In the book I am following the following circuit is given:


simulate this circuit – Schematic created using CircuitLab

The book states that as soon as we apply power to the circuit, the LED should start flashing.

However, this doesn't happen in my circuit. As soon as I apply power to my circuit, the LED flashes once and stays off after that.

What might be the reason my circuit does not work? I am giving my circuit below:

Since I don't have exact resistor values, I have made R2 out of 10K Ohm and 4700 Ohm resistors. And I have made R3 out of two 10K Ohm and one 6800 Ohm resistors.

I don't know which manufacturer's 2N6027 (PUT) I am using but AFAIK their pinouts are always the same.

enter image description here

I am using an AC to DC adapter at 6V as my power supply.

What I have tried up to now:

  • Replace R1 with 100K Ohm. Didn't work. Same behavior as before.
  • Try transistor's (a user) circuit. Didn't work. This time LED didn't flash even once. The following is transistor's circuit:

enter image description here

  • Increase the cap.
  • Increase the cap and decrease R3.


  • Current at anode starts at 5uA as soon as I power the circuit, and drops to 0uA about a course of 5 seconds and stays at 0 afterwards.
  • Current at cathode becomes 1uA momentarily as soon as I apply power and then drops to 0 and stays that way.
  • \$\begingroup\$ Replace R1 with 100k or 220k. The cap may be too leaky to fully charge. \$\endgroup\$
    – jippie
    Jan 9, 2016 at 10:54
  • \$\begingroup\$ @jippie Replaced with 100k. That didn't work as well. Like before, as soon as I apply power, LED flashes once and never again. \$\endgroup\$
    – Utku
    Jan 9, 2016 at 11:05
  • \$\begingroup\$ @Andyaka Make: Electronics book. I could upload the original schematic in the book but I am not sure that would be legal. \$\endgroup\$
    – Utku
    Jan 9, 2016 at 11:53
  • \$\begingroup\$ @Utku: this article explains that for your original circuit the PUT stays on until the anode current drops below the 'valley current' (about 100 uA). Can you pop your multimeter in the LED leg on uA / mA and see if the current ever drops to zero? \$\endgroup\$
    – Transistor
    Jan 9, 2016 at 11:56
  • \$\begingroup\$ I was reading your schematic wrong - it should work just fine - make sure you have gate and anode correctly wired. \$\endgroup\$
    – Andy aka
    Jan 9, 2016 at 11:57

1 Answer 1


As far as I can see on the photo the circuit seems OK, except

  • the capacitor is the wrong way round: the white strip identifies the negative pin.

  • I can't verify whether the LED is the right way round. Bridge the left LED pin to the + with a 470 .. 4k7 resistor, if it lights up it is OK.

  • you don't show the power connection. Be aware that some solderless breadboards have a break in the middle of the power strips.

  • \$\begingroup\$ White part visible in the photo is not the white strip but something else. I guess the cap is correct because I have connected the longer leg to anode of the PUT, this is visible on the photo. LED is correct as well because it flashes once once I apply power to circuit. Also it never flashes when I flip the LED. For power, I am using only the upper half of my breadboard. \$\endgroup\$
    – Utku
    Jan 9, 2016 at 10:53
  • \$\begingroup\$ Your LED flashes once because it is being powered by the capacitor. C1 charges up through R1 and when the anode voltage gets high enough the PUT turns on discharging C1 through the LED. The PUT must be staying on and R1 value is so high that it can't light the LED or charge C1 again. See my circuit. \$\endgroup\$
    – Transistor
    Jan 9, 2016 at 11:04
  • \$\begingroup\$ Ignore my comments. I found a schematic similar to the OP's and his should work. I've deleted my answer. \$\endgroup\$
    – Transistor
    Jan 9, 2016 at 11:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.