Flashing LED circuit problem

[Finch]

I need a circuit that will make 2 LEDs flash when a micro switch is triggered. It will flash on and of for, say, 10 secs (not that bothered) and then will either be switched off with another switch, or stop by itself(again don't mind).
Any ideas what I should do/use?
I hope I've explained that OK, and I'm new so I hope it's OK if I just go straight ahead and post this.

 

[Brian Hoskins]

Hmmmm. I'm getting the impression here that you're asking for help with something when you don't fully understand the question yourself.

What do I mean by this? Well, firstly you state that you want two flashing LEDs to be triggered by a micro switch. That part is fair enough. But then you go on to mention that you're not really concerned with how long they flash for, although you do throw 10 seconds up into the air. Personally I think you should know how long you want your own project to operate for! It doesn't have to be accurate of course; for example you could say: "I want the LEDs to flash for 10 seconds after the microswitch is pressed, +/- 2 seconds". That would be far more specific.

You then go on to mention that you're not too concerned about how the LEDs are to stop flashing. Either it can be via another switch (a reset switch) or the flashing LEDs can time out by themselves. Well... which one is it that you want?

My initial advice here is for you to have a sit down and think about your project, and decide how you would like it to operate. Then, when you've done that, have a think about how this might be accomplished. Post your ideas (even if it's just ideas in terms of discussion of methods) and then I'm certain you'll get positive responses.

Hope this helps,

Brian

 

[Finch]

Hi, thanks for replying.

I wondered what I would use to stop it if I either used a reset switch, or had it on a timer, and wanted to see how either would work/ fit into the circuit. You're right I havn't fully finalised it, I am still looking at how I could do it. I wanted a clearer idea of my options.

How long it flashes for isn't really important, but I'll say about 10 secs if I use a timer. It shouldn't be an issue if I use a reset switch.

Hope this clears up what I'm trying to ask.

 

[Boncuk]

Hi Finch,

you still haven't made up your mind if the LED should be lit as long as there is no more button pushed (flashing) or flash for the given 10secs time period and extinguish regardless of a button push.

Boncuk

 

[Brian Hoskins]

Ok. Well, it looks like your project can be split up into three small problems.

1. You need a circuit to flash the LEDs, unless you're just going to use "flashing LEDs" in the circuit? Do both of the LEDs need to flash at exactly the same time or does it not matter as long as they both flash? If They need to flash together then you'll need a simple clock circuit for them. What sort of on / off periods are you looking for?

2. You need a delay circuit that will enable the flash circuit to operate for your 10 seconds. This could be accomplished via simple counters to divide a clock signal down.

3. You need a way of causing the circuit to reset once the delay time has elapsed. This should be very simple.

Have you any idea how any of this might be accomplished yet? I'm willing to help you with some ideas, and I'm also willing to help you with a circuit that isn't functioning the way you expect.

Brian

 

[Finch]

Right, thanks for that. This is going to be where I show what a complete beginner (but going throught the sites on FAQ) I am, so sorry for any mistakes.

For part one, yes I want them to flash together. I was thinking of a flash rate of about twice a second, which seems to be the norm for most flashing LEDs. I did find a circuit to flash to ordinary LEDs but I don't know how to put it in a post (how would I?).

It involves two transistors, two capacitors and four resistors. It is simply connected to a power source and the LEDs flash. I was thinking of starting it with a push-to-make micro switch(would it need to be on a latch)

For the delay, I was told to use a 555 timer, but your way sounds simpler. How would counters to divide a clock signal work?

Thanks for your time.

P.S. Boncuk, I'll remember that advice, and I think I've answered your question in here(?).

 

[Ubergeek63]

OY... a single NAND schmidt trigger. A feedback resistor to one input with a cap to ground gives you an oscillator and an RC across the rails on the other input gives you a delay.

Not extremely accurate but extremely easy. A 74HC132 is a quad NAND schmidt that leaves you 3 NAND gates to tie off or play with.

 

[Brian Hoskins]

If you used a counter as the delay, you would still need a clock source so a 555 timer or similar device would still need to be utilised for that.

I think I would tackle this problem by using a 555 timer to control the flashing of the LEDs, and use the output of the 555 timer to also clock a couple of cascaded counters. The cascaded counters would then provide your delay.

Alternatively, you could investigate using a resistor - capacitor charging circuit to trigger a FET or an Op-Amp comparator as your delay.

Have a look on the net about 555 timers, there is loads of information out there on how to use them. Also have a look at counters, and you might also have some luck doing a search for dedicated delay ICs - perhaps someone makes an IC with an adjustable delay within the range you want?

Get back to us and let us know how you get on!

Brian

 

[Finch]

OK, will do thanks.

I'm researching those suggestions now.

 

[Finch]

Hi,

I've found a circuit I can use that uses a 555 timer (but I haven't tried to make it yet so I don't know how well - or not - it works).

The only thing is, I am not sure how to reset it. Do you have any suggestions please?

 

[jst3712]

Hi,

I've found a circuit I can use that uses a 555 timer (but I haven't tried to make it yet so I don't know how well - or not - it works).

The only thing is, I am not sure how to reset it. Do you have any suggestions please?

You could make use of the 555's 'Reset' pin (4) - Connecting it to ground briefly usually does the trick if it's setup as a delay/timer ("multivibrator"), but remember to use a pull-up resistor so the pin is not left floating.
If it's going to be used as a flasher ("astable"), a different approach is needed.

I'll leave the rest to the pro's Good luck

Jason.