# Turning on eight LEDs

I have one trigger which is a 200 ns pulse and a clock with 12.5 ns width and 50% duty cycle.

I have made 8 pulse each 5 V and 12.5 ns width.

I need to turn each LED for each of my pulse so when first pulse comes the first LED turns on and the second LED will turn on when the second pulse comes but the first LED will remain on.

I want to use some transistor but I cant make it works. i can use FF but not IC.

edit: i need to turn on the leds each at time and make them stay on until at any time i get new trigger and then they will turn off and will turn on each at time again. i just need take my 8 pulses and every pulse will turn on one led at time and keep them works until next trigger. in the question they said transistor can help. i know FF can be usefull here. Timing diagram.

               _   _   _   _   _   _   _   _              _   _   _
Pulse    ___| |_| |_| |_| |_| |_| |_| |_| |_____.....__| |_| |_| |...
__________________________________________   ________
LED 0    ___|                                          |_|        ...
______________________________________       ____...
LED 1    _______|                                      |_____|
__________________________________           ...
LED 2    ___________|                                  |__________|

• A timing diagram is better than a lot of words. Please check my edits. How do you want to turn off your LEDs? Mar 14 '20 at 21:02
• actually i dont must to turn them off but if you have any easy idea i can use it will help. Mar 14 '20 at 21:11
• why do you need to turn on the LEDs sequentially? Mar 14 '20 at 22:04
• Unless you’ve special LEDS or aggressive drive and wiring, the LED on time is going to be around 50 ns. electronics.stackexchange.com/questions/86717/…. It would help your question if you said a few words about why you want this circuit to really do. Mar 15 '20 at 13:42
• Is it possible that ns should be ms? If this is to be seen by humans then ms is a million times more likely. Mar 15 '20 at 17:41

simulate this circuit – Schematic created using CircuitLab

Figure 1. A shift-register made with 'D' flip-flops.

How it works:

• Every time the clock goes high (V+) the data on the 'D' input appears on 'Q'.
• All the clocks read at the same moment so REG2 won't see the change on REG1 until the next cycle.
• When SW1 is switched to ground the LEDs will turn off one by one starting at D1.

Timing diagram.

               _   _   _   _   _   _   _   _
Pulse    ___| |_| |_| |_| |_| |_| |_| |_| |___
_________________________________
LED 1    ___|
_____________________________
LED 2    _______|
_________________________
LED 3    ___________|

• actually i cant use clock or counter. Mar 15 '20 at 5:24
• Why not? The first line in your question says you have a clock. I did not use a counter. I used D flip-flops. Mar 15 '20 at 9:09
• Well I have one trigger and I made eight pulses with the clock. Now I can use only the 8 pulses and 5v source. Also i need a solution which is using transistors Mar 15 '20 at 10:44
• Why do you need a solution which uses transistors? Is this a homework question? Mar 15 '20 at 12:52
• @IdanAra_18 You can make a flip-flop from transistors. Mar 15 '20 at 14:44

You can do that with a shift register (for example 74HC595) or a decade counter (?). Search also for other models than those mentioned as example, and chose the one you prefer. If it's just an indicator led (max 8mA) you can drive them directly form these ICs. If they are power leds, consider adding a transistor mosfet to increase current and voltage.

• If you mean the CD4017 then that isn't suitable. It only turns on one LED at a time. (It's a Johnson counter, not a shift register.) Mar 14 '20 at 22:08
• Please let us know where we can get a 74HC595 that will run at 80MHz. Where can we find a CD4017 that will operate at 8MHz, much less at 80MHz? Who makes a CD4017 or 74HC595 that can provide 20mA to an LED? Links to the datasheets would be much appreciated. Mar 14 '20 at 22:09
• Where does the 20mA come from? Anyway LED's don't need 20mA these days. A high efficiency LED can work perfectly from 1mA. Mar 14 '20 at 22:13
• Transistor. I didn't remember exactly. You must be right. Maybe there are simple counters with increment instead of one at a time? Does have to be a sophisticated shift register but it depends what you want to do. Mar 14 '20 at 22:19
• Eillot Alderson Where does the OP speaks about such frequency. At such speed he can turn everything on at the same time as well. Doesn't make any sens. Most IC's (not all) provides up to 20 mA, that why I wrote "max". As Oldfart said, SMD leds can be lit with 1 or 2 mA. Mar 14 '20 at 22:22