I joined just because I'm having sort of a dilemma. I need to design a flash circuit for a tiny SMD based project, and I have a way of doing it, but I want to see if I can make it better and I haven't had any luck for days.
Basically I'm using a 555, in a monostable configuration, which would actually be okay but I want to do better if I can.
I want two simultaneous flashes from the LED, but I'm having no luck. I've tried running parallel 555's, but no luck either.
I also tried an RC circuit, but I dislike how the LED discharges slowly, plus I can't get multiple flashes out of it, which is why I'm using the 555 approach. I like how the 555 just flips the LED ON\OFF and doesn't fade with the cap's discharge. Furthermore, the SMD chip and total solution is pretty tiny.
So basically how do I make this circuit flash the LED twice? Is it even possible with the 555?
PS: I'm sure my values and placement are probably baffling everyone right now. I'm not an EE, but I am learning fast, so if there's other corrections, let me know by all means 🙂
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Best Answer
Here are the corrections you asked for, and some caveats:
1) a 555's trigger must be de-asserted before timeout or else the output will just follow the the button-press.
2) 1000µF is pretty ungainly and wastes a lot of power when - for the same time constant - it could just as easily be 1µF by making R1 820k ohms, a standard 5% value.
3) 9k isn't readily available and the value of R2 isn't at all critical, so it could be a standard 10k 5% resistor or anything else between about 1k and a couple of megohms.
4) For an application like this, C1 can be omitted. –
5) With a 3.3V supply, a 7555 can only source a couple of mA before the output drops about a volt, so your LED brightness might be disappointing, even without a ballast resistor, if it lights at all.
6)Using a gated astable, as mentioned in the other answers, will result in the first flash being about 50% longer than the second.
7) using a free-running astable and capturing only two whole pulses per button press will get real complicated real quick.
All that having being said, here's one which uses a single 555 - albeit at a higher voltage than yours - where the button has to be held down for at least the entire 2 pulse event and the first pulse is longer than the second. In the real world, R4 might/will have to be adjusted to assure a 2 flash output.
If you want to play with the circuit the LTspice circuit list is here, and if you don't have LTspice, it's available, here, for free.