Flash a LED twice

Hi All,

I have been on the forums a fair bit, but I had to change my username becuase I couldn't remember my password for my old account.

Anyways, I have been trying all day to find a way to flash an LED, but only twice. I thought I'd probably have to use a 555 timer or something similar, but I have a constant 12v coming along a wire, need to solder a circuit and some LED's in the middle so as soon as the 12v is flowing the LED's will flash twice, maybe even sound a buzzer. Once they have flashed, the 12v will have to continue flowng through this circuit.. but leave the LED's off.

If anyone can help it would be much appreciated.

Thanks in advance,

Assuming you don't want to use a PIC, it can be done (amongst many other ways) using two 555 timers. The first is a monostable, that when the 12v flows, it triggers the monostable, which turns on the second 555 (wired as an astable), but it only turns it on long enough for the LED to flash twice.

As I say, there are many other ways to do it, this was just one that came to mind.

I'll draw the diag for you when I get the chance

Tim.

Just a thought - use a 555 to generate pulses at the desired flash rate - use the pulses to drive the LED directly - or to control a driver for the LED.
Then add a simple binary counter that is also tied to the pulse train. Tie the output(s) of the counter to and AND or NAND gate to serve as a decoder of sorts - when it sees '1' '1' output it's state changes - let that state change interrupt the current to the LEDs so they stop flashing. Ideally the interruption should occur at the end of the second pulse. If fast enough, using a 3 input gate, it could shut off as the third pulse is starting. You'd also have to disable the counter so it holds at the stop point - maybe they make counters with an enable/disable line - or the gates may be available that can enable/disable. On power off all would have to return to a workable state.

If I had to do this I'd drag out my Radio Shack books with simple circuits. Seems like a simple but interesting challenge. Have fun.

For my ideea, I was thinking - why use two 555s?

why not just have a non-retriggerable monostable, connected to a flashing LED?

this diag should help- just connect the flashing LED to the output.

Blaitently though, Stevez's idea is much better, and is much neater.

Stevez: you say, "if fast enough", LEDs take a while to light up, so it probably shouldn't be a problem, expecially considering the sped of modern logic ICs

In summary: use Stevez's idea, it much better.

P.S. can the circuit use the power from the 12v or not? if so, where is it coming from?

grrr .... my "fast enough" comments were kind of a catch-all thing. Wasn't sure if states change on leading or trailing edge. Also was thinking that if counter saw 11 and shut the LED off before you could see it blink that could be a problem hence the thought about shutting it off at the start of '3'. I guess I know enough to worry about it but little more. Pondering the simpler stuff like this during quiet moments here at work helps me to learn. This would actually be a nice little exercise to help learn more about digital logic - challenging but not overwhelming.

ok stevez, fair enough.

yes, I agree, you would have to catch it on the start of '3', because otherwise it would shut it off too fast.

The reason I wasn't worried about it was because (for some reason) I naturally turn to 4000 series, and they usually switch on leading edge. Also, I have built circuits like this in the past, and I know that you can 'catch' the output (that is lighting up an LED), and cut off the supply to this LED before the LED has time to light. Thus, past experiences told me not to worry about this (even though you do know more than me about this... :lol

Well, I setup the following circuit in Crocodile Clips, and I can't get it to work. The reason I'm trying to make this circuit is so I can flash indicators on a car, and also sound the horn.

What am I doing wrong?

Well all I really need is a 12v pulsing circuit that only pulses twice then stops. Pulse length shouldn't be too long, maybe 1/10 of a second?

Thanks for your help so far,

Tim.

Explain what you think should be happening, step by step. I can see that you push a button - and that you have a series resistor in line with the LED. What do you think is going on in the 555?

It should be flashing the LED twice then cutting the circuit off.

What have I done wrong?

Lets,
Your timing cap is way too small at 0.1nF (1us pulse). Even 0.1uF is way too small (1ms). Use 10uF for a 100ms pulse that you can actually see.
The 555 needs pin 2 (trigger), which should be high with a pull-up resistor, to go low to start the monostable.
Pin 4 (reset) should be tied to the positive supply.
Then each time the button is pressed, the LED will flash only once.

If you use this monostable to control the reset pin of a flashing astable oscillator, then the LED might flash twice, depending on your timing since they aren't syncronised, but the 1st flash will be longer than the 2nd.

Thanks for the advice, as I understand it I have to change the capacitor value. Any ideas how i would get it to flash twice.. its basically to simulate a car alarm so I'm open to any suggestions, as long as the circuit isn't going to cost me more than $40 to make.

Thanks in advance,

Tim.

I think this will work, but I haven't actually tested it. Let us know how it works (or doesn't) if you decide to build it.

You can change the pulse width by changing R1 and/or C1. I show Q0 and Q2 as the outputs that activate the LED, but you can use any two outputs that are separated by at least one output, i.e, 1-3, 0-8, 5-7,2-6, etc. Don't use the output that feeds back to CE, as it locks up and stays high until the supply drops to zero. You can choose any output to connect to CE so long as it is greater than one that causes the second flash.
You can change the LED brightness by changing R5. Iled=(12-Vled)/R5.