Generate pulses with a capacitor

Thread Starter

AMIRAAM

Joined Dec 31, 2019
37
Hello everyone

I am new to electronics but I know some basics!!

I know that the current do not flow through the capacitor when it is full charged. That will cause the Led to be off after moment

Please what I have to add to be sure that the capacitor is completely discharged before another high state

Also if there are any other notes please I will appreciate any help

Thanks

circuit 04.png
 

ericgibbs

Joined Jan 29, 2010
12,976
hi AMI.
This simulation shows the action of your circuit.
The LED is Lit for only a fraction of a seconds.. [ the LED current plot]
What are you trying to do.?
E

Added the Diode, increase On time to approx 0.1 sec.
 

Attachments

ericgibbs

Joined Jan 29, 2010
12,976
hi Bob,
I don't think he is trying for a relaxation oscillator, just a button push, wait , button push ....

Hopefully he will tell us.
E
 

Audioguru again

Joined Oct 21, 2019
3,208
I used some 2N6027 things 50 years ago when they were called a PUT.
Maybe Amiraam does not see that in his circuit nothing happens after one LED flash if he keeps pulsing the button because the important diode is missing. EDITED.
 
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Ian0

Joined Aug 7, 2020
2,294
I’ve seen them called programmable unijunctions as well, but they’re thyristors, so they have three junctions.
Apologies for being very literal!
 

Thread Starter

AMIRAAM

Joined Dec 31, 2019
37
Hello everyone and thanks for your replies

BobTPH said
:
Simplest way is to use a Unijunction transistor.
BobTPH said:
He is trying to create a relaxation oscillator.
It would be good if Unijunction transistors are always available (something can be easily replaced)

Here I am not
trying to create a relaxation oscillator or something for excellent frequency stability (555, Crystal Oscillator …) but only simple delay timer

AlbertHall said:
Add a diode across the base emitter, cathode to base.
Ericgibbs said:
hi AMI.
This simulation shows the action of your circuit.
The LED is Lit for only a fraction of a seconds.. [ the LED current plot]
What are you trying to do.?
E
So please can you explain to me how can the diode discharge the capacitor when the low state is applied
 

Audioguru again

Joined Oct 21, 2019
3,208
When the input is high then the capacitor is charged to 4.4V through the forward biased base-emitter and 2k of resistances.
When the capacitor is charged then it has no more charging current and the LED turns off.
Without the diode the capacitor does not discharge.
With the diode then the capacitor discharges through 1k resistor R2 and the diode when the button is pressed.
 

BobaMosfet

Joined Jul 1, 2009
1,786
Hello everyone

I am new to electronics but I know some basics!!

I know that the current do not flow through the capacitor when it is full charged. That will cause the Led to be off after moment

Please what I have to add to be sure that the capacitor is completely discharged before another high state

Also if there are any other notes please I will appreciate any help

Thanks

View attachment 237670
learn in a structured way- so much faster and gives you a solid foundation:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3
 

sparky 1

Joined Nov 3, 2018
544
Another relaxation oscillator using a neon bulb (Ne2) as an active component.
(The circuit uses less than 1mA or low current and higher voltage.)
Some mistakingly refer to something like this as a voltage only circuit, raising the voltage is important and it can run for years but not magic.
 
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