you could press 1 every second...
.
Am i on fire? OHHH NOOOO!!! I'M ON FIREEEEEE

make....from scratch?

google "555 timer" and "monostable oscillator"
thats a one shot timer. Good enough if you want a light to stay on for 20 seconds after pressing the button or something like that.
"astable oscillator" if you want a repeating blip ever X seconds (like the ticking of a watch)

though thats not the most accurate method. If you want a more accurate clock get a crystal and use some counter chips to divide the frequency down to whatever you want to count (from 3Mhz down 1Hz or whatever)

if you want to use a calculator as a display hook up the clock signal to the = key and do "+1=", so every time you press the = it adds +1 to whatever the number is

edit: simple timer
http://en.wikipedia.org/wiki/Hourglass
2 bottles, some duct tape and some water or sand or whatever other substance
Last edited by seljer at Nov 3, 2007,
For what do you need a timer on a calculator?
Quote by vigenharutyunya
I just need a timer to set the time when to turn on a light and so...

can't you just buy one of those ones that plug directly into the socket?
these type of things http://www.m-99.co.uk/Timer_Plug/timer.jpg (I'm sure today they're all digital and whatnot)
they're like less than \$15 and will solve your entire problem instantly

you're not going to DIY anything for much less unless you already have all the parts

edit:
for the rocket.....maybe. Depends on what kind of timer you need.

and a counter and a clock/timer are 2 reasonably different electronic devices...sure they work well together but its a different concept behind them
Last edited by seljer at Nov 3, 2007,
<html>
<script type="text/javascript">
var c=0
var t
function timedCount()
{
document.getElementById('txt').value=c
c=c+1
t=setTimeout("timedCount()",1000)
}

function stopCount()
{
clearTimeout(t)
}
</script>