# Help needed in understanding this LED chaser circuit

Here is a circuit called LED chaser that I found the other day. I watched some videos on Youtube and saw that its function is to blink the LEDs in some kind of sequence:

But I don't fully understand what the circuit does or how it works. In the video the LEDs seem to blink in groups in somewhat random order (this could be cool for a disco light application). Is this the function of the circuit?

I also have a hard time getting my head around how this circuit works. There are the sections that consist of an LED with a series resistor, a transistor, and a capacitor. These sections are connected together with some kind of biasing resistors (?) going into the bases of the transistors. Then finally the collector of the transistor at the final section is connected back to the base of the first transistor as well as to the ground. But how do these sections work? I just see the transistors turning on when the circuit is powered, conducting current, lighting the LEDs.. Then the capacitors do something and I just get confused.

Help will be much appreciated! Googling hasn't produced any explanations.

• re-arrange the leds into this order: 1 3 5 2 4 Aug 14, 2018 at 2:49
• The reason they are in groups on the video, is because the video uses a modified version of this circuit, using just 3 transistors and 3 capacitors, and 3 leds per group. Otherwise it's the same principle. Aug 29, 2018 at 23:55
• The Youtube design stresses the Vbe with about-7V then ramps up to turn on, which causes the collector to turn off the next stage for the same RC time constant. My design eliminates this by using emitter follower to drive the LED. tinyurl.com/y8xwtmgw I also included a switch and variable supply from 5 to 9V ( 30 to 65mA) Aug 30, 2018 at 5:05
• Each stage is a 1-shot triggered by the previous stage and finally looping around after an odd number of stages, which then causes 2 or 3 LEDs to be on all the time as they cascade. But 2 adjacent LEDs can NEVER be OFF. Aug 30, 2018 at 5:12
• Re the circuit in the tinyurl: Will it work with electrolytic caps? I tried the down log in both directions. Can't reproduce the result, the lights are all on, no pattern. Oct 17, 2021 at 2:14

This is a fun circuit! You will enjoy figuring this out. A couple of clues for you...

1. The 470 ohm resistors are current limiting resistors.
2. LED's operate on current flow, not voltage.
3. Now, think about the RC time constant and what it is that controls the rate at which the capacitors are charged up enough such that the voltage drop across the caps rises sufficiently to turn on the transistors.
4. Finally, note that this circuit can ONLY work with LEDs and not with light bulbs.

Enjoy!

• why not with light bulbs? Aug 14, 2018 at 2:50
• Would it work with a light bulb and a diode in series? Points 1 and 2 I am aware of, but they don't seem to help me much.. I also know about RC time constants since I know how the BJT astable multivibrator works, but I'm still confused about this one. Aug 14, 2018 at 7:02
• I haven't really been able to wrap my mind around this circuit, could you provide more assistance or should I offer a bounty? Aug 28, 2018 at 10:34
• Have you tried simulating it? Looking at the voltages around the circuit could be a big help. Sep 2, 2018 at 19:35

A few notes to ponder:

1. KD Mann is basically correct in that once started it will run as long as it has power.

2. There is the assumption that applying power will trigger at least one stage (LED) to come on, then time out which triggers the next stage.

3. It is possible that if all resistors were equal in a very tight tolerance and all transistors had the same beta, or hFE, the circuit may not oscillate as expected. This circuit depends on slight imbalances to start up.

4. If a 'no start' situation exist it can be avoided by making sure the LED and base bias resistors have differences of several percent. Likely the transistor with the lowest value base resistor will come on first, so unless you lower the value of one base resistor the startup will not be predictable. However it will always tend to start up with the same LED for the reasons I mentioned.

5. The circuit is a loop that once started will not stop unless you remove power. Each stage triggers the next stage but the capacitor makes sure it times-out to trigger the next stage. Do not be surprised if you apply power and the starting LED seems to be random or fixed.