• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Ttl output noise

qwertyqwq

Member
Hi all.
I build a nor gate circuit using 74hc02.
The circuit doesnt need a scheme cause its simple. 74hc02 is quad two input nor gate and im just using one of them. The first input of the gate is 25kHz pulse signal which is coming from 74hc14 pulse sharper. The pulse generating from arduino. And the other input is just a 5V constant voltage which is coming from a push button.
Let me tell about my power supply. The power supply is adapter looks like pc adapters and its giving 12vdc output. Cause of the supply range of ttl ic's , i builded a current boosted zener regulator just like this;
index.pngThe regulator circuit has just few millivolts ripples and its giving about 5.12V. And there is also a 10uF electrolytic capacitor after the regulator. Circuit is builded on pcb board and soldered .
Okey, here is my problem. When i hook up my oscilloscope to input signal ,here it shows to me; WhatsApp Image 2020-05-07 at 00.01.24.jpeg
and when i look at the output of nor gate , it shows this; WhatsApp Image 2020-05-07 at 00.01.25.jpeg
As you can see there are lots of noise on my signal. The weird thing is when i touch my finger to 12V or 5V or even Gnd , noises are getting lost and pure signal is showing on my scope. After i stop touching the points , about 2-10seconds later the signal is getting same noises and dont know where it is comming from !.
At first i thougt it need decoupling caps and i soldered two 100nF ceramic capacitors directly on Vcc to Gnd pins of my 7402. But the result didnt change at all. After some searching also added to resistor on output to ground pin (about 500ohms) signal is still acting same. And the other weird thing is , when i physicaly shake my board, noise are incearsing !!!

NOTE: On scope screen there is 0.7v offset because of i added a diode to Gnd to ground for reverse polarity protection.

If you have idea what is going on , please let me know. Thanks .
 

crutschow

Well-Known Member
Most Helpful Member
Where is the oscilloscope ground connected?
The noise symptoms suggest its floating.
 

audioguru

Well-Known Member
Most Helpful Member
74HCxx is high speed Cmos, not old fashioned TTL.
Did you build the circuit on a solderless breadboard with wires all over the place? Guaranteed oscillations like you show.
 

Latest threads

EE World Online Articles

Loading

 
Top