voltage for the music IC or sounder to make a sound. Figure 2 is a printed circuit diagram. Comments on the experiment: The thyristor uses MCR 100-8, and L1, L2, and L3 all use 6V small bulbs. Connect the lines according to the diagram. When K1 is pressed, the light L1 lights up, but when K2 is pressed at this time, L2 also lights up, and the same happens when K3 is pressed, and the answering function cannot be realized. Analyzing the reasons, when SCR1 is turned on, the measured potential of point A is 2.2V, and the potential of point B is about 1.2V (due to the discrete nature of the forward voltage drop of the diodes, the forward voltage drop of the two diodes is less than 1.4V). There is a voltage of about 1V between AB, so the voltage may cause SCR2 or SCR3 to turn on. So R3 is adjusted. When R3 is adjusted to 1.5kΩ, after SCR1 is turned on, SCR2 and SCR3 are not turned on, and the answering function described in the article can be realized. In addition, in the power indication branch, it is recommended that R1 be calculated according to the following formula: R1 = (U1-U2)/I, where U1 is the power supply voltage, U2 is the voltage drop of the light-emitting diode, usually 1.8V, and I is the luminescence The operating current of the diode is usually 5~15mA. Therefore, it is appropriate to select about 5kΩ for R1 in the figure. 