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| 74LS14 testing circuit pls |
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| Zero999:
--- Quote from: Ian.M on April 13, 2018, 09:12:27 pm ---So you've only got about 3V input amplitude, which means that if you are running the logic from 5V, you *NEED* real bipolar TTL or LS TTL or 74HCT not 74HC logic. --- End quote --- One way round that is to add a diode to the pull-up resistor. It's a bit of a crude hack and only works at fairly low frequencies. |
| Ian.M:
The other dirty trick is to lift the ground of *ALL* the 3.3V logic by one diode drop, which can work reasonably well with faster signals. However it barely makes the typical 70% guaranteed logic '1' threshold, so isn't great for noise immunity. Also, beware of 5V logic with 80%/20% thresholds, as even with a diode drop boosting it, the 3.3V logic wont reliably cross the 80% threshold unless you overvolt the 3.3V logic and/or undervolt the 5V logic. |
| joeqsmith:
--- Quote from: Chriss on April 12, 2018, 08:59:15 pm ---Hi! Can somebody share with me a simplest testing circuit for a 74LS14 Schmitt Trigger pls. I have some strange situation with this ic's and I cant figure it out. I give power to the ic as in the datasheet described. I put a LED through a 330Ohm resistor to the Y1 and set a +5v to A1 and the led is all the time on. No matter if I set a +5v or GND to the A1. Pin. Also to other gates too.. So, does I mess something or are my ic's bad? Some of them with the same setup didn't lite up the LED. I just swap the ic in the same circuit... --- End quote --- Picture of your setup may have helped. Could be wired wrong. Some time ago, I played with a bunch of old DIP inverters. I started out trying to replicate someones setup using some whiteboard. https://www.eevblog.com/forum/projects/waveforms-in-a-74ls04-ring-oscillator/ |
| Chriss:
Ok. I have some questions: 1. Is my ic fake or not? 2. Is it worth to buy the HCT version and use that new technology for upcoming projects or can I use also the old 74LS14 series? I digged through tons of data sheets, resources on the net but found millions of variations of circuit made with the 74ls14 which are actually crossing between the theory and real world hardware circuits. So I found also a blog where somebodi I think from Porugal wrote about a light detector circuit where actually my circuit was used. I mean the guy who wrote the text on that blog didn't copy - paste my circuit but it is the same meaning how they made his electronic light switch. If my circuit could kill the ic an/or are wrong then millions of other circuits are still wrong in the world, what are not impossible. No one datasheet show any so to say "tipical" schematic how to use the ic. Can we figure out a tipical wireup for this ic what could work in real world? What a kind of crappie ic is this when it can't handle a LED diode. Because of 20mA max current they will be killed.... Than it is a "must have" to put a transistor to the output. Without the teansistor a such of ic can be really easy killed. Alk these is to shitty. At the end I come to a mind not to use this crappie ic's and interchange them wit an LM339 and finishing the job. So, what is your opinion about a circuit how to use this crap inverter to switch the input from a photo interrupter and the output should send the signal to a uC. Sent from my GT-I8260 using Tapatalk |
| ebastler:
--- Quote from: Chriss on April 14, 2018, 07:12:03 pm ---Ok. I have some questions: 1. Is my ic fake or not? 2. Is it worth to buy the HCT version and use that new technology for upcoming projects or can I use also the old 74LS14 series? --- End quote --- Ad 1. It seems unlikely that it is a fake, but given its age and not-so-mainstream origin and package, maybe it has not aged well and has degraded over time. Have you tested it with (a) clean power supply, decoupled with suitabe 100nF capacitor, (b) strong, well-defined input test signal from pullup resistor and switch to GND, and (c) checking the output on an oscilloscope, without a load connected? It is not clear to me from the thread if you have done tests where all three conditions were met. If the anser is "yes", ad the chip still did not behave, then it is probably bad. Ad 2. A key benefit of an HCT chip would be that you can get one from current production, ruling out the possible degradation of your Hungarian LS chip. Also, the higher input impedance of the HCT may be helpful if you feed it a wimpy signal from a photosensor. So yes, I would give it a try. |
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