I am endeavouring to build a count down timer from discrete components like a 16f877a and 595's or CMOS 4000 series.I would like to use a 7 seg display to show minutes remaining. I have searched the web for a suitable count down timer cappable of counting 90 minutes with a simple flash or piezo alarm when the set time elapses. I would like to be able to program it for 60 or 90 minutes based on a vessel reaching 100 degreees C. Any body have a schematic that utilizes a 16f877A, the rest of the components can be sourced if needed and I dont have it in my current bench stock which is pretty extensive. Any help on this quest? Thanks in advance for any pointers or directions anyone may have.

Bob

 

Here's a project that I participated in that used a PICAXE to do what you want. I would guess you could adapt it to a PIC. It's long, so you might want to start at the last post.

http://forum.allaboutcircuits.com/showthread.php?t=79034&highlight=traffic

A photo of my final working prototype is in post 108.

 

Trtacecom,
That is perfect!!!! I have a bunch of Picaxe 40 X series chips so I am in fat city. I havent looked at the BOM list yet but probably have most or all of the other components in my bench stock already. I've been bulking up my bench stock for a year or so and have progressed to stocking commmon CMOS 4000 series, a few 74 series like 595's and of course project boards, caps, resistors and misc connectors and terminals. I knew when I got started with electronics I would probably end up needing all kinds of weird components so I have dedicated a monthly allowance to buying various components in bulk quantities. Kind of like 5 NE555's for .99 on Ebay and deals like that, it.s amazing how far you can stretch a 50 dollar monthly allowance if you shop carefully and use the Buy it now and free shipping options from the Thai and chinese parts houses. So far no knock offs or parts that dont meet the specs and I test each one beforoe I install it into a circuit. Thanks a ton for the referal and help, it makes what I am building much easier to integrate since I am already familiar with Picaxe and have all of the communication and down load cables and software already.

Wheelchair Bob

 

I worked with Tracecom on the post he listed. Lotta work, but it was a lot of fun too.



I'd make the following suggestions using our project as a template:

1. If you don't need them, leave out the buttons both physically and in the code. This will free up some processing time.
2. If you only need to select between 60 or 90 minutes, I suggest adding a toggle, rocker, or slide switch. You could use a SPST and connect one end to GND and the other to the uC. Then add a pull-up resistor to the uC pin. When your program starts, have it look at the state of said pin to determine if it is high or low and whether to countdown from 60 or 90 minutes.
3. If you only plan to display two digits for the minutes and you're using a 40-pin PICAXE, skip the $11 MAX7221 and wire the display to the uC directly. If you're digit height will be much bigger than 0.8", use something like a ULN2003 with a common anode display between the uC and the display.
4. Since it sounds like you have a nice collection of CMOS ICs, you can save a little money using a CD4060 with a crystal, a flip flop and a few parts in place of the DS1307 to generate the 1 Hz clock signal. BillM has a good circuit I've used part of, but this site has the same thing just focused on the clock circuit. You can also use a 4013 in place of the 4027.
5. Adding a temp sensor to the program shouldn't be hard. I believe Tracecom has a post or two showing how to program and interface a temp sensor to the PICAXE. You can have the program start by reading the temp sensor continuously. Once 100 degrees is reached, you have the program start the countdown (if I understand how you want this operate).

Let us know if you have any questions.

 

Elec Mech,
Thanks a bunch to you and Tracecom for the hint. I have been learning Picaxe for a while and have a few 40M2's so when things slow down for me a little again I will probably build the project you and he have described. Yes I only need to count 60 or 90 and no other variables and the temp is based on boiling water reaching a boil to start the timer. So wtih the info you two have pointed me to I will start building soon I hope. Thanks for the help, Us beginners really appreciate it a bunch!!!!

Wheelchair Bob