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I've seen thousands of standard led chaser circuits online but I haven't seen one in which I can shut off all LED's in a non-mechanical way.

I'm wiring one pin of my chaser up to a microcontroller so that it controls whether or not the LED chaser operates or not. In the schematic, the pin is labelled as GPIO.

The reason I'm not using a micro for the entire chaser design is because I'm doing a major project with someone else and they don't know coding, so I want the other person to adjust the speed with control knobs. That part I have no problem with.

My circuit is a modification of the circuit found at: https://electrosome.com/wp-content/uploads/2016/09/LED-Chaser-using-4017-and-555-Timer-Circuit-Diagram.jpg

Here I'm just showing one LED and one stage.

Assume that one LED with one resistor is actually a set of 30 LED's with 2.0V voltage drop and 30 330 ohm resistors connected in parallel

Here's my question. Is the NPN transistor with emitter connected to ground in the right position to switch off a large array of LED's and are the resistor values to the NPN base correct?

And could I also copy the transistor to LED arrangement (circled in green) and apply it to each Q output of the 4017 and tie the NPN emitters in the LED arrangements together and to the NPN collector of the grounded emitter transistor?

In the end I want to make 5 sets of 30 LED's bright and have each set light up one at a time in sequence and have the option to shut the whole thing off without manually disconnecting power.

LED chaser with automatic shutoff

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  • \$\begingroup\$ The drivers in this voltage range have an RdsOn of 300 Ohms so no R is needed, just a common emitter switch to the Common cathodes to permit disable/enable. \$\endgroup\$
    – Tony Stewart EE75
    Dec 27, 2018 at 2:35
  • \$\begingroup\$ Using an MCU doesn't preclude "adjustment with knobs" and since you have one in there already... \$\endgroup\$
    – Chris Stratton
    Dec 27, 2018 at 4:24
  • \$\begingroup\$ Yes, I think this should work. Have you tried it in a simulator? \$\endgroup\$
    – user253751
    Feb 21, 2020 at 17:09
  • \$\begingroup\$ Please accept the answer by Jack Creasey. (and read my note down there, please.) Please ask a new question if you are still searching/tweaking/wondering. Thank you, @Mike \$\endgroup\$
    – MicroservicesOnDDD
    Jul 9, 2021 at 2:54

1 Answer 1

Reset to default
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The HCT4017 already has everything you need to turn off the LEDs.

Just interface your MCU to the MR (master reset) line, this will enable you to turn off all the Q(1-9) outputs simultaneously. It does mean that you can't use Q0 of course, since that would be ON when reset.

Since your MCU is going to be less than 6-9V capable you do need to isolate the higher voltage drive.

Perhaps something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Update: as pointed out not using digit "0" causes a pause in the chaser cycle. I included a fix for this by adding Q3

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  • \$\begingroup\$ Will that put a dead cycle in the chase each time it reaches the end and resets to start over at the unused Q0? \$\endgroup\$
    – Chris Stratton
    Dec 27, 2018 at 4:23
  • \$\begingroup\$ @ChrisStratton Not quite, if the reset is removed by the MCU there will 0-1 clock cycle delay before the "1"LED comes on since the 555 is still clicking away asynchronously ….but at the end of a full cycle ( 9 --> 0) there will be 1 cycle pause with no LEDs on. If the OP uses the"0" LED then there will 1 LED on when MR is activated. I'll put a fix in the circuit. \$\endgroup\$
    – Jack Creasey
    Dec 27, 2018 at 6:07
  • \$\begingroup\$ It's exactly the period with no LED on in the ordinary cycle that I was referring to. \$\endgroup\$
    – Chris Stratton
    Dec 27, 2018 at 6:15
  • \$\begingroup\$ @ChrisStratton Yup I got that. Fixed in the circuit, though you still have a 0-1 cycle delay before the 0 --> 1 digit change after the the update. You could prevent this by resetting the 555 ....but that creates another problem ….the first cycle is always longer (2/3 instead of 1/3 trigger). These circuits are always full of compromises. \$\endgroup\$
    – Jack Creasey
    Dec 27, 2018 at 6:22
  • \$\begingroup\$ I'm trying to avoid having the LED's in the led chaser shut off temporarily when it is set to the on state. I also considered your idea of connecting a Q output to reset via diode and connecting a GPIO input to reset via diode... As it may be overkill, I think I may need to apply a logic gate input to Q0 and a logic gate input to the GPIO pin and the output to the LEDs. hmm... \$\endgroup\$
    – Mike -- No longer here
    Dec 27, 2018 at 7:11

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