Long time delay (6hrs ish) - HELP

I'm running some leds off a 4.8v nicad pack but need to turn them off after
a 6 hr delay can anyone help please. The nicads voltage spans 5.2v down to
3.6

thanx
David

 

Is the problem battery life? Or is there some other reason to turn it off?

Its pretty easy to notice when the battery gets low, and turn off power to
the LEDs...

see http://home.comcast.net/~rcmonsen/turnoff/turnoff.html

Regards,
Bob Monsen

 

I would initially wonder if the NiCad pack has sufficient amp hours to run
"some" LEDs for 6 hours. Have you checked the capacity of the pack to
ensure that the current drawn by the LEDs can be supplied by the pack for
the required amount of time?

Assuming that the answer to that question is "Yes", then I suggest using a
CMOS 4521 combination oscillator and counter. Pin 1 provides a divide by
16,777,216, so if you set the oscillator to run at 777 Hz, that will give
you a pin 1 output after just about 6 hours. the CMOS chip will use minimal
power, so it won't run down your battery significantly by itself.

I'm still a bit concerned about the current drain from those LEDs though.

regards

John

 

Bob
A bit of background. I make LED lights out of chrome tubing and bits of
slate/ wood or whatever I find and give them to friends/family as presents.
Most people haven't a clue about nicads etc so cannot 'manage' them. So, to
some extent yes battery life is an issue. The general idea is that the cct
will effectively stop the cells being destroyed by being discharged too far.
It will also stop them coming on if VCC is too low which is a wake up call
to charge the cells!!

One other thing... I have a cct that will gently fade in the lights (just a
nice effect) so I'd want the on/off cct to control that too. Does your cct
do that?

Thanks very much for your help

David

PS I can send the cct to you if reqd

 

John

A bit of background. I make LED lights out of chrome tubing and bits of
slate/ wood or whatever I find and give them to friends/family as presents.
Most people haven't a clue about nicads etc so cannot 'manage' them. So, to
some extent yes battery life is an issue. The general idea is that the cct
will effectively stop the cells being destroyed by being discharged too far.
It will also stop them coming on if VCC is too low which is a wake up call
to charge the cells!!

One other thing... I have a cct that will gently fade in the lights (just a
nice effect) so I'd want the on/off cct to control that too. Would your cct
do that and if so do you have a cct diagram?

Thanks very much for your help

David

PS I can send the cct to you if reqd

 

Peeps.

I built my first lamp last July using 4 x 4000mcd blue LEDs and turn it on
since then for about 1 hr a week. I still has the same two IKEA D cells as
it started with with no dicernable reduction in brightness...

David

 

John

Can't find the original post but can see lots of subsequent discussion!!!
ANy decisions reached?

David

 

I would suggest the used of a small PIC with an Analogue to Digital
converter (A-D)
The 12F675 - 8-Pin or 16F676 - 14-Pin cost is about $1.50 from agents in
small quantities
I prefer the 16F676, it costs about the same as the 12F675 but has many more
pins to
play with and the Flash PICs allows in-circuit reprogramming.
(Just as the friends get bored, you can add new tricks to your gadget
by reprogramming the PIC.)

The NiCad's 3.0 to 5.2 volts falls nicely within the PIC's operating voltage
range
Using the internal oscillator and an external FET or Transistor to drive the
LEDs
will result in a very low parts count.

You would probably require a very low current, voltage reference of say
1.25v
to set a threshold on one of the analogue inputs. As the supply drops, the
A-D
will register an apparent increase in voltage as the batteries discharges
- just pick the correct value to shut down.

With the PIC you can
a) Use the A-D to measure the battery voltage and shutdown the system
to sleep mode (few uAmp - probably less than the NiCads self discharge
voltage)
b) Use a single simple pushbutton to switch on and off.
c) Have all sorts of fun with light effects and multiple LEDs.
d) Can indicate (with LED flashes) when batteries are fully charged.
e) Automatic turn on and off at specific times (needs accurate clock)
f) Automatic turn off after x hours
g) .....(just be creative - you don't seem to have a problem

You could even use four NiMH batteries or three rechargable Alkalines.
Charging these batteries is series can be a problem.

Gerhard

 

Gerhard
'Fraid I haven't a clue what a PIC is! Explain please I'm very interested in
its apparent flexibility...

David

 

A minimal and *cheap* microcontroller, with perhaps as few as
eight pins. Program it do do as you will.