I am trying to build a 4 LED sequential turn signal using the ne555 timer module and some comparators. I am having trouble with my next steps when trying to figure out what is wrong with my design. Below is a picture of my schematic. In falstad, it works exactly how I would want it to work. In real life, after actually ordering components and putting it together on a bread board, all of the LEDs turn on and stay on. I am assuming this has something to do with my comparator resistors not being at the right value. At first glance, is there something I am missing or doing wrong? Advice on where to go would be greatly appreciated. The switch next to the 10uF capacitor is to imitate what the 10uF should be doing in real life. I believe falstad only uses ideal capacitors so they are always charged. I am open to different designs as well. This will eventually be used to power LEDs with a fwdV of 2.9V @ 350mA. enter image description here

  • \$\begingroup\$ So you are making a high going pulse that "slowly" rises and the comparators will change their output as the voltage of the pulse rises. In real life, what does the voltage ramp look like? \$\endgroup\$
    – Aaron
    Nov 17, 2021 at 15:43
  • \$\begingroup\$ What comparator and transistors did you use for the build? \$\endgroup\$
    – Aaron
    Nov 17, 2021 at 15:44
  • \$\begingroup\$ The time constant of 1k and 10u is 10 milliseconds (neglecting the current stolen by opamp inputs). Let's assume that this duration is divided into 3 equal time slots for each sequence: 3.3ms = 300Hz. The human eye can catch lower than 20-25Hz, so you cannot distinguish the sequential illumination of the LEDs. Plus, you should always be careful while working with breadboards. \$\endgroup\$ Nov 17, 2021 at 15:48
  • \$\begingroup\$ @Aaron For the comparator, I am using the LM2902N and I am using BT169D,112. (Ordered these off of mouser). \$\endgroup\$
    – Myles
    Nov 17, 2021 at 16:37
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    \$\begingroup\$ @RohatKılıç So, you are saying that it could be working but it could be switching so fast that it just looks on to me? \$\endgroup\$
    – Myles
    Nov 17, 2021 at 16:38

1 Answer 1


I converted your negative image into a normal positive, then cropped and enlarged it. Your 1k feeding the capacitor, 10k feeding the resistor divider, 510 and 390 ohms resistor values are 20 times too low. sequential

  • \$\begingroup\$ Okay, I will order those components and try that out. Why are they 20 times to low? Wondering so I can understand what I am doing. \$\endgroup\$
    – Myles
    Nov 18, 2021 at 16:54
  • \$\begingroup\$ When I simulate that in falstad, none of it works. The resistance in the 1k and 10k when upped by 20 times is too large for current to flow. \$\endgroup\$
    – Myles
    Nov 18, 2021 at 17:34

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