| Electronics > Beginners |
| 74LS14 testing circuit pls |
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| StillTrying:
On the scope pic the input signal is only just going LOW enough to reach the 14's lower hysteresis level, which can be quite low ~0.6V, it could do with going a bit lower. "Could it be does the IC's was killed from ESD?" In theory yes, in practice no. Edit: While I'm closing down old windowz, I post this here. :) |
| Ian.M:
I just dug out a crusty old Motorola 1981 date code CERDIP 74LS14 and breadboarded up a simple test jig. I used longer jumpers for the gate input and output for ease of moving them around the chip. Its currently blinking away at about 6Hz. LED steady ON - input not connected or stuck high or output stuck low LED off - input stuck low or output stuck high LED blinking - gate O.K. You need such a low value of feedback resistor R1 to guarantee the output low can pull the input down to the logic '0' threshold. Its a little bit abusive as when the output goes high, its pulling about 8mA to 10mA from it initially to charge the capacitor, but as that's only for a brief part of the circuit, and the IC can withstand a single output direct short to ground for up to one second, its unlikely to cause any issues unless the IC is really crappy. I wouldn't however use this circuit long-term in a permanent build. The same circuit will do for testing any inverting Schmitt trigger input bipolar TTL gate - just tie extra inputs high (via 1K) or low to reduce the gate's logic function to a single inverter, and apply the test jig to one input at a time. If you want to test a CMOS gate, decrease C1 to 4.7uF and increase R1 to 33K. It is *NOT* suitable for testing non-inverting or non-Schmitt input gates. If your PSU wires are longer than a few inches, put 100uF across them at the breadboard. |
| Chriss:
Ian.M Thank you, I will try your method later. Here is the IC how they look like: Is this maybe a fake one? |
| ebastler:
--- Quote from: Chriss on April 13, 2018, 08:08:17 pm ---Is this maybe a fake one? --- End quote --- Certainly a vintage one - from 1985?! Where did you get these? The EL manufacturer prefix does not ring a bell for me; but I certainly don't claim to know them all. The body material looks unusual (color, sharp edges). Maybe from the former Eastern block countries? Anbody? In any case, this seems like a chip someone dug up from the bottom of a long-forgotten box; so it might well be fishy... |
| Chriss:
--- Quote ---Personally, in future I would look at AHCT family chips... --- End quote --- I found this one as equivalent in AHCT version: SN74AHC14 but the main caracteristics are the same as the LS. Can you pls give me a model of AHCT? which would be nice for me. I have to use the IC to convert the photo interrupter signal to a logic Hi-Lo signal... I got that ic from a local electronic component store in Hungary. |
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