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A project, help


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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


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


I am enclosing the schematic. please help.



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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.
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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.
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