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| 74LS14 testing circuit pls |
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| Chriss:
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... |
| Zero999:
Are the LED and resistor connected from the output to 0V? If so, your existing circuit sounds like the correct way to test it and barring any mistakes, such as: the IC's 0V in not being connected or confusion over which input goes to which gate, the IC is bad. |
| Chriss:
Yes. The LED is connected exactly as you described. Some gates are didn't recognize anything, some gates are non-stop on or off. .. Sent from my GT-I8260 using Tapatalk |
| ebastler:
If you have tried multiple ICs which do not work, it seems likely that your test environment has a problem. Do you have a clean supply voltage? (How do you generate the +5V?) It is advisable to have one electrolytic capcitor on your breadboard, e.g. 470µF, and one small 100nF capacitor at each IC. The capacitors are connected between +5V and GND to filter the supply voltage. If you want to test the behavior of the Schmitt trigger in more detail, you can connect a potentiometer between +5V and GND. (Use any value between 470 Ohm and 10 kOhm.) Then connect the middle pin of the potentiometer, which will have an adjustable voltage, to the Schmitt trigger's input. As you turn the potentiometer up and down, you should notice the hysteresis of the Schmitt trigger: The voltage where it turns the LED ON is higher than the voltage where (coming down) it turns the LED OFF again. |
| TomS_:
--- Quote from: Chriss on April 12, 2018, 08:59:15 pm ---74LS14 --- End quote --- As I understand it, TTL inputs effectively "float high", so applying a logic high and removing it does not alter the state of the input, which is perhaps why you see no change. Try adding a pull down resistor to the input and then apply a logic high to it and see if that helps. |
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