Why is my D-latch not working?

Thread Starter

Ajgor

Joined Nov 16, 2023
16
dlastch.png

Hi, i have been trying to o a all transistor binary counter using d latches but my pcb doesn't work. When i test it on simulation program everything is all right. Can you help me see where i have made a mistake?
 

Thread Starter

Ajgor

Joined Nov 16, 2023
16
What will that change?

Go back and compare the logical elements in the logic diagram with your transistor version.
The only difference i can see is the lack of buffers but i don't think they change anything, but maybe this it the issue:

1700172786064.png Is this resistor screwing me?
 

michael8

Joined Jan 11, 2015
407
What simulator are you using?

The input to your transistor NOR gates is directly to the base of the transistor and the emitter is grounded. So the maximum
base voltage will be the Vbe of the transistor (and the current isn't limited). This will limit the output voltage of the stage driving
it to a "high" voltage of around 0.7 volts, much lower than Vcc.
 

Thread Starter

Ajgor

Joined Nov 16, 2023
16
What simulator are you using?

The input to your transistor NOR gates is directly to the base of the transistor and the emitter is grounded. So the maximum
base voltage will be the Vbe of the transistor (and the current isn't limited). This will limit the output voltage of the stage driving
it to a "high" voltage of around 0.7 volts, much lower than Vcc.
Thanks! i will try to break it down and understand what you said haha. For the simulation above i used https://logic.ly/demo/
 

Thread Starter

Ajgor

Joined Nov 16, 2023
16
So the solution will be to add like 2 inverters before Q17 the Vbe of it will be higher?
1700248618995.png
 
Last edited:

michael8

Joined Jan 11, 2015
407
So you are NOT simulating at the transistor level. The problem is that your basic transistor NOR gate is wrong so simulating
the logic using NOR gates doesn't show the problem but building with real transistors does.

Your NOR gate (built with transistors) should accept gnd or Vcc on each input and not load the inputs too much.
The output should switch between close to ground and Vcc.

PS: I'd prefer schematics with inputs toward the left, outputs toward the right, Vcc at the top and gnd at the bottom.
Your 1k collector resistors should go straight up and the NOR output should be to the right (not vertical).
nor.png
 

Thread Starter

Ajgor

Joined Nov 16, 2023
16
So you are NOT simulating at the transistor level. The problem is that your basic transistor NOR gate is wrong so simulating
the logic using NOR gates doesn't show the problem but building with real transistors does.

Your NOR gate (built with transistors) should accept gnd or Vcc on each input and not load the inputs too much.
The output should switch between close to ground and Vcc.

PS: I'd prefer schematics with inputs toward the left, outputs toward the right, Vcc at the top and gnd at the bottom.
Your 1k collector resistors should go straight up and the NOR output should be to the right (not vertical).
View attachment 307816
nice, i will try that on breadboard ;)
What program do you use? does it simulate at the transistor level? (i use falstad also)
 
Last edited:

michael8

Joined Jan 11, 2015
407
I use LTSPICE it simulates at the transistor level (and runs locally and is free).

In your #1 post diagram the transistors Q17 Q18 Q19 Q20 don't have resistors on their inputs so they
load down the outputs driving them. Also the LED4 doesn't have a resistor so it's going to load down
the output driving it. Perhaps the LED should have it's own driving transistor.

Also in #1 where are the D latch inputs?
 

Thread Starter

Ajgor

Joined Nov 16, 2023
16
I use LTSPICE it simulates at the transistor level (and runs locally and is free).

In your #1 post diagram the transistors Q17 Q18 Q19 Q20 don't have resistors on their inputs so they
load down the outputs driving them. Also the LED4 doesn't have a resistor so it's going to load down
the output driving it. Perhaps the LED should have it's own driving transistor.

Also in #1 where are the D latch inputs?
1700253649696.png
It actually suposed to be a d flip flop. I am trying to do a binary counter (i will add the resistors to Q17-20)
1700254050562.png
Now it works in falstad, but the led doesn't wanna turn off, but i think its the program fault because i can't do such a quick pulse
 
Top