06.07.2020
FAQ

Video Switch

China offers switches, circuitry solutions often use expensive or rare in our area chips of RF switches. And I decided to show originality and make a switch on electromagnetic relays. The advantages of the relay are obvious: galvanic isolation, as well as the minuses: current consumption. Initially, I wanted to make switching using a relay, and control on TTL logic, but only 12V relays were available. I was too lazy to do two power supply, coordination, therefore, I also made the logic of turning on the inputs on the relay too. Thus, the result was a switch assembled completely on a relay, without microcontrollers and even logic circuits.

I sketched the scheme in just 20 minutes in Proteus. The diagram shows that the number of relays, and hence the inputs, can be increased to the desired number. I decided to stop at four. One operation requires a relay with two contact groups. It will be completely occupied by “logic.” The algorithm is simple: by pressing a button we turn on the relay, which will continue to be in self-holding mode. That is, we supply the initial current with a button, and after switching on, power is supplied to the relay winding through a closed contact. The second contact opens the common voltage supply circuit for the buttons, thereby preventing the inclusion of another input. In order for the relay to turn on in the self-holding mode, the accumulating capacitance is present in the circuit, otherwise the relay starts to work like a generator. The capacitance value and resistance are selected experimentally, so that the capacitance is large enough so that the relay is guaranteed to turn on. However, the self-discharge time of the capacitor through the resistor should be about 0.5 s, so that the next time you press other buttons, the inclusion does not occur. To select another input, you need to press the “reset” button and select the appropriate channel. The reset is implemented through a normally closed relay, which supplies power to the entire circuit. By clicking on the button, we blur the power supply and all relays that are in self-holding will turn off. For switching N-lines we connect the required number of relays, the power of which is connected in parallel with the control relay.

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