# LED count down timer

#### brozizds

Joined Aug 15, 2010
135
Howdy,
I tried to get help with an old project but got no responce. So here is my new one. I would like to diy a varible count down timer on a PCB with a green LED at the start of the timer and a red LED and piezo buzzer at the finish. An LED numeral timer would be great! Times would vary on different timers from 15 min. to 12 hrs. may be. Can anyone help with this or am I asking to much? I apppreciate all your help in the past and on this one!
Thanks Jim

#### elec_mech

Joined Nov 12, 2008
1,500
Hi Jim,

Bertus replied to your other post. I saw it, but I wasn't sure what you were asking for - another circuit design or PCB design help?

In response to this post, what you're asking for isn't difficult. If you have microcontroller experience, that is the way to go to reduce parts count, cost, assembly time, etc. Assuming you don't want to go with a microcontroller, you can do this completely with digital ICs.

The link Bertus sent in response to your earlier post is helpful. Using the same IC, here is another circuit by Bill: http://forum.allaboutcircuits.com/showpost.php?p=374112&postcount=12.

Adding digits to display the time isn't hard, but it will add a lot of hardware to your circuit. Are you looking to display only hours and minutes or seconds too? HH:MM (four digits) vs HH:MM:SS (six digits)?

How big do you want the digits? You can go with the readily available 7-segment displays from 0.30" high to 5" high or make the digits as big as you want using individual LEDs mounted to a plastic board.

Do you want the green LED to stay on until the end of the timing cycle when the red LED and buzzer go off or only for a second or two when the timer is first started?

#### brozizds

Joined Aug 15, 2010
135
Thanks elec mech for the reply and I just saw bertus's reply TY. I would love to get a PCB design for this circuit http://forum.allaboutcircuits.com/showpost.php?p=281440&postcount=27 which works great and was developed by Bill M. I have butlt 8 or so of these but hard wired all and it took very long hence the reason for PCB. But I am open for sugestions. I got interested in possibly led numeral after a pm with Bill. I like the above design because parts are easy to get and not to complicated to build.(freebie for wifes brother)and I think could be fit onto a small PCB. Thank You Again, Jim (the newbie)

#### Wendy

Joined Mar 24, 2008
22,138
I'm thinking of designing a simple LED numerical count down timer. I think I hear the sounds of a Guinea pig! Interested?

It isn't too complex, around 7 chips or so. I actually designed and tested a prototype for a stadium timer almost 10 years ago, we regularly get requests for instructions from other folks on the site (students).

Since you are into carpentry you would also have some fun with the box, large LED displays are practical from discrete LEDs.

I'll draw complete plans, you build it, and we'll post it on the complete projects if it turns out to be worth something. I have most of the parts you'll need, so other than labor it shouldn't be that expensive.

First of all, lets define what you need. My concept was for a martial arts timer, which can count down from 3, 2, or 1 minute, has a start, pause, and reset button, and a buzzer when it hits end of count. An additional option is a light bulb on the top of the box.

What is it you are needing?

I think I missed your other post.

#### brozizds

Joined Aug 15, 2010
135
howdy Bill,
Yepper I'm interested and thanks. These timers would be simalar to our original http://forum.allaboutcircuits.com/showpost.php?p=281440&postcount=27 same purpose for cooking. The 8 I did are still up and running. Now I need about 8 more for the baking area but as in my first thread I would like them to be variable. Is this possible? From a few mins. to possibly 12 or so hrs. But there will be 8 so I can make #1 timer from 5 to 15 mins. #2 timer from 10 to 20 mins. and so on. What do you think ? and can we squeeze it on a fairly small PCB?
Thanks Jim

#### Wendy

Joined Mar 24, 2008
22,138
Count down either involve a PIC (which is small) or discrete logic (which isn't).

A really simple way to do it is a pot with a calibrated numbers in back of it. Being analog (as were your other timers) it won't be extremely accurate. Another way is a rotary switch, with 8 positions. These would be small, and a PCB would be super simple, most of the work for these kinds of circuits is in the case, the box they are packaged it. You probably have figured that out though.

I'm willing to help how I can whatever you choose to do. I can't be of much help with the PIC or Arduino, since I have never worked with them, and software is hard work (for me at least). In this I am fossil, I like the old ways.

The chip count I gave could be more for a numerical display, so it looks like you probably don't want to go that route. You might want to try one, but this is going to be a bit of work, and if the 555 route works (with improved controls) then we can go that path.

Something we could do with the 555 is a bargraph, a analog type display that will show percentage of time passed. It will take one or two extra chips, but it would be almost as good as a numerical display. It would be pretty small overall too.

#### brozizds

Joined Aug 15, 2010
135
thanks im not really looking for super accurate and I do like the pot for adjusting like the last ones with green led on at start and while running and red led with piezo buzzer at the end and could we have the piezo pulse with the red led so the sound is diff than the others I built?
Thanks Jim How bout the PCB layout would that be a problem?

#### Wendy

Joined Mar 24, 2008
22,138
For this making a PCB is easy. Now we are discussing features. For some reason this is the longest phase (basically a combination of negotiation and trying to understand someone elses thought processes). It is also where things go wrong generally.

I know you are helping a barbecue place (a family thing). If the people can be trusted to set the knob right this would be simplest way. If not then you are better off with switches, it is hard to screw up turning something on or off. Either way the front panel will take up much more space than the circuit inside. A good exercise at this point is to start doodling what you want it to look like.

I know of some nifty way to make labels, which will come in handy later.

Is the bargraph still on, or is it off the project?

