Electronic – Solution for accepting 3-24v as digital high for microcontroller

There were some hobbyist experiments (in the early '80s, I believe) with decoding variable speed digital data with the intent of being able to distribute code to accompany magazine articles by printing it as bar-codes. The reader (person, not machine) could then scan it into their machine with a hand-held scanner. It was assumed impractical (until shown otherwise) due one's inability to hand-scan at a uniform speed.

The solution ended up being for the decoder program to initially collect enough white-black to black-white transition times to discover the mean wide-bar and narrow-bar times, assign '1's and '0's, respectively, to the collected data, and continue decoding the incoming stream while simultaneously updating the wide-bar and narrow-bar mean times to account for changes in the wand speed over the bars.

The same technique was applied to decoding hand-sent Morse off the air, with a simple circuit going high and low with the receiver's audio output, fed to a similar algorithm.

Your hardware decoder would need to be similarly adaptive.

As an aside, an interesting issue came up when the words 'T5' or '6E' appeared frequently in the decoded text. Operators naturally developed keying habits on common words, and would key (and understand) f/ex, the word 'the',

Dah - dit dit dit dit - dit

as

Dahdididididit,

which the decoding algorithm, doing its best with slightly uneven element spacing and a result string that didn't match any known Morse character, got one of the above digraphs.

You might find some of the articles in early issues of Byte Magazine at the library.

How about this?

^{simulate this circuit – Schematic created using CircuitLab}

To light the lamp, one of the switches must be closed and the other open. Note that if both switches are closed, a lot of power will be wasted in the resistors but the light will be fully off. Note further that one may have to use a very small bulb and may have to use a higher voltage or reduce the resistors to get much light, but one should ensure that the voltage squared divided by the resistance does not exceed the resistor's power rating (for example, if you used 12 volts and 22 ohms, you would need to use 5-watt resistors). Alternatively you could replace the resistors with light bulbs and shelter them so their light isn't visible.

If you want a transistor circuit, here's half of a circuit I designed for my parents' car some decades back when I was about eight (I don't remember the actual resistor values; the transistors were some sort of TO-3 package and not 3906's; the components given should be suitable for demonstration purposes--the real one used a lamp rather than an LED and resistor). An electrical engineering grown-up friend helped with the design, but I designed the overall concept.

^{simulate this circuit}

The left-side input is wired to one of the turn signal flashers on the car; the right-side input is wired to the brake light. The lamp is the left light of a trailer. The right-side flasher and trailer lamp are wired similarly. Note that positive is on the bottom. Your son's challenge is to figure out what the diodes on the bottom are for (consider the above description of what the circuit was connected to).