I was watching the video on Forever Flasher circuit in Advent Calendar of Circuits 2011 series by vk2zay. Below is the circuit I am talking about. However, there are some things I understand and most things I do not understand about the working of the circuit.

Things I understand and do not understand:

When the circuit is powered up, Q2 is turned on a little through R1 and R2 which turns on Q3. When Q3 is turned on, Q2 is turned on more with the current coming from the collector of Q3, through C2 and R5. So they are complimentary pair as vk2zay said in the video. When Q3 is on, it turns Q1 on through R4. When Q1 is turned on, base current that were supplied to Q2 through R1 and R2 is no longer supplied because bottom end of R1 is in the ground potential.

In this position, where Q1, Q2 & Q3 are on, currents flowing are: from Q3:C to R7 to GND, from Q3:C to C2 to R5 to (Q2:B & (R2 to Q1:C to GND) ), Q3:C to R4 to Q1:B to GND. These are the ones I understand but I do not not how C1's current flows and comes from where and goes to where. But here, I think the voltage at "out" and Q3:C which is supply voltage, adds up.

After some point Vc2 goes up to about 0.9 volts which turns Q2 off. So it turns Q3 off. What happens to the charge and voltage at C2? How and to where it discharges? At this point, how and from where to where will Ic1 go?

I just added QTLP690C as a high forward voltage LED. Assume the forward voltage of the LED is 2 to 2.5 volts.

Sorry if I sounded complicated. I would appreciate any detailed information on this. Also, any tips on how to understand how these kind of circuits work?

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  • \$\begingroup\$ RE "I do not not how C1's current flows and comes from where and goes to where." -- It looks like you have the circuit set up in LTSPICE. If you run the transient simulation and probe the currents through the branches connected to the c2 or out nodes, you should be able to answer this yourself. \$\endgroup\$
    – The Photon
    Commented Dec 30, 2011 at 16:15
  • 1
    \$\begingroup\$ @The Photon, I know to where they flow, but I do not know why. That's why I am here. LTspice will tell me the output of a very complex circuit but won't tell me how it works. \$\endgroup\$ Commented Dec 31, 2011 at 9:36

2 Answers 2



  • I understand but I do not not how C1's current flows and comes from where and goes to where. But here, I think the voltage at "out" and Q3:C which is supply voltage, adds up.

The output if driving an LED would depend on Vsupply being < Vf_LED_On but > VfLEDOn/2. When Q3 is off C1 charges to vsupply via Vsupply-R3-C1-R7//R4//Q1_be.

When Q3 turns on the left side of C1 goes from 0 to Vsupply so the right side of C1 goes from Vsupply to ~= 2 x Vsupply *(as voltage across a capacitor cannot change instantaneously and left end was raised by Vsupply so right side must be too.

2 x Vsupply acrosss D1 turns it on if VF_D! < 2 x Vsupply.

This is a bad wayto drive D1 as it provides large chort current pulses. IF the circuit is driving D1 at about mean rated current then current peakswill be >> I rated. Moderm LEDs only allow a small margin - which this will exceed if LED is near mean rated current.

BUT cap C1 is charged vi R3 + R7 etc. If Vsupply = 1.5V then Icap charge = 1.5/27k = very very very little. .


Ocsillator action:

Q1 Q2 Q3 are effectively inverters.

Remove C2 temporarily

Force Q1b low -> Q1C high Q1C high -> Q2b high -> Q2 on. q2 on -> qQ2clow -> Q3 on -> Q3C high.

So the 3 Q's are high low high

And Force Q1b high -> Q1C low -> Q2B low -> Q2 off Q2 off-> Q2C high -> Q3 off -> on Q3C low

So the three Q's are low high low

This like 3 CMOS inverters in a row with output connected to input.
As their are 3 you get net inversion.
Connect out to in as here and it tries to "chase its tail" With pure R biasong it will settle dowm to some stable point/

BUT R5 + C2 connect across an inverting point - when one end is high it is driven by a low input and when low it is drive by a high input. This provides the negative feedaback to ensute ris system is unstable and will always chase its tail.

This is not a very "exciting" circuit. It id not obvious that it does anything overly well. But, it may.

Consider this circuit:

Try simulating that. Make L1say 1 mH to start. Can be say 100 uH or whatever.
Play with R1 & C.

Does it oscillate.
Note this is for 1 cell. Q2 grounds Q1's base - noyt good for high Vsupply

Components may be re-added as desired - this was a stripdown attempt AFAIR.

If R2 C1 time constant is long - say 0.5 second, you have a LED flasher. If R2 Ca time constant is short you have a boost converter and steady LED driver (it seems visually).

I "invented" this circuit in about 2000 and published it on-web in ?2001?.
I say "invent" as Q1 Q2 oscillator was used by Noah to power a deck light. I simply added L1 which was wholly obvious -and very successful. I would be completely non-surprised if this had been invented by 3,456 people, 10 years before I suggested it, BUT I've never seen any earlier versions. Several close version turned up inside 6 months of my publishing the original. No doubt a fluke - they didn't mention me :-).

enter image description here


enter image description here
[Image is a link to the simulation]

This is a nice opportunity to play with the falstad simulator, as it's a good way to visualize circuit behaviour.


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