trying to build 5v led timer

[Johny pc]

hi I am looking to get help building a 5v circuit timer to turn leds on and off every 12 hour. the circuit would need to repeat over and over. All and any help would be greatly appreciated.

 

[betwixt]

This can be extremely simple or extremely complex.

How many LEDs and what power are they? (a signal light or football stadium lights)
How accurate has the timing got to be? ('fairly' accurate or to the microsecond)
Is the 12 hour period set by clock time or can it start at any time of the day?
How is it to be powered?

Brian.

 

[jiripolivka]

This can be extremely simple or extremely complex.

How many LEDs and what power are they? (a signal light or football stadium lights)
How accurate has the timing got to be? ('fairly' accurate or to the microsecond)
Is the 12 hour period set by clock time or can it start at any time of the day?
How is it to be powered?

Brian.

I would use an electronic clock or watch as a precise timer.Extracting one pulse per 12 hours can be done by finding output pulse or generating one from a "mechanical" handle, by a contact or an opto-detector. The cost is minimal, time accuracy excellent and one can use a relay to switch low or high power load as needed.

 

[Johny pc]

This can be extremely simple or extremely complex.

How many LEDs and what power are they? (a signal light or football stadium lights)
How accurate has the timing got to be? ('fairly' accurate or to the microsecond)
Is the 12 hour period set by clock time or can it start at any time of the day?
How is it to be powered?

Brian.

I am looking to power 5 or more 3.6v leds I would like it to be powered by usb. the time does not have to be to accurate. as for when it starts I would like for it to start when its powered on and reset its self when its powered off. I need the circuit board to be no bigger than 4 by 4 inches but the leds will not be on the same board . as for the cost I am trying to keep the price down as low as i can.

 

[betwixt]

OK, further questions:

What supplies the USB power? You have not specified how much current the LEDs require so if these are high power types you will run into problems if the USB power comes from a computer USB socket. These are 'current limited' when first powered up and the device plugged in has to tell the computer the maximum it can demand from it before more current can be allowed. The problem with that is do be able to do that there has to be a full USB interface circuit and some 'intelligence' in your circuit to negotiate the amount of power it can ask for. This makes the circuit more complicated and WILL need a microcontroller with software. If the computer is switched off, you may also lose some or all the power of course.

If the USB power is coming from a wall adapter you might be in luck as these tend to be 'dumb' power supplies. Some, especially those designed to charge mobile devices may have a fixed power limit built into them though.

If at all possible, I would suggest you avoid USB as a power source and in view of the long periods the LEDs will be on, a mains AC power source would be a better solution by far.

The simplest design for what you want is a two transistor oscillator but these are very susceptible to variations in temperature and supply voltage, you may find the timing varies by as much as one hour or more between cycles. The other simple option is a tiny microcontroller configured as a timer. This may be as small as a 6-pin IC and it will give timing accurate to say +/- 20 seconds in 12 hours. They cost ~$0.50 US. All you need is one transistor and two resistors per LED so for a 5 LED system you would be looking at maybe $2.00 US and it would fit on a board much smaller than 4 x 4 inches.

Brian.