Electronics > Beginners
Reading schematics that don't show inputs explicitly?
mindcrime:
Sometimes when browsing online tutorials, or datasheets (test circuits), etc., you'll find a schematic that does not explicitly label the input(s) of the circuit. See, for example, Figure 5.1.3 on this page:
http://www.learnabout-electronics.org/Digital/dig51.php
I assume that they don't show these because it's supposed to be so obvious that there's no need to label those points explicitly... but this bothers me on a fundamental level. Maybe it's just a mindset thing, but I always favor "explicit over implicit" and I don't like trusting things to faith, or common lore, or hunches, etc. And so when I see one of these, I generally just move on, as I'm not interested in trying to infer what the schematic author was trying to say (but couldn't be arsed to actually say).
That said, it's so common that I guess I have to suck it up and deal with it. So what's the big secret? How do you know where to apply voltage on something like this? And likewise for other "no explicit inputs called out" type schematics?
Whales:
For that particular example you need to take it literally: this circuit does not have an input. It oscillates on its own (like a cold human :P). Not all circuits have an input, unless you count their power pin (ie turning them on).
(On a more general note: yes assumptions by people writing technical works can get really annoying, especially when people give you funny looks for asking. I've been told before "you should go back to school!" and such things, the urge to murder has been strong.)
fourfathom:
Yes, this one has no input, it's an oscillator -- output only. With this schematic if you want to build it you are supposed to look up the 74HCT04 and find the input, output, ground, and power pins. You will need to know that you should terminate unused input pins to power or ground (there will be four unused inputs.) You will need to figure out what type of crystal is appropriate (not all xtals will work in this circuit.) But this schematic is really supposed to be an example, not necessarily a construction project.
mindcrime:
--- Quote from: Whales on August 03, 2019, 11:25:24 pm ---For that particular example you need to take it literally: this circuit does not have an input. It oscillates on its own (like a cold human :P). Not all circuits have an input, unless you count their power pin (ie turning them on).
--- End quote ---
Right, that I get. I don't mean a "signal input" necessarily... I'm including the power (Vcc) as an "input" in this context. And this particular example doesn't even show where that goes. I'm assuming you just apply the power to the base of IC1a because I don't see how it makes sense otherwise, but it's frustrating that they don't tell you so definitively. :-(
mindcrime:
--- Quote from: fourfathom on August 03, 2019, 11:52:58 pm ---Yes, this one has no input, it's an oscillator -- output only. With this schematic if you want to build it you are supposed to look up the 74HCT04 and find the input, output, ground, and power pins. You will need to know that you should terminate unused input pins to power or ground (there will be four unused inputs.)
--- End quote ---
That's a good point. Since they do call out the specific IC, it makes sense that they're basically "incorporating by reference" it's datasheet. Somehow that flew completely over my head when I first glanced at this.
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