[vivek_pv]

Hi all,

Now I have a project.

Let me tell like this :

1. There is a chip which emits a +5v signal (of very sort span) when one condition is

satisfied. Like a blink.

2. I want to take that "one blink type signal" and switch on an LED.

3. If I use, NPN and give that signal to base, then the ckt will be on as long as the signal

persists.

4. But I want to hold the LED "on" unless anybody resets it.

5. This I have made using relays. (using feedback type)

6. I have tried the same topology using transistors. (Collector to base) but that's not

working.

I am enclosing the schematic. please help.

regards,
v

View attachment 39491

 

[audioguru2]

The circuit in your box needs to be a flip-flop. A blink flips it on and another short signal resets it.

 

[vivek_pv]

Can you please give me the design of this "flip flop"?

regards,
vivekanand

 

[audioguru2]

Can you please give me the design of this "flip flop"?

For more than 40 years, when I needed a flip flop then I used a flip flop IC. You can look in Google if you want to make your own.

 

[vivek_pv]

How if I use 7400 (used as SR type F/f)? for this?

 

[audioguru2]

I haven't used an old 7400 for more than 40 years. I made set-reset flip-flops with them but since I have used Cmos and flip-flop ICs. Look for a set-reset flop-flop that uses TTL gates in Google.

 

[vivek_pv]

What is the difference between TTL & CMOS in working (as digital gate ics)?

Is CMOS superior to TTL?

 

[audioguru2]

What is the difference between TTL & CMOS in working (as digital gate ics)?

Google will tell you more than me, but Cmos has a very high input impedance, has a very low supply current, works from 3V to 15V and 74HCxx high-speed Cmos is just as fast as 74LSxx.

Is CMOS superior to TTL?

I think so. Intel and other computer chip makers also think so.

 

[vivek_pv]

Tell me which type of Flip Flop should I use to serve the purpose.

SR F/f with 7400 and 4011 was unsuccessful.

 

[audioguru2]

Look at "set, reset flipflop" in Google. It shows that a NAND gate flipflop won't work with your high signals, it works with low signals. You need a NOR gate flipflop.

The signals to a gate flipflop must be momentary. If an input signal stays then the other signal won't work.

 

[vivek_pv]

Ok.....

But if my set signal stays for 3sec or still longer, then what type should I use.

Another thing the input signal is of +3V.

 

[audioguru2]

Ok.....

But if my set signal stays for 3sec or still longer, then what type should I use?

Use capacitor coupling. It will give a short duration input pulse no matter how long the input stays. Then the NOR gate flip-flop's inputs also need pull-down resistors.

Another thing the input signal is of +3V.

Then the supply voltage for the gates of the flip-flop should be the same.
Ordinary Cmos gates work with a supply voltage down to 3.0V. 74HCxx high-speed Cmos gates work with a supply voltage down to 2.0V.

 

[vivek_pv]

How about this ckt?

View attachment 39520

 

[audioguru2]

Gate inputs must be connected to a DC voltage, so I added pull-down resistors for a logic low at rest.
You had the LED connected so "set" would turn it off the first time. I have it so it turns on.
A "power-up reset" circuit could be added.

View attachment 39522