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Simple timed LED? Answered

i need a way to turn on an LED after a certain amount of time (e.g. 5 minutes) then keep it on until a momentary switch is pressed.

a very low power consumption if possible, because it's going to be wrist mounted.

thanks in advance



8 years ago

The problem with a 555 solution is the long period you require. You would need to use the CMOS version of the chip to get a time of 5 minutes (300 seconds) and you would need high value capacitors which are physically large and / or low tolerance and sensitive to temperature variations.
There used to be a ZN1034 long period timer chip which I used to use for this type of thing, but they are no longer produced.
For low power consumption and wrist-mounting, I think the microprocessor route is the best. There are many to choose from, but I would use the surface mount Picaxe 08m module which is tiny (around 0.8" x 0.6") and would run off a lithium coin cell. You would also need to buy the programming cable and program the beast, but help is available on Instructables to get you through that stage.  Have a look at my PicAxe based Instructables for more info on this.


8 years ago

a piece is missing from your puzzle...

what is the desired effect, from the start:

do you want the light to default on or off when the circuit powers up?

what do you want to happen after 5 minutes?

what do you want to happen when the button is pressed?

The way you describe it, it stays on for 5 minutes, or shuts off when a button is pressed.


Answer 8 years ago

sorry if i wasn't clear enough:

when i turn the circuit on, the LED is off, and a timer starts counting down five minutes, when the timer reaches zero, the LED turns on, and will remain on until a button is pressed, that turns the LED off and re-sets the timer to start the whole thing off again

hope that's clearer, thanks


Answer 8 years ago

okay, so a 'reminder to do something every 5 minutes' kind of thing...

That's easy with a 555 timer. It can be put into 'monostable', or 'one shot' mode where its got a timer for one state (call it 'off'), and totally stable in the other state ('on') -- when it powers on the timer starts, the output is off, and it keeps going until the threshhold is reached to turn on -- where the on state saturates the circuit and tells it to stay on, and not oscillate.


read up on the 555, it's very versatile, and easy to work with.

If passive ic's aren't your favourite, pick up some microcontroller like the arduino, which you can program to perform more complex tasks (this one is far below what the arduino can perform)...