# LED timer (for 2 LEDs) in a daily cycle

#### arizonasun

Joined Aug 28, 2011
4
Need to turn 2 red LEDs on at 7 pm, and off at 10 pm every night (on three hours, then off 21, then repeat). We have tried a 555 circuit, but that only executes once, not every day. We have also tried to program and then take apart an Orbit water timer, but outgoing current to the valve was negligible. Help would be appreciated.

#### Wendy

Joined Mar 24, 2008
22,328
You need something better than a 555, lots better, unless you want to use sunset and sunrise sensors. I've designed something similar for other people using a 4060 that you might find handy, or you can buy an off the shelf clock that has a feature that can be adapted.

When I have more time I'll pull up some other ideas. The problem is the precise time, it implies a true clock.

#### arizonasun

Joined Aug 28, 2011
4
There doesn't need to be a clock with a precise time: you can just turn the 3/21 cycle on at 7:00. Also, the 3 hrs doesn't need to be exact at all. Could you please give a source of programmable clocks that can be hooked to LEDs (like you mentioned), or your 4060 circuit? Thanks for the reply.

#### Potato Pudding

Joined Jun 11, 2010
688
Look up a CD4541B programmable timer.

#### Wendy

Joined Mar 24, 2008
22,328
Instead of thinking of this as one timer, think of it as two. One is a 24 hour timer with an alarm, the other is a 3 hour timer that is triggered by the alarm.

A wrist watch would be ideal for the first. It has an alarm you could use as a trigger, and they are cheap.

A 3 hour 555 monostable could be built, but it would use some very large components, and would probably be failure prone. It would be simple to build however, a 555 one resistor and a few capacitors.

I will draw a possible schematic if you would like.

A better 3 hour timer would be several chips, and digital in nature (a 555 is a analog/digital hybrid). If you want something different I can draw that up to.

I've never used 4541, so I can't speak as to how good or bad it is. I've used it's cousins before (the CD4500 series is a large family of digital chips).

#### swap.bhikule

Joined Aug 25, 2011
3
you can use IC4040 or IC4020. Both are counter clock ICs.Or use IC555 as a timer. for the value of resistor and capacitor use formula t=1.1*R*C. where t is time when LED glow. with both ICs use relay as a aswitch.

#### swap.bhikule

Joined Aug 25, 2011
3
you can use IC4040 and IC4020. But you need one switch to restart the operation....

#### Wendy

Joined Mar 24, 2008
22,328
you can use IC4040 or IC4020. Both are counter clock ICs.Or use IC555 as a timer. for the value of resistor and capacitor use formula t=1.1*R*C. where t is time when LED glow. with both ICs use relay as a aswitch.
Uhh, no. Both the CD4020 and CD4040 are simple ripple counters. They will divide a frequency by a set amount, but they are definitely not clocks, though they can be used as part of a larger circuit (about 3 chips and several diodes).

I've had a chance to look up a CD4541 data sheet, it is a timer with a built in counter. Assuming you can get the part it is a lot more promising.

#### wayneh

Joined Sep 9, 2010
17,020
Programmable, digital light controllers are less than $5 if you watch the ads. I pick 'em up every holiday season for free after rebates. Why design and build something to avoid this? If time-of-day precision is not terribly important, I'd still look to use the photocell approach to start a ~3-hr countdown. It will false trigger during storms, though. #### Wendy Joined Mar 24, 2008 22,328 Programmable, digital light controllers are less than$5 if you watch the ads. I pick 'em up every holiday season for free after rebates. Why design and build something to avoid this?

If time-of-day precision is not terribly important, I'd still look to use the photocell approach to start a ~3-hr countdown. It will false trigger during storms, though.
Actually that can be beat with simple RC integrators.

#### iONic

Joined Nov 16, 2007
1,650
I like the wrist watch idea as it is the most precise with respect to time and very frugal with respect to energy. Using the 24Hr alarm on the watch can trigger the 555 for 3 hrs with tweaking.

#### Potato Pudding

Joined Jun 11, 2010
688
I've had a chance to look up a CD4541 data sheet, it is a timer with a built in counter. Assuming you can get the part it is a lot more promising.
They are available through Mouser and others for $0.20 to$0.50 range.

#### arizonasun

Joined Aug 28, 2011
4
Thank you, Bill. Could you please share the 555 schematic? Does it have a way to repeat daily, or does it rely on the watch? About the photo cells, our original circuit (the one that doesn't repeat every day) says that you have to trigger it momentarily, and it indeed doesn't work when you hold the connection that triggers it. With that in mind, how do you get the photo cell to just send a burst, and then stop until the next night?

#### JingleJoe

Joined Jul 23, 2011
186
Thank you, Bill. Could you please share the 555 schematic?
The 555 schematic in question is a monostable circuit, which turns on for a determined period of time when triggered.
http://www.kpsec.freeuk.com/555timer.htm#monostable
how do you get the photo cell to just send a burst, and then stop until the next night?
Don't use a photo cell, use a light dependant resistor (LDR) and some variation of op-amp based voltage crossing detector;

This circuit could be used to activate the 3 hour timer at a set light level, additionally coupled with a 555 monostable, it could create a trigger pulse.

#### Wendy

Joined Mar 24, 2008
22,328
That schematic for the 555 is definitely flawed, a signal conditioner is required, not optional for these kinds of circuits.

I have an experiment and theory of operation here...

555 Monostable

Before I draw something from scratch, are you going to use a digital watch or a IC 24 hour timer?

The IC version has a problem, you need to reset the timer at the start of the 24 hour period, which means only one time of day can you set it.

The clock, on the other hand, is completely settable, you set the time, then you set the alarm time.