Electronics > Beginners
Not understanding behaviour of HCF4093BE: Schmitt Trigger NAND gates
javi14:
Hello everyone!,
this is my first post here, so I hope I can explain my question in an undestandable way!
My purpose is to use a HCF4093BE to blink some leds and build a led display. With this IC, there are 4 outputs, so I expect to be able to ligth 4 arrays of leds, forming 4 characters (displaying a person's name, for example). I attach the datasheet of this component.
I have tried the IC using a single NAND gate first, and I checked that everyting worked. By adding a capacitor and a resistor, I could get a beautiful oscillator. Then, it was time to add the other outputs to check that everything worked correctly. I am using transistors to drive de LEDs because I expect to drive many of them with a single output (thus increasing the current needed). I attach the schematic (note that I don't have 4 transistors of the same type, so I have mixed them) and the circuit (I am using a 5v 1A USB supply to power the circuit)
hcf4093.pdf (348.84 kB - downloaded 77 times.)
When I powered the circuit up, I expected to see all 4 leds blinking. Maybe their frequencies would be different, as I expect the passive components to not be perfect. But for my surprise, the LEDs were blinking with an unexpected behaviour! They are following some kind of pattern, but then changing it randomly, so I really don't understand what is going on. I have recorded it and uploaded to Youtube, so you can see the leds blinking quite "randomly".
https://youtu.be/Qxqr4AZS23k
I will be grateful if someone can explain this to me, and provide me with a solution to this (if it exists), as I would want to blink them synchronously. The reason why I am using separate outputs, is because I want to be able to turn on and off some otputs, while keeping others on.
I guess that I have fallen into a trap for young players!
Thank you!
EDIT 1: Added a 100nF ceramic cap between VDD and GND, as close to the IC as possible. The behaviour of the circuit seems to be the same.
rstofer:
With a 100k base resistor, I doubt that the transistors are turning completely on. Stop the oscillation and measure the voltage between collector and emitter. Ideally, you want to see the number around 0.2V or whatever VceSat is for the particular transistor.
wraper:
--- Quote ---But for my surprise, the LEDs were blinking with an unexpected behaviour! They are following some kind of pattern, but then changing it randomly, so I really don't understand what is going on. I have recorded it and uploaded to Youtube, so you can see the leds blinking quite "randomly".
--- End quote ---
I don't see anything unusual. Each LED blinks independent of others, if you see any "pattern" at some points of time, it's just because oscillator frequencies are very close but not exactly the same.
james_s:
I don't see any power supply decoupling, it's possible that your power rails are glitching and causing one channel to affect the others.
javi14:
--- Quote from: wraper on January 05, 2020, 05:09:50 pm ---
--- Quote ---But for my surprise, the LEDs were blinking with an unexpected behaviour! They are following some kind of pattern, but then changing it randomly, so I really don't understand what is going on. I have recorded it and uploaded to Youtube, so you can see the leds blinking quite "randomly".
--- End quote ---
I don't see anything unusual. Each LED blinks independent of others, if you see any "pattern" at some points of time, it's just because oscillator frequencies are very close but not exactly the same.
--- End quote ---
But arent they oscillating tooo differenly for the passive component errors? I expected to see it more slightly similar.
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