Jan Luthe

Joined Jan 10, 2015
89
I am trying to understand this circuit. I understand: current R1 is 0.02 ma, current at B1 ocurrs only if there is current at B2, no current flows thru D or E after capacitor is full, max current thru R2 40ma. I would appreciate, in laymans terms, what is happening when the circuit is started (LED off) and then what is happening when when LED flashes. Some approx current numbers for each phase would be good. Explanation how R2 works as I am only familiar with a current limiting resistor in line with the LED. Also how/when the capacitor discharges.

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Danko

Joined Nov 22, 2017
1,606
Circuit in post #1 not works.
D1 current is not limited.

EDIT: circuit below will work from voltage not more than 5 V!
Collector and emitter of Q2 are swapped.
Now circuit works, but D1 current still not limited.
Shape of pulses is not good.

EDIT: circuit below will work from voltage not more than 5 V!
Added resistor R3 in series with D1.
C1 and R1 values are corrected.
Now D1 current limited at appropriate level.
Shape of pulses is good.

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Jan Luthe

Joined Jan 10, 2015
89
thank you for your reply. I did breadboard the circuit that I provided and it does work .(I assume I did it correctly). I still need help in understanding it.

Jony130

Joined Feb 17, 2009
5,445

Danko

Joined Nov 22, 2017
1,606
thank you for your reply. I did breadboard the circuit that I provided and it does work .(I assume I did it correctly). I still need help in understanding it.
Yes, it works sometimes, when current gain β of transistors is quite low.
If collector and emitter Q2 are swapped, then β of Q2 is about 3 instead 30...150. Then stability of circuit is guarantied.
Also pay attention to current of D1 in your circuit which is 285 mA instead permissible 20 mA max.
EDIT: recommendation about swapping is true only for circuit, feeding from power supply with voltage not more than 5V!

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Jan Luthe

Joined Jan 10, 2015
89
Learning a bit at a time: Beta the same as HFE? Where is D1? My D, I believe, has a max current of .02 ma thru R1.

Danko

Joined Nov 22, 2017
1,606
Learning a bit at a time: Beta the same as HFE?
Yes, it is.
Where is D1?
D1 is LED on my diagram
My D, I believe, has a max current of .02 ma thru R1.
See how current goes: +9V --> E1 --> C1 --> anode of LED --> cathode of LED --> -9V.
In your circuit LED during every pulse connected directly to 9V source (short circuit).

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Jan Luthe

Joined Jan 10, 2015
89
Jony130: thank you for the link. I will be studying the attachments.

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Jony130

Joined Feb 17, 2009
5,445
Your original circuit can be redrawn to see the "classic" circuit. I also fix the capacitor polarity error.

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Jan Luthe

Joined Jan 10, 2015
89
Thank you Jony. I will study the new classic circuit after I have studied the previous speaker schematic.

AlessandroLamba

Joined Feb 27, 2023
12
Your original circuit can be redrawn to see the "classic" circuit. I also fix the capacitor polarity error.
Probably a resistor is also needed in series with the LED

Danko

Joined Nov 22, 2017
1,606
To prevent multivibrator accidentally latching and for extending range of frequency adjusting,
I eliminated direct coupling between transistors, using capacitor C1.
Below is circuit of metronome, which generates pulses 2.8 ms duration.
Frequency of pulses changes from 343 to 20 BPM (beats per minute),
when resistance of pot R changes from 0 to 1 MΩ.
Circuit starts work from voltage of V1 = 1.5 V (at least in simulator).

________

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