I am a beginner in electronics and have been playing around with simple LED circuits and using Ohm's law to determine what size of resistor to use to limit the current to the LED. I have also been studying how resistors and capacitors work in series and parallel. I think I have at least a fundamental grasp of these theories. I have since moved on to diodes and now transistors and how they are used in simple circuits, again using LEDS. I found the following circuit which uses two transistors, capacitors and resistors to alternately flash two LED’s but I am struggling to understand how it works.

Dual LED Flasher Circuit (CircuitLab)

enter image description here

How I think it works is this:

  1. Voltage is applied to the base of Q1 from capacitor C2 thus switching it “on” which allows charge to flow through to LED1 which lights it.
  2. At the same time, capacitor C1 is charged.
  3. When C1 is charged it begins to discharge into the base of Q2 thus switching it “on” which allows charge to flow through to LED2 which lights it.
  4. At the same time, C2 is fully discharged which switches Q1 “off” causing LED1 to go off.
  5. Eventually, C2 is again fully charged and the cycle repeats.

But I have a few questions which I cannot seem to answer.

  1. I believe that the resistors R1 and R2 are the current limiting resistors for the LEDs, but what are resistors R3 and R4? Do they control the rate at which the capacitors charge and discharge? Or do they have some other purpose that I am not grasping?
  2. There seems to be a “chicken and egg” situation here. When the circuit is unpowered and both capacitors are discharged, what causes either of the transistors to be “switched on”? Where does the voltage for the base of either of them come from?
  3. And a more general question: What "triggers" the capacitors to discharge?

I have read articles about how a transistor works at this link: http://amasci.com/amateur/transis.html and it has helped some, but the simple circuit above is still fuzzy to me.

Can someone help me to understand this so it “clicks” in my head?

I have also looked at the question below but it has not cleared it up for me.

How does this 2 LED flasher circuit work

  • 1
    \$\begingroup\$ I added the schematic as an inline image. You click on the picture button and then give either a web link or a local file. \$\endgroup\$
    – Adam Haun
    Sep 9, 2015 at 20:41

4 Answers 4


I specialize in the clicking of brains.

I remember trying to figure this circuit out when I was around eleven. (The one in my old book used light bulbs, put in Halloween-mask eyes.) Here's my version below. The main trick to these is to re-draw the schematic so it reveals familiar patterns.

With yours, first turn it rightside up, and you'll see that it's actually two amplifier stages connected by capacitors. And, the signals are connected in a loop.

re-drawn blinker circuit


Below is the rightside-up circuit without that looped capacitor connection:

Blinker wo/C2

Each transistor is wired as a common-emitter amplifier stage. Each amp will both invert the signal and also make it larger. If we apply a pulse to point A above, a large upside-down pulse will appear at point B. And this inverted pulse goes through a coupling capacitor C1 to point C, which is the input of the next stage Q1. Q1 inverts it again, and the twice-amplified signal appears at the output at point D. All together it's a 2-stage amplifier. Probably you could hook up a microphone and loudspeaker, and use it that way.

So, what happens if we connect the output directly to the input? Positive feedback. Then any small pulse will go through the loop from point A to point D, getting bigger each time. Or in other words, it will break into oscillation.

With C2 restored, we'd expect to see high-frequency sine waves, where their frequency is caused by the time-delay through the whole loop of amplification. It's similar to when you hold a microphone a bit too close to an auditorium loudspeaker. But in reality, this whole system has way too much gain, so it gets overloaded and goes nonlinear. It won't make a high-frequency sine-wave oscillation, instead it clips into big slow square waves. (I guess it's more like holding your auditorium-microphone directly against the loudspeaker cone.) In that case the output slams all the way to 9V and to zero, and the speed of the oscillation is determined by charging and discharging of the two capacitors, which mostly happens through the 100K resistors.

Once you know how it works, you can sit down and plot out the various events. Start out with one capacitor zero volts, and the other charged to 9V, then figure out what happens next, then next after that. You might have to go through several cycles to see how it settles into constant blinks. All the components are symmetrical, so it blinks equally back and forth like a logic flip-flop. Its official name comes from that: non-stable or "a-stable" flip-flop blinker circuit.

