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Looking for help with a flash circuit Answered

I'm looking for help designing a circuit that can slowly ramp up the brightness of a blue LED whilst charging a small flashbulb as you'd find in a disposable camera please. Once it peaks, flash, then stay from the beginning. I can also saw it being possible to run a stepped version where more than one LEDs are lit in sequence using capacitors, but I'm unsure I'm new to electronics and don't know enough to figure this out. Is this possible to do with only simple components (ie avoiding IC chips if possible) Thanks for any help! https://sarkariresult.onl/mobdrohttps://pnrstatus.vip/

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Jack A Lopez

4 weeks ago

It occurred to me that I did not answer the part of your question about how to "slowly ramp up the brightness of a blue LED whilst charging [the flash circuit's big capacitor]"

One way to do this is to watch the voltage on the big capacitor, and produce a current signal proportional to this voltage, to drive the LED.

The main problem is the voltage on the camera flash capacitor is huge, having a maximum of around 300 volts.

So a voltage divider, with two resistors whose ratio is about 100:1, could reduce that 300 volts to about 3 volts.

However the resistors in this divider will have to be very large, or else they will quickly discharge the big capacitor, and prevent it from charging. I am thinking 10 Mohm and 100 Kohm would be a good choice. What is the R*C product for a R=10 Mohm and C =100 uF? Answer: 10*100 = 1000 seconds, which is like 17 minutes. So I think that is pretty good.

However the output impedance from that divider is large, around 100 Kohm, so something with even larger input impedance is needed to follow it.

An op-amp follower would probably work, since those have huge input impedance.

An emitter follower, made from a single transistor, or from 2 transistors in a Darlington pair, might work too. The input impedance to that follower, is the resistor between the emitter and ground, multiplied by beta (the natural current gain of the transistor).

I drew some diagrams of this. I will attach those to this reply, and also point you to this page, which shows a similar circuit, using an emitter follower as a current source (or perhaps as a current sink) for a single LED.

http://www.pcbheaven.com/userpages/LED_driving_and...

Wikipedia calls this circuit, "Common collector"

https://en.wikipedia.org/wiki/Common_collector

which it turns out is essentially another name for "emitter follower."

Did I mention the LM3914 bargraph display driver? No? Well I think that one is the usual go-to IC, for a cheap and dirty way to convert an analog voltage to a number of illuminated LEDs.

https://www.alldatasheet.com/view.jsp?Searchword=L...

Although, you did say you were allergic to ICs, and I am not sure what to think of that. Espcially for op-amps, because those are ICs too, and they seem to be handy for working with analog signals.

I have had a certain amount of success with the TLC272, and TLC274, especially for signals that are slow, almost DC. Those are actually the same op-amps, but in packages with either 2 (dual) or 4 (quad) op-amps per chip, respectively.

https://www.alldatasheet.com/view.jsp?Searchword=T...

voltage-controlled-current--using-bjts.jpgvoltage-controlled-current--using-op-amp.jpgop-amp-follower--following-voltage-from-a-voltage-divider.jpg
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Jack A Lopez

4 weeks ago

I think this page,

http://www.talkingelectronics.com/projects/XenonFl...

gives a good overview of how simple flashtube circuits work.

Looking at these circuits you will notice there is a big electrolytic capacitor, wired in parallel with the flashtube, and it is that big capacitor which stores the energy that drives the flashtube.

Thus it is more correct to say, "charging a capacitor, " than it is to say, "charging a small flashbulb..."

Also, I think the words, "flashtube", "flashlamp", "strobetube", or even "camera flash"

https://en.wikipedia.org/wiki/Flashtube

are used more often than "flashbulb." Moreover, I think the word "flashbulb" might be used to refer to the old, very old, single use, disposable devices described here,

https://en.wikipedia.org/wiki/Flash_(photography)#...

in section titled "Flashbulbs" in the article for "Flash (photography)"

I apologize for being so pedantic, but I think you will find better search results if you can manage to use the same words as the authors who are writing about these topics.

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Downunder35m

4 weeks ago

Why not just get the circuit out of an old camera?
Around here there is still a few shops that do old school film development and these guys are happy to give you the scrap hpousings with the flash circuit.
There is also lots of online places where people reverse engineered these flash circuits if you want to build one instead.

Principle is simple:
The battery voltage is chopped up and goes into a tiny transformer.
Output of the transformer chrges the capacitor.
The led is just wired on iether the input or output side of the transformer with a resistor.
As long as the capacitor is not fully charged the LED does not get enough power to fully light up.
Once the capacitor is fully charged the LED stays on as the capacitor no longer drains the power.
Automating this would mean to add a small transistor to the LED.
Once the voltage is high enough (as before) the transistor will switch and close the circuit that was formerly the push button.

But it might be easier to just get an adjustable strobe light instead for a few bucks.
They come with a potentiometer for the flash speed already....