Why This Blinking LED Circuit Not Working?

Thread Starter

adityanaiyyar

Joined Sep 6, 2021
5
I built this circuit on my own thinking that it would work as blinking circuit. But the LED is not glowing at all ! and obviously its not working as LED blinking circuit. I want complete explanation of this circuit.

Thanks !
blinnking.jpeg
 

Dave Lowther

Joined Sep 8, 2016
102
I built this circuit on my own thinking that it would work as blinking circuit.
How did you think it should work?
But the LED is not glowing at all ! and obviously its not working as LED blinking circuit. I want complete explanation of this circuit.
You might see the LED flash on briefly when you connect the 9V. As soon as the capacitor charges the transistor will turn on making a short across the LED which will then be permanently off.
 

Thread Starter

adityanaiyyar

Joined Sep 6, 2021
5
How did you think it should work?

You might see the LED flash on briefly when you connect the 9V. As soon as the capacitor charges the transistor will turn on making a short across the LED which will then be permanently off.
Thanks Mr. David. Could you please tell me , Why this capacitor does not discharge again via 47K Ohm Resistor?
 

peterdeco

Joined Oct 8, 2019
397
Welcome to the forum. Your capacitor is receiving a steady positive through the 47K. The voltage will rise and cause the transistor to conduct. There is nothing to discharge the capacitor. If you put your LED in series with the collector, the LED will turn on and stay on.
 

Ian0

Joined Aug 7, 2020
3,533
It does not oscillate because it does not have enough phase shift. A single RC network has a maximum phase shift of 90°. You need more than 180° to make it oscillate, so you need three RC stages to make it oscillate. Look up Phase Shift Oscillator
 

ericgibbs

Joined Jan 29, 2010
13,889
Hi adit,
The circuit is stable at some point, note the LTS sim
E
Update;
This sim shows the effect more clearly , from 0V power up.
ESP_ 746 Sep. 06 11.21.png
 

Thread Starter

adityanaiyyar

Joined Sep 6, 2021
5
Welcome to the forum. Your capacitor is receiving a steady positive through the 47K. The voltage will rise and cause the transistor to conduct. There is nothing to discharge the capacitor. If you put your LED in series with the collector, the LED will turn on and stay on.
Thanks! Could you please Recommend me basic led flasher/blinker circuits.. Links or Schematics will do.
 

Thread Starter

adityanaiyyar

Joined Sep 6, 2021
5
Welcome to the forum. Your capacitor is receiving a steady positive through the 47K. The voltage will rise and cause the transistor to conduct. There is nothing to discharge the capacitor. If you put your LED in series with the collector, the LED will turn on and stay on.
Also Kindly suggest Best Circuits Simulators.
thanks!
 

DickCappels

Joined Aug 21, 2008
7,906
I should have mentioned that in the two transistor circuit in post #11 the base resistors can be 3.3k and up (until it doesn't work any more!) as these resistors affect the frequency -to be more precise, the percentage of time each LED is on. The capacitors should start with about 10 uf and go from there.
 

Ian0

Joined Aug 7, 2020
3,533
And for the real vintage component fans, it can be built with a real unijunction such as TIS43.
I would also suspect that one could be built with a diac, but the supply voltage would be quite high as the usual diac break over voltage is 32V. I think that’s how the reverse biassed transistor is behaving in @DickCappels ’s circuit of post #11
 

DickCappels

Joined Aug 21, 2008
7,906
Yes, the transistor with the breaking down emitter-base junction is acting like half a diac.

1630959633090.png

Two transistors can simulate a whole diac, but usually at a much lower voltage.
 

Ian0

Joined Aug 7, 2020
3,533
Yes, the transistor with the breaking down emitter-base junction is acting like half a diac.

View attachment 247394

Two transistors can simulate a whole diac, but usually at a much lower voltage.
There's also a trigger device that's meant for the job called 2N4991 or BS08D with a guaranteed breakover voltage.
They used to be cheap-as-chips and we used them to trigger the crowbar circuit in audio amps, now they're £6 a pop!
 
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