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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