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Created February 15, 2023
Last modified February 15, 2023
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I made this schematic to illustrate my answer to the SE EE question, "How to create an on/off indicator for a SPST in a circuit?"


I made this schematic to illustrate my answer to the SE EE question How to create an on/off indicator for a SPST in a circuit?.

I used my answer not only to solve the OP's problem from seven years ago but also to show the difference between a straightforward and inventive solution. The former is based on the use of a standard NOT logic gate while the latter is based on some unexpected circuit trick. I had the chance to come across such a clever trick 40 years ago (see my paper and my answer), and now I used it to solve the OP's problem. This once again shows that ideas are eternal and immortal; only their implementations die. That is why it is important to see the ideas behind circuits.

The trick is very simple but significant because TTL and ECL gates are built on it. It is known under the name "current steering" and is based on the following: if we connect two diodes with different forward voltages in parallel, the current flows through the diode with the lower voltage.

In this case, this means that if we connect a (green) LED with a lower forward voltage to a lit (red) LED with a higher forward voltage, it will turn it off. And since red LEDs usually have a lower forward voltage than green ones, we can artificially increase it by connecting an ordinary silicon diode D in series (a trick widely used in TTL gates). Let's consider the circuit operation.

The button is OFF. Current flows through the red LED and it lights up. The total voltage across the string of two diodes is about 3V (I have connected the voltmeter only as a voltage indicator).

The button is ON. The green LED is connected in parallel to the diode string and sets the total voltage across it about 2V. As a result, the current is diverted through the green LED and the red LED turns off.


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