That's just one tack on an explanation. Paraphrasing Feynman: if you don't have three or four separate approaches to explain something, you don't really understand it.

This circuit can use very, very low DC voltage yet still keep running. Besides LED blinkers, I've seen it used as various beepers, signal injectors, even analog-synthe instruments (with tiny capacitor values, like 0.01uF etc.) A similar circuit with a big iron transformer can generate 120VAC 60Hz, for an electric shocker or as an automotive "Power Inverter" for low-power appliances. Or use coils wrapped on a CRT ferrite HV flyback transformer and make a mini Tesla Coil or a 20KV power supply. The exact same circuit is in a solar-powered pendulum toy, where the LEDs are replaced by electromagnet coils, with a tiny ceramic magnet on a little pendulum getting kicked back and forth when light shines on the solar cells (with four 1cm solar cells in series, for about 2V power supply in sunlight, far less w/indoor fluorescents.) Or, use very large resistors on the base connection, small capacitor values, and add a few-inches pickup wire to one transistor base, to create the world's cheapest Theremin or touch-sensitive audio generator.

  • \$\begingroup\$ Thanks to all for the answers. I will have to read (and re-read) these several times and combine it what I have already learned. \$\endgroup\$ Sep 11, 2015 at 18:51

A couple of things to think about.

Firstly even though all of the paired transistors and caps etc are same specied values. Their true characteristics will vary in the case of caps it can be 80%. So this difference is what makes the circuit unstable and results in the leds flashing.

Secondly caps act like a short when discharged and an open circuit when charged. So imagine they are like a tap being alternately closed then opened. So think about how this action would affect the base of your transistor and thereby the current flow between the collector and emitter.

When you combine both of these factors you end up with an oscillating current path.

  • \$\begingroup\$ Also. Your circuit uses a different design because it is using a different type of transistor. So although the principles are the same there are subtle differences. Plus no diode/transistor is perfect. there will be some leakage current that kickstarts the process. \$\endgroup\$
    – BenG
    Sep 9, 2015 at 22:10

This circuit is actually a astable multi-vibrator.It acts like a two switch which gives the complement outputs.The basics is that when one transistor is in saturation region,the other transistor will be in the cutoff region,so the LED in cutoff region will glow and the LED in saturation region will not glow.On the next cycle,the cutoff transistor will goes to saturation region and the other will be vice versa and the output get inverse.The frequency depends on the capacitor connected in it.Mostly both the capacitor connected will be same value.

The same circuit without capacitor and with some modification,they will be used to store one bit of data in static RAM.But they are built using MOS technology.

  • \$\begingroup\$ Your explanation regarding the LEDs associated with the transistors in saturation and cutoff is backwards. The LED associated with the transistor that is saturated glows, the LED associated with the transistor that is in cutoff is dark. \$\endgroup\$ May 12, 2019 at 2:07

In my explanation I will use the electron flow theory because electrons are 1800 times lighter than protons. When Q2 is conducting c1 is charging through LED1, R1 into the -plate of C1, out of the +plate into the base of Q2 through the emitter to +9 volts. When c2 finishes discharging through r3, r3 pulls the base of Q1 down turning Q1 on pulling it's collector high placing the 9v in series with c1 reverse biasing Q2 turning it off. Meanwhile charging c2 through LED2,r2 into the -plate of c2 , out of the + plate into the base of Q1 out of the Collector to the +9 volt source. When c1 discharges the whole process will keep repeating. I forgot to say the capacitors are in backwards. This is the way I see it.

The schematic on the right in the number 1 answer is the only correct schematic with the caps installed with the right polarity.

  • 3
    \$\begingroup\$ "In my explanation I will use the electron flow theory because electrons are 1800 times lighter than protons. This reads as though you think that electron flow is faster or different to conventional current. You could do with a few paragraph breaks in your text. Use 2 x Enter. Welcome to EE.SE. \$\endgroup\$
    – Transistor
    Jul 18, 2017 at 21:21
  • \$\begingroup\$ I never said anything about the speed of electrons, only the direction that they flow in order to make it easier to understand how they charge and discharge. \$\endgroup\$
    – Cannonball
    Jul 19, 2017 at 14:49

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