Weird one transistor LED-blinker circuit - Page 1
Forum section: electronics / beginners

author: erikj
author_rank: Contributor
timestamp: November 20, 2018, 11:30:38 pm
content: I came across this weird one transistor LED-blinker circuit with a backwards connected BJT.It works, and the LED strobes nicely at a few Hz.But how exactly does the circuit work?I have also been playing around with the values a bit. It seems to run with a capacitor as small as 10nF, and up to a frequency of about 50kHz.What I have found out so far is that the transistor seems to act as a Zener diode at about 8.5V but with decreasing voltage for increasing current, forming a negative resistance region.This could be part of what makes it oscillate, but why does the transistor behave in this way?

author: james_s
author_rank: Super Contributor
timestamp: November 20, 2018, 11:52:56 pm
content: That looks like a relaxation oscillator, a more common version was often built using a neon lamp which performed the function of both the LED and the transistor there. The transistor will behave open circuit until the voltage across it reaches the breakdown value where it conducts, turns on the LED which drains the charge from the capacitor and the cycle repeats.

author: Wolfgang
author_rank: Super Contributor
timestamp: November 21, 2018, 12:15:21 am
content: Hi,the circuit uses the same mechanism as untriggered avalanche pulsers. The breakdown voltage is greatly reduced by putting the transistor backwards, but it is still a transistor.Googling "avalanche transistor" has a lot of info on this mechanism

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 21, 2018, 12:34:31 am
content: http://www.kerrywong.com/2014/03/19/bjt-in-reverse-avalanche-mode/

author: Wolfgang
author_rank: Super Contributor
timestamp: November 21, 2018, 12:35:55 am
content: Yes, only they dont turn the transistor upside down, and they normally have an external trigger for the pulses.This creates higher voltage pulses.I have also built free-running ones, like here:https://electronicprojectsforfun.wordpress.com/pulse-generators/avalanche-transistor-pulsers/

author: erikj
author_rank: Contributor
timestamp: November 21, 2018, 05:49:48 pm
content: Very good answers, thank you!

author: Ian.M
author_rank: Super Contributor
timestamp: November 21, 2018, 07:33:03 pm
content: Neat. However there's more than one way to flash a LED with only one transistor. e.g. see attachedLTspicesim of asqueggingJoule Thief.

author: Zero999
author_rank: Super Contributor
timestamp: November 22, 2018, 12:06:29 pm
content: Another, more convoluted way of flashing an LED using just one BJT, is to use a phase shift oscillator. It's of no practical use these days, as the extra capacitors and complexity makes it more expensive, than just adding another transistor. It wouldn't surprise me if this kind of thing was done way back, when BJTs were expensive, although a unijunction transistor would've probably been used, so perhaps not.Quote from: Ian.M on November 21, 2018, 07:33:03 pmI like the flashing Joule Thief circuit. It's much more useful than my idea, as it also performs a boost converter function, so a white LED can be run off a single 1.5V alkaline cell.