#### brozizds

Joined Aug 15, 2010
135
Bill,
thanks I really need a PCB layout for our original timers cause I have 8 more to build. I would be ok with those timers. I've been having probs with my comp. TY jim

#### elec_mech

Joined Nov 12, 2008
1,500
Jim,

If you only need eight boards, I'd recommend using a prototype board. See my reply, post #3, in your original post: http://forum.allaboutcircuits.com/showthread.php?t=59475.

No need for the time and expense to design a custom board. All you need is the prototype board (~$3 per board, ~$1-1.50 per circuit), your parts, and solid 22 AWG wire (insulated being ideal, but you can go without as well since none of the jumpers cross each other). If you get the board from RadioShack, you can pick up a three pack (different colors) of 22AWG solid for under \$8, P/N: 278-1221 if you don't already have some. I'm reasonably sure elexp.com also sells solid 22AWG too. Then you just need a soldering iron and solder.

#### Wendy

Joined Mar 24, 2008
22,138
This is his next eight boards, he's already wired 8 previous up, and I suspect he's thinking in terms of it being something that keeps coming up. PCBs are easy once you have a pattern, and are enormously simpler to make than protoboards of any strip.

I have an idea for a 4 switch bank that will allow for 1 to 15 minutes. I'm still stuck on the bargraph too.

#### elec_mech

Joined Nov 12, 2008
1,500
Bill,

You mean using a bargraph to indicate remaining minutes in place of a digit? Like a fuel meter, I like it!

Do you mean a 4-position DIP switch to change the time or something else? If the former, could you use a rotary switch with BCD output instead? It would be relatively easy for an end user to understand.

I haven't made PCBs by hand before. I have made a layout in Eagle and sent it off to a PCB house before, which seems a little bit time and labor intensive in my opinion for this application. However, if you're talking about home fabrication, I can see your reasoning.

I'm a bit intimidated doing that myself - I don't love the idea of using chemicals I have to later store and properly dispose of. However, that's not too big a deal. The transfer of a layout to the copper has always seemed to be a big hurdle in the research I've done on the subject. I will have a look later at your tutorial though.

Still, even if you already have all the materials for making your PCBs including printer, transfer materials, copper board (which a quick check shows will cost a little less than a prototype board), acid, drills, etc., the time involved to make and drill each board can get up there, especially for a lot of boards. If I were doing several boards with a lot of parts, I might make my own, but for a few with so little parts, I still like prototype boards. Just a difference of preference of course. If the OP wants to learn to make his own boards, this is a great circuit to do it with.

#### brozizds

Joined Aug 15, 2010
135
thanks guys, ok Bill I'll bite on the bar graph and I know you need a (piggy) lol but I still would like a PCB layout for our old ones lol. I will deffinetly build the pcbs if you can get the layouts to me. Hay elec mech I followed Bills tutorial on PCBs and mine came out perfect for a much harded circuit. Try it its fun! Thanks Jim

#### Wendy

Joined Mar 24, 2008
22,138
OK, several points, with a fancy timer the PCB would work for the simple timer too. You just leave the parts off that you don't want.

I could come up with a simple PCB when we do the fancy one. The PCB is simple enough it would be practical to hand draw the pattern on the board.

The bargraph feature would use the ramp function of the 555, where the capacitor is slowly charged up on the monostable. There are several ways to do this, the LM3914 would probably be the easiest, while it could also use something like the LM393.

Be that as it may, it isn't heard.

Still using a common 9V power supply, or 12V? I would need to know early in the design.

#### brozizds

Joined Aug 15, 2010
135
Sounds great Bill as far as power supply what ever you design in the circuit is fine BG sells both and being I plan on mounting all 8 timers on a mounting board (same as last time) I'll only be using one pwr supply.
Thanks for the great imput! Jim

#### MMcLaren

Joined Feb 14, 2010
842
Bill,

Got a summary of features? I was thinking about providing a PIC or MSP430 example design with the same features, just for fun. I suspect that some of the features you're considering would be much easier to do with an mcu. For example, you only need four I/O lines to drive a 10 element bar/dot display.

Regards, Mike

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#### Wendy

Joined Mar 24, 2008
22,138
Off hand I would say that is up to Brozizds. I don't have a problem with it either way, and it is a common request from beginners. Even if he doesn't use it someone will eventually, I would be glad to post the circuit in my blog (after redoing a Completed Projects thread). It could be a good first project for someone.

Here are the requirements as I currently understand them.

1. Variable timer (1-16 minutes). This can be with 4 switches in binary (1+1/2/4/8) or a pot.

2. Bargraph as elapsed time indicator.

3. Multi-choice audible alarm, where individual sounds could be made so the personal can distinguish which alarm has gone off. Think the three note alarms at McDonalds and you get the idea.

4. Two LED indicators, red and green.

That is the deluxe model.

The basic model is a simple 555 circuit. It uses a 555 set for one time, two LEDs, and a sonalert. Turn it on and it starts timing. I will definitely be redoing it with a PCB. Their is a dedicated chip for bargraphs, the LM3915, that I would use for a display like yours.

To Brozizds, I would still like the voltage. At the moment I'm assuming 9V, but I would prefer 12V. Give me a number please.

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#### brozizds

Joined Aug 15, 2010
135
Bill
12V is fine and I will do both if you would like standard and deluxe. Thanks guys for all the imput. Jim

#### Wendy

Joined Mar 24, 2008
22,138
Lets change the subject a bit. You want to change the sound of the output of this gadget. Not too easy with a sonalert, those are fixed. You can add beats and beeps, but that is it.

With 3 chips I can play a short musical phrase, but that seems a bit complex to me. If you have any ideas what you want this would be a good time. A second 555 with a on/off rate would offer some variation, but not very much. The three chip method I'm thinking about would drive a speaker, not a sonalert.

A 555 has a variable voltage while timing that lends itself to something like a bargraph. It could as easily be a meter.