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I have 4 LEDs that are rated for 3v-3.2v 20mA. Each LED will be wired in Parallel to a 5v power supply.

When power is applied to the circuit, I would like each LED to turn on with a delay.

The first LED with a 0.1 second delay, the second with 0.2 second delay, the third with 0.3 second delay, and the 4th with 0.4 second delay.

What capacitors, and resistors, will I need to make this possible?

LED gradually lighting up over time is fine.

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  • \$\begingroup\$ Do you expect the LEDs to cleanly change from off to on, or do you want them to gradually light up over time? \$\endgroup\$ Sep 23 '20 at 0:17
  • \$\begingroup\$ You can do this AFTER A FASHION with R & C only but it's very poorly defined - they will "ooze on". Adding a simple package of gate makes it work well. 1 x hex scxhmitt inverter IC. 74xx14 or CD40106 or ... \$\endgroup\$
    – Russell McMahon
    Sep 23 '20 at 0:19
  • \$\begingroup\$ To answer Elliot Alderson's question, the LED gradually lighting up over time is fine. I edited my initial question to include that statement. \$\endgroup\$
    – Gambit
    Sep 23 '20 at 3:17
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In this circuit the subsequent LED won't start its RC timer until the previous LED lights, so all time constants can be the same (0.1 sec, approx 1M and 0.1uF, adjust as necessary). The buffer needs to be non-inverting (could be a quad OR or AND) and capable of supplying the LED current (4 * about 5mA each). The LEDs will snap on at even intervals, not fade in.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Sorry, schematic shows 1uF caps - should be closer to 0.1uF as I mentioned in the text. The delay time is close to simply R * C. Too lazy to update schematic. A 74HC08 running on 5V would be adequate for the buffer (tie inputs together). \$\endgroup\$
    – td127
    Sep 23 '20 at 20:29
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This kind of problem is exceptionally well suited to the use of a microcontroller.

A PIC12F508 and a bypass capacitor (plus the LEDs and one resistor per LED to limit the current) is all that is required. You get typical accuracy of +/-1% on the timing without adjustment, and commensurate stability, both of which are hard to achieve with inexpensive components in a simple circuit.

Downsides include a bit of learning curve, and then necessity to purchase a programmer.

enter image description here

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    \$\begingroup\$ Or an (gasp!) Arduino & a USB cable. \$\endgroup\$
    – Russell McMahon
    Sep 23 '20 at 9:30
  • \$\begingroup\$ I have even previously built a PIC programmer based on an Arduino, it wasn't terribly difficult but the protocol wasn't very well documented and I found some differences from the specification. \$\endgroup\$
    – user253751
    Sep 23 '20 at 10:40
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You can do this AFTER A FASHION with R & C only but it's very poorly defined - they will "ooze on". Adding a simple package of gate makes it work well. 1 x hex Schmitt inverter IC. 74xx14 or CD40106 or ...

Many of these will work.

Drive current is about 8 mA max for any of these but you can probably get somewhat more in practice by overloading somewhat - Vout is not a legal logic level but works as LED driver. Output buffers can be added - eg cheap transistor and resistor if more drive needed.

Here is a through hole version in stock at Digikey. NTE4584B

Datasheet here


Use one of these circuits per LED.
Up to 6 available in the one IC.
T ~~~= RC time constant.
With C = 1 uF ceramic and R = 100k time ~= 1E-6 x 100,000 = 0.1 S.
Actual time depends on gate hysteresis level.
I have not checked this as it always need adapting to specific family - easily resolved by "playing".

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ I think you got your R1 and C1 backwards - your LED turns on immediately and gradually turns off. \$\endgroup\$
    – td127
    Sep 23 '20 at 3:38
  • \$\begingroup\$ @td127 True ! :-). Thanks. I probably started off with a grounded LED in mind. Will fix. \$\endgroup\$
    – Russell McMahon
    Sep 23 '20 at 9:23
  • \$\begingroup\$ The 74xx14 is an inverting buffer. \$\endgroup\$ Sep 23 '20 at 21:09
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If you power an LED using 5V and a 330-ohm resistor, it will flow about 9 mA and give you okay brightness. If you add a 330uF capacitor across the LED, it will give you an RC delay of about 0.1 sec (330uF * 330ohm). For each 330uF capacitor you add, it will delay another 0.1 sec (stage 1 1-capacitor, stage 2 2-capacitors, etc). Unfortunately, as you add stages/capacitors, LEDs will fade-in and appear sluggish -- you said this is okay -- so that's my answer.

A more advanced circuit is: I like what others had written here about using a Schmitt trigger buffer chip. However, the 74LS14 chip acts is an inverter and will behave oddly. Instead, I like using the 74LS125 non-inverting buffer. Similar to this schematic below. Note the RC time constant of 1uF * 100K = 0.1 sec.

Good luck with your project/learnings! Staggered LEDs using 125 buffer.  Time constant 0.1 sec per stage.

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