Electronics > Beginners
Weird one transistor LED-blinker circuit
erikj:
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?
james_s:
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.
Wolfgang:
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
SiliconWizard:
http://www.kerrywong.com/2014/03/19/bjt-in-reverse-avalanche-mode/
Wolfgang:
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/
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