First, background on me:

I have taken electronics courses and understand the basics, but have never successfully built anything more complex than a flashlight. IE: Use small words.

Project:

I want to make a countdown timer that meets the following criteria:
Small/handheld (small flashlight size is best)
Battery powered (preferably AA/AAA)
Rugged enough to be dropped and tossed into a bag without falling apart

The timing task to be completed:
-Single button
-Every time the button is pushed, a new 7 second countdown starts
-If the 7 second countdown is completed, a buzzer goes off for 1/2 second (approximate). I prefer a vibration, like a cell phone on vibrate.

This is for refereeing a sport that has a repeated 7 second timer. Every time a player catches a disc, 7 second countdown starts. If they don't release the disc before 7 seconds, it is a penalty. This happens hundreds of times in a game. As an official, I want something I can hold in my hand and just push a button every time someone catches. I can't look at the device or trust hearing a sound, so I want it to vibrate in my hand when the 7 seconds has elapsed. Every time I push the button, the countdown needs to start over. The timing will never change, so I don't need options on the length of time, it will be fixed at 7 second.

So, two questions:

Is this a good first project for someone who has some idea of what they are doing, but not a whole lot?

What does it take to build this?

Feel free to refer me to "Timing Basics" or something like that to read stuff that is already written.

 

Very basic and very doable.

As always, lowest current consumption for longer battery life would be a prime objective.
Running off two AA batteries would be preferred.

You can do this with:

one 8-pin MCU
or two TLC555
or one TLC556

You will need a cell-phone vibrator motor which are readily available.

 

Is it sufficient to calibrate the 7s time from a standard clock?

If the player gets rid of the disc before 7s what do you want to happen:
1. Push the button to cancel with another push to start the 7s countdown?
or
2.Push the button to restart the 7S time countdown?

 

Yes, calibrating it from a standard clock would be fine. There will be error of tenths of a second based on the ref pushing the button each time regardless, so if it is off by .01-.1 it won't matter.

Option #2: Every time a new player catches the disc, I push the button to restart the 7s countdown.

 

While C1 is charged the LED will not get any current. Once it drops below a certain level the LED goes on. One can control how long that takes by choosing the values of C1 and R1. The formula is (approx) 7 seconds = 5 * C1 * R1. When you press the button, then C1 charges. Actually, reverse the order of C1 and R1 and have the lower switch go between them. That way it charges faster. The charge rate would be 0.01 second = 5 * C1 * R2. However, R2 has to be large enough to protect the LED and the JFET from too much current. Actually, just move R2 ahead of the battery so switch is between R2 and the battery.

 

A fixed a few thing in the diagram

 

A single CD4060 and 2 gates will give you an output that goes up after 7 seconds, another one you gate with the first one to get the 1/2 second pulse for the motor, and an led that blinks once per second for tuning. Pressing a reset button zeros the counter; it restarts when the button is released.

ak

 

I will come up with some thing over the weekend.

 

Below is the LTspice simulation of a 7s one-shot circuit with a 1/2s vibration at the end.
I used a CD4047 CMOS multivibrator because it is low-power and is retriggerable, as you wanted.
You shouldn't need a power switch since the current draw is only a few microamps when quiescent.
You can get the pinout numbers for the package you buy from the data sheet.
Don't forget power and ground for the 4047 which aren't shown.
The transistor driving the vibrator motor can be any logic-level (not standard) N-MOSFET device with at least a half-amp capacity.
The 500kΩ pot, U3 is to adjust the 7s delay.
I used 3 cells for power but you likely could get by with 2 cells, although the operating margin would be less as the cells discharge.
If you use two cells, make sure the MOSFET you pick fully turns on with Vgs=3V (check the Vgs used for the ON resistance value in the data sheet).

 

I Think this is a great circuit.

 

I made a few more adjustments to my idea. Battery =9V, T1=2N3819, D1=two 1N5225B effectively a 3.7V zener, C1=220uF, R1=5K, R2=0, R3=4.7K, R4=1K. I bread boarded it and it works, but these values are only approx and would need to be fine tuned.

 

Unfortunately I am now realizing there are some flaws in my design.

 

Wow... thanks for the designs.

I can tell I am a little out of my league on this, but I have a friend or two who are a bit better, so I will have to get them to help me turn these diagrams into something, but I think I have enough to get started at least.

Thanks, and I'll update you when I make some progress!

 

I was thinking along the lines of a monostable multivibrator such as the design provided by crutschow.
I was going to use both halves of the monostable. I'll get around to it some day.