This was around 1972 when I lived in Flanders, NJ with my parents, younger brother Bill, and older brother Jim.
Bill had an 8-track player, but it didn’t have any indicators as to which of the 4 stereo tracks was playing. All it had was a square white button that would change to the next track. I offered to open it up and look at the mechanism to see if it would be feasible to add a display. At the time I was thinking about maybe 4 LEDs or something.
Much to my amazement, there was an unconnected terminal strip with four dry contacts to a common. I also found a positive power supply. It looked to me that the mechanism was the same as would be used in a “higher-end” player with indicators. The only problem was that there wasn’t room on the front bezel for 4 indicators. But there was just enough room for a single 0.3″ 7-segment LED display.
At around age 14, this was my first try at an electronics design. Up until then, all my electronics experience came from a Radio Shack 100-in-one experimenter’s kit. These things were a collection of components line an antenna, germanium diodes and transistors, relay, resistors, and capacitors. It came with circuits to build an AM radio, a photo-cell detector, audio amplifier, etc. By the time I got done with it, I had destroyed both transistors and figured out how to wire the relay to self-oscillate with a 9-volt battery and send the signal to the secondary of the audio transformer. Yeah, you could definitely feel the voltage on the primary side!
Needless to say, I had no idea what Ohm’s law was. But that wasn’t going to stop me, because I didn’t know what I didn’t know… But, how hard could it be? It’s just a bit of logic to convert one-of-four to 7-segment. I knew about diodes (or at least the first-order approximation of them), and that the display LEDs lit up when you applied voltage (note that I didn’t say current). So I drew up a diode logic diagram that would display the digits 1, 2, 3, and 4 based on which of the transport contacts were closed. I confidently ordered a display and a SIP diode array from Poly Paks and got to work wiring it up. It worked for a brief moment before the magic smoke was released. What could have gone wrong?
My friend Bruce McIntyre from high school to the rescue. He knew things like Ohm’s law. He explained things like forward voltage drop and current needed for an LED segment. So, I need resistors, then? You mean I will need to use my calculator? So I added series resistors to the original diode logic diagram and son-of-a-gun it worked! I drilled a large hole in the bezel and epoxied the display and diode SIP in place. It worked until Bill got rid of the player many years later.
So, at this point I’m comfortable with diode logic and current-limiting resistors. Just don’t ask me how a transistor works…