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 :)

555 Monostable Flash

  • 1
    \$\begingroup\$ Two timing parameters means two timers. And I think you meant "sequential". \$\endgroup\$ Oct 13 '14 at 3:57

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.

enter image description here

  • \$\begingroup\$ When I make R1 820 Ohms, and then C2 1 microF, it barely stays lit. I might as well use a straight up momentary on it :P \$\endgroup\$ Oct 13 '14 at 17:59
  • \$\begingroup\$ @user3226378: "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." Note that 820k ohms is 820,000 ohms... \$\endgroup\$
    – EM Fields
    Oct 13 '14 at 18:11
  • \$\begingroup\$ I missed the K on your first post. That makes way more sense now. Thanks! :) \$\endgroup\$ Oct 13 '14 at 18:20
  • \$\begingroup\$ So how can I get the max voltage to the LED as possible. The ones I'm looking at are rated at 3.3 V \$\endgroup\$ Oct 13 '14 at 18:29
  • \$\begingroup\$ Do you have a supply voltage higher than 3.3V that you can work with? \$\endgroup\$
    – EM Fields
    Oct 13 '14 at 18:39

If you just want to use 555 devices then you could wire the LED to a 555 in Astable configuration and use a 555 in Monostable configuration (as your diagram) to control the RESET (pin 4) of the Astable 555. So the monostable enables the astable for the required time for 2 flashes.

See here for a 555 tutorial.

Also: You need a resistor in series with the LED. 100 to 300 Ohms or so depending on the brightness you need. C2 is very large. You should reduce it by a factor of 100 and increase R1 correspondingly.

  • \$\begingroup\$ You gave me just what I needed. I had the mono\astable design going, but I couldn't figure out what pin connected to what. Thanks a ton! \$\endgroup\$ Oct 13 '14 at 17:16
  • \$\begingroup\$ Also, what if I were to wire it into a 556? Wouldn't that inevitably be smarter? \$\endgroup\$ Oct 13 '14 at 17:17
  • \$\begingroup\$ Yes. A 556 is dual 555, so that would be a smart thing to do. \$\endgroup\$
    – akellyirl
    Oct 13 '14 at 17:31
  • \$\begingroup\$ Lastly, how come I'm losing 0.4 volts to the LED? My voltage is 3.3, but upon triggering it it shows 2.9 (Just in the monostable circuit above) \$\endgroup\$ Oct 13 '14 at 17:33
  • \$\begingroup\$ You need a resistor in series with the LED. 100 to 300 Ohms or so depending on the brightness you need. \$\endgroup\$
    – akellyirl
    Oct 13 '14 at 17:36

A circuit that flashes more than once is "astable". What you want is your monostable to trigger an astable, let it run (for X blinks)... then disable it.

look up an astable 555 circuit, and use it to drive the cathode (negative end) of the LED.
When the anode (positive end) is held low, the LED has 0V - or 3.3V of reverse bias.
When the monostable pushes the anode high, the LED will start to blink. The number of blinks depends on the timing for the astable, vs the monostable.

Alternative: you can buy Blinking LEDs on EBay. The blinking circuit is built in. If you used one, you could use your cuicuit "as is".


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