7555 pulsating led

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MrDEB

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designing a Statue of Liberty lapel pin for the 4th of July 2023'
have two similar schematics for a pulsating led (white 3mm led) but the capacitors are electrolytic caps that are larger than 3mm in diameter (have only 1/4 inch height restriction. THINKING of maybe using a titanium cap or? need to order parts but the capacitor size?
both of the schematics are basically the same.
 

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I guess you know that increasing the timing resistor (as R2), the capacitance for the same pulsing LED decreases.
I mean, I wonder what prevents you increasing R2 to 470K for example and use 10u instead of 470u.
 
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And both are particularly poor examples - I'd like to think that you've paid a little attention in all the other threads you've created, but obviously not - why are you still putting LED's in the emitters of transistors?.

Your circuit could be drastically reduced, by using a simple 8 pin PIC - one PIC, one resistor, one LED - that's it, no large capacitors, no need for a transistor, and infinitely more effective. If you wanted slightly smaller still, then use an 8 pin SM PIC, or even one of the 6 pin SM PIC's, which still would do what you want. Essentially the same VERY simple software in either case, just to flash an LED ON and OFF.

Or for the 555 circuit, dump the transistor anyway, as a 555 has plenty of capacity to drive an LED, or at least put the LED in the collector.
 
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Kerim said:
I mean, I wonder what prevents you increasing R2 to 470K for example and use 10u instead of 470u.
Click to expand...
The LED driver transistor is an emitter follower. My guess is that the OP wants the LED to fade up and down in brightness rather than blink on and off crisply.

The issue is that the transistor's base current is not coming from the 555's output stage, which is designed to do this, but through the timing resistor. Increasing the timing resistor by 50x might starve the transistor. It definitely will change the timing period.

Assume the LED has a Vf of 3.5 V, the transistor Vbe is 0.6 V, the transistor beta is 100, and run the numbers.

ak
 
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This project uses a modern low current Cmos ICM7555, not a battery-eating TTL designed NE555 or LM555.
With the transistor emitter-follower circuit ramping voltage to the LED then the battery will last only for a short time.

If the Cmos 555 has its Discharge pin controlling the capacitor then the timing resistor can be high resistance and the timing capacitor can be low capacitance and be tiny. The LED with its current-limiting resistor can be driven with on-off pulses of at least 10mA from the Cmos 555 output and the +6V without a transistor.
 
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the schematics I found examples using Youtube etc.
Would love to use a pic but have never used anything other than an 18F
will post links to the youtube videos if I can find them.
post#2 sounds usable as do all the suggestions.
Would love to use an LM3909 but finding one. I did find a link to building one using the datasheet. but the cost would be IMO higher than using the 7555.
 
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I agree that the TS wants a fade, since the transistor is connected to the timing circuit, not the output.

To allow a higher value charge resistor, the transistor can be a Sziklai Pair, sometimes called a complimentary Darlington (a standard Darlington has a higher Vbe voltage and gives a flat spot at the bottom of the sweep).
LTspice simulation below:
Note that the transistor peak base current (red trace) is <1µA so the timing is not significantly affected.

 
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by a FLAT SPOT you mean the led would turn off briefly?
this would be acceptable.
All I want is to simulate a torch that the statue of liberty is holding
I attempted to run a simple simulation in EASyEDA but the tutorial video is hard to understand and tooo fast.
 
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As usual, the You Tube videos have no schematics then have wiring errors.
They use an NPN transistor but wrongly connect it as a PNP transistor.
The collector and emitter pins are mixed up.
 
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Just a chip and a R and a small cap :



You program operation, by dragging blocks out design area and config and you
are in business. In fact I could have easily added "heartbeat" with a few more blocks
to pulse LED on slowly then same for turn off. Easily could add light level control to
save power. The program is mBlock, takes your block configuration and turns it into
Arduino code and programs the Arduino chip. The code it generates shown in right hand
window. Chip to use ATTINY85 in this example.




Gets you into basic programming, a Nano board ~ $3, mBlock is free, lots of videos. Kids
in 6'th grade using mBlock to program robots.

Other examples, here is a more sophisticated timer done in mBlock :

Pulse Sequence Generator / Smart Timer

Often users, developers, hobbyists need timed and qualified event generators, many resorting to 555 timers and the like. The venerable 555 has had a long run but its limited in accuracy and capability. This approach uses block language to create...
www.electro-tech-online.com

Here is a talking voltmeter (done in another block language, once you know one its easy to use other variants) :

GUI Codeless Processor Design

Micro design has for years relied on ASM and C programming and other languages. Often many tasks are fairly simply but these tools were focused and quite a learning curve, especially ASM and C and C++. There are a number of new GUI based tools that essentially take out the "strong typing"...
www.edaboard.com


Regards, Dana.
 
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most anything can go WRONG.
I want to use the 7555 as the LM3909 are $$$
yes, a pic would be easier but the cost is a factor.
I could just have the LED light up but there goes the WOW! factor
These lapel pins are 30mm x 110mm and 1/2 inch thick. The circuit is sandwiched between 2 -1/8" plexiglass with a 1/4" cavity for the circuitry.
 
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