vivek_pv Posted August 4, 2006 Report Share Posted August 4, 2006 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 Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 4, 2006 Report Share Posted August 4, 2006 The circuit in your box needs to be a flip-flop. A blink flips it on and another short signal resets it. Quote Link to comment Share on other sites More sharing options...
vivek_pv Posted August 4, 2006 Author Report Share Posted August 4, 2006 Can you please give me the design of this "flip flop"?regards,vivekanand Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 4, 2006 Report Share Posted August 4, 2006 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. Quote Link to comment Share on other sites More sharing options...
vivek_pv Posted August 6, 2006 Author Report Share Posted August 6, 2006 How if I use 7400 (used as SR type F/f)? for this? Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 6, 2006 Report Share Posted August 6, 2006 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. Quote Link to comment Share on other sites More sharing options...
vivek_pv Posted August 6, 2006 Author Report Share Posted August 6, 2006 What is the difference between TTL & CMOS in working (as digital gate ics)?Is CMOS superior to TTL? Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 6, 2006 Report Share Posted August 6, 2006 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. Quote Link to comment Share on other sites More sharing options...
vivek_pv Posted August 11, 2006 Author Report Share Posted August 11, 2006 Tell me which type of Flip Flop should I use to serve the purpose.SR F/f with 7400 and 4011 was unsuccessful. Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 11, 2006 Report Share Posted August 11, 2006 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. Quote Link to comment Share on other sites More sharing options...
vivek_pv Posted August 12, 2006 Author Report Share Posted August 12, 2006 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. Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 12, 2006 Report Share Posted August 12, 2006 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. Quote Link to comment Share on other sites More sharing options...
vivek_pv Posted August 12, 2006 Author Report Share Posted August 12, 2006 How about this ckt? Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 12, 2006 Report Share Posted August 12, 2006 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. Quote Link to comment Share on other sites More sharing options...
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