Hi,
Please help, I am pulling my hair out!

I have built a 4021 NOR gate latch based on a schematic that is readily available online.

I want 4 inputs and 4 LEDs for outputs.

I am using it as 2 separate SR latch circuits.

When I build 1 half all is fine. That is one input going high drives an LED high, driving the other input high drives the other LED high.

That is all fine.

If I build the other half all is still fine.

The problem comes when I build the full circuit up, that is 4 inputs, 4 LEDs as 2 SR latch circuits.

One latch circuit will work but the other will not. One of the LEDs seems to take priority over the other. It is lit, therefore high. When I drive the other input high the other LED goes high and then other turns off. Now comes the problem. One LED will not latch, allow that input to go low and the other LED goes high, ie it does not latch.

I am using 10k pull downs on the inputs.

Any ideas?

Maria

 

Can't really tell without a schematic.

A guess: Do you have resistors in series with the LEDs to limit current?

 

Can't really tell without a schematic.

A guess: Do you have resistors in series with the LEDs to limit current?

Yep

 

What does the rest of your circuit look like, including power supply connections?

How abotu a link to the schematic on the web that you are using?

 

What does the rest of your circuit look like, including power supply connections?

How abotu a link to the schematic on the web that you are using?

I will have a look for it.

 

Could you show the actual schematic?

 

CD4021 appears to be a shift register. SR = Shift Register? SR = Set/Reset?

What part did you actually use?

 

Forgive my error

I meant, 4001 NOR gate

 

That makes a lot more sense now. If we can see a schematic of what you made we might be able to figure out why it doesn't work.

 

That makes a lot more sense now. If we can see a schematic of what you made we might be able to figure out why it doesn't work.

Cool. As soon as I can I will post the schematic. Thanks

 

4021 is 8 bit shift register, while 4012 is 4-input-NAND.

Which chip did you use?

 

 

Thats the schematic, I am using a CD4001

 

What is the value of the current-limiting resistor in series with the LED? A CD4001 can source or sink only a few mA. If you try to draw more current the output voltage changes, perhaps to a value below the point at which the LED will light.

 

Hello,

Connect a 0.1uf cap from the V+ pin to the V- pin of the chip.

eT

 

hi M,
What are the LED types and their series resistors.?
E

 

Hello,

Connect a 0.1uf cap from the V+ pin to the V- pin of the chip.

eT

Already tried that

 

hi M,
What are the LED types and their series resistors.?
E

Standard 5mm with 10k with a 12vdc supply

 

You should not drive an LED from a logic gate and expect the output of the logic gate to be a valid logic level.
Use a separate LED driver to drive an LED.

 

A 10k is too high a value,and what are the Colours of the LED's.?