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The title is referring to a youtube video if a 555 and CD4017 led larson scanner but this circuit only allows the LED to be on at the same time interval.

Is there a circuit which can help to activate three LEDs consecutively for different time intervals. For example, LED 1 will on for 3s (the other 2 LEDs will be off), followed by LED 2 will be on for 2s (the other 2 LEDs will be off), then finally LED 3 will be on for 5s (the other 2 LEDs will be off) and this cycle repeats itself.

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    \$\begingroup\$ Johnson counter will easily count to 10 with OR gates with a 1 Hz clock like 3,2,5...s \$\endgroup\$ Jul 7 at 3:30
  • \$\begingroup\$ @TonyStewartEE75, hi sir, do you mind if you could show me a schematic of it if you are not busy :) ? \$\endgroup\$ Jul 7 at 3:47
  • \$\begingroup\$ i'm busy, would you like to think about it design it \$\endgroup\$ Jul 7 at 3:55
  • \$\begingroup\$ @catherinetan A twisted ring johnson counter made from 5 FFs provides 10 states. If you light up LED 1 when the last three FFs are 0 and light up LED 3 when the last FF is 1 (tie LED 3 directly to last FF) and light up LED 2 otherwise, then you have it. \$\endgroup\$
    – jonk
    Jul 7 at 4:20
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    \$\begingroup\$ @catherinetan This only works for your single, concrete example as given in the question. If you change the problem, the logic changes. If you want a generic solution where you can program in the period and the timing, then you have an MCU or else some kind of "plug-board" computer. \$\endgroup\$
    – jonk
    Jul 7 at 4:31

2 Answers 2

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The typical NE555 circuit has a timing capacitor that will be discharged by a resistor connected to pin 7 and charged by another resistor connected to the positive supply. You can modify (shorten) the charge time with additional current from the outputs of the johnson counter.

Connect each counter output with a different series resistor and a diode to the timing capacitor of the NE555. The diodes are needed to avoid current flow back to the inactive outputs.

Use resistors in the range 2 kohm to 100 kohm for a first attempt and use a larger timing capacitor, because this method produces only individual shorter clock periods.

enter image description here

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  • \$\begingroup\$ Could you add a schematic of that? Even a pseudo one? \$\endgroup\$
    – Passerby
    Jul 7 at 15:48
  • \$\begingroup\$ @Passerby just added \$\endgroup\$
    – Jens
    Jul 7 at 16:31
  • \$\begingroup\$ Ah, parallel the led output with the wire going to the discharge pin. \$\endgroup\$
    – Passerby
    Jul 7 at 16:49
  • \$\begingroup\$ @Passerby D1-D3 are not LEDs! You add them to the outputs, so you decide how bright they will be. Do not take more than 4 mA, CD4017 has weak drive strength. \$\endgroup\$
    – Jens
    Jul 7 at 16:55
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The 10 off 15k resistors isolate the 4017's outputs from each other as well as providing base drive for the transistors. The transistors are basically acting as NOR gates, if any input to a transistor's base resistor is high then that transistor's collector is pulled low turning on an LED.

The 680R resistors limit LED current to about 10mA.

The 7555 is running at just about 1 Hz.

LED Driver

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