You have a lot of time on your hands

 

There are several ckts online for single xstr flasher if you have some 2n2222 npns.

 

When the LED threshold voltage is reached the LED will then discharge the capacitor down to its threshold, the LED that is.

This is a critical flaw in your understanding of an LED.

First, the voltage at which the LED lights is called its forward voltage, abbreviated Vf. In electronics, a threshold voltage is a parameter of a field effect transistor, a comparator circuit, and some other things.

This explanation is based on my interpretation of your text description of a circuit. If that sounds shaky, it is. Electrical engineers did not invent the schematic diagram, but we can't "take a dump" without one. (apologies to Rear Admiral Joshua Painter in The Hunt for Red October) My guess is that you are describing an R-C circuit with an LED in parallel with the capacitor.

Sticking with theoretically perfect components, an LED has only one Vf, just like most power rectifiers and signal diodes. For a typical small red LED it is around 1.8 V. So in the circuit you describe, the capacitor will charge up to Vf, at which time the LED will conduct and illuminate, with its brightness set by the current through it, which is set by the resistor going to the power source. At this point, the capacitor stops charging, it's voltage is stuck at Vf, and things just sit there until power is removed.

The idea that something in a circuit has two action points is not new, and a whole section of oscillators and flashers is based on the circuit concept called hysteresis. This is a circuit in which the threshold voltage for action changes depending on the action, which is what you describe in post #1. This can not be achieved without something other than Rs and Cs. It can be a simple as a neon bulb or more complex like a comparator, a Schmitt trigger logic gate, or a 555.

ak

 

check this out... it references our famous @DickCappels



https://www.homemade-circuits.com/how-to-make-single-transistor-led/

 

He was referring to: http://www.cappels.org/dproj/simplest_LED_flasher/Simplest_LED_Flasher_Circuit.html

 

There are several ckts online for single xstr flasher if you have some 2n2222 npns.

I know. They are the ones I refered to as recipes in my initial posting. I have watched them. I don't want to know if it works. It appears to. I am more interested in why it works. The fundamental principles behind it.

 

The circuit in post #7 requires no transistors, and should be the simplest led flasher with a twist!

 

The circuit in post #7 requires no transistors, and should be the simplest led flasher with a twist!

You would thinks so. I have built a circuit that does use a ttansistor. I was youying with the idea of pulsing the base terminal. I guess you could say that I am trying to build a pulse generator from first principles. If I can do that then I can apply it to anything really.

 

A circuit with R + C + LED will not flash repeatedly. It will eventually reach a steady-state DC condition.
This is because the LED does not suddenly conduct. What you need is a device that has sharp turn-on characteristics.

Some simple devices that will oscillate are neons, UJT, and tunnel diodes. Getting such circuits to power an LED is a different story.

 

That is what I saw when he first posted. This is how it is supposed to work. Quite a different matter than all in series. I can see this possible working but didn't try it in LTS.

View attachment 182997

Been a few days. I was waiting for the 2.2mF to arrive so I could do it to spec as per diagram. Still no joy though. LED just stays lit.

 

Are you beginning to see the problem yet?

You seem to think that an LED does not conduct any current until it reaches some specific voltage, and then it lights up. Further, to make your flasher work, it would have to continue to conduct, even at a lower voltage than the original threshold and stop conducting at some different threshold. An LED does not work this way.

The way an LED will behave when put in parallel with a capacitor that is charging, is it will conduct a small current once the voltage is above 0, rising ever more steeply until you start seeing the light, then, at some point, the current through the LED will equal the current through the resistor and no further change is possible, you have reached a steady state.

The way an LED will act when in series with a resistor and capacitor is it will initially draw current through the resistor. The current will decrease as the capacitor charges, leaving less and less voltage for the LED until the capacitor is at the supply voltage and no more current can flow.

Adding more capacitors and resistors will not help, they will always reach a steady state. To make a flasher you need a component that drastically changes its behavior after a certain voltage or current is reached. That is what things like transistors can do.

Bob

 

Nah try some more capacitors...

If we used an analogy:
in parallel the capacitor is like a tank... once it fills the power just keeps flowing to light the LED... You need to dump the tank if you want to turn off the overflow again...

in series the capacitor is like a tank that fills up then no wheel turning if no flow (LED). You need to dump out the tank if you want to flow again.

 

I was rooting for ya but the more experienced guys here were correct about it not working. Goes to show ya not everything you find on the net is correct. Put a 2N2222 in the circuit and get it oscillating. Or try the 555 timer. Both are something to get familiar with at some point and lots of net info on both and both very inexpensive in small lots (5 - 10). Gook luck!