# Delay LED turning on

I’m trying to think of a way to have many LED in serie to turn on one by one.

Ex: 3 LED connected in serie, LED1 turn on when power is apply to the circuit then about half a second later LED2 light up and another half a second later, LED3 turns on.

If they were in parallel, I would have use capacitor/resistor to delay the moment they turn on.

Is there a way to do it ? (My guess is that it’s not possible as the current flow in the circuit at the same time but I’m not an expert so wanted to check with the pros)

Thanks!

• use a microcontroller if you have many LEDs Commented Jan 15, 2023 at 0:21
• They won’t be in serie with a microcontroller. If I have 50 LEDs, I’ll then need a microcontroller with 50 output pins. Commented Jan 15, 2023 at 1:13
• @Dominique 49 leds could be driven as a 7x7 grid using only 14 outputs. Search for “LED matrix”. Otherwise I’d recommend address able leds in a string like the ws2812b Commented Jan 15, 2023 at 1:37
• If you had 50 LEDs at 3V each you'd need 150V minimum. Is that what you had in mind? Commented Jan 15, 2023 at 2:14
• (It's (connected) in series - or is it (mounted) in a row?) Please add to your question a ballpark figure for many. Commented Jan 15, 2023 at 6:31

use capacitor/resistor to delay the moment they turn on.

This idea in itself is not likely to work very well, if at all. I highly recommend trying to do it anyway, as experience is a valuable lesson. Basically, the LED will turn on slowly as the voltage rises, and the resistor you need to get it slow enough wouldn't pass enough current to light the LED to a useful level.

That didn't stop me from thinking about the question :)

You can make an LED in series turn off by shunting current around it. Those fancy words just mean "short-circuit the LED". As you understand the current to be the same through the series circuit, the other LED's would light up. The problem, then, is that there is no voltage across the shorted LED, so it's hard to put any kind of circuitry there.

But wait! If we put a capacitor across the LED, we start with a short--all current is going through the capacitor in order to charge it. Then, when the voltage reaches the LED's voltage, the LED turns on and the current goes through that way, even though the capacitor isn't taking any more current.

simulate this circuit – Schematic created using CircuitLab

The trick that will give this thing a chance of working is the constant current source (I1). This eliminates any issues with the fact that voltages in the circuit are all changing as it charges up. And you don't have to use any resistors to set LED currents. I chose 10mA because the actual capacitors will end up being very large, so the lower the current, the slower their voltage will rise.

I still don't think the results will be all that good but given the current source and the limiting effects of the LED's I think it actually has a better chance than the parallel version. Again I suggest trying it if only for the experience. I may even try it myself just to see!

By the way, the schematic maker wouldn't let me turn the current symbol over, so the arrow points down. So I changed it to -10mA, just because.

Oh, and you can buy things called Individually Addressable LED strips. You drive them from a microcontroller. This probably counts as "cheating", but that's for you to decide. What you get is the ability to control the color and the brightness of each and every LED on the strip!

• You're low in capacitance by about 100:1. Commented Jan 15, 2023 at 2:13
• Thank you, I really appreciate your answer (people have the bad habbit to try to find other « ways » which does not help me with that kind of « to learn » question. I know well how adressable LED works and I know how to manage them with a microcontroller ;) Commented Jan 15, 2023 at 2:29
• Just for the extra explaination, I got that question after receiving slow blink LEDs. I taught about having a lot of them (maybe 30) connect to AC (with a cap that would limit the current to about 10mA and a full bridge rec) but I would have like them to not be the same colors so having them turned on at different time would result in having different colors at the same time). I’m recycling old chrismas light, replacing all burned lights with LED, for fun and to learn. Thanks again for your answer, I’ll try it just to see if that works 😉 Commented Jan 15, 2023 at 2:34
• (I taught about having a lot of them thought) Commented Jan 15, 2023 at 6:27
• Consider the tolerance in capacitance you need to have a 48th LED light subsequently to LED 47, but before LED 49. Now, consider somewhat homogeneous delays. And the transition from not lit to lit as swift as for LED 2. Commented Jan 15, 2023 at 6:28

The fun way was to let the emission of one LED trigger turn-on of the next one.