Hi, I'm a student trying to build a timing circuit that after a switch is closed counts for 2 mins, beeps(piezo or to a female mono jack connector), counts for another minute then beeps, counts for two minutes then beeps, counts for another mintue then beeps, then another two minutes then beeps again and turns off. Then does it again when switch is closed. Ideally having green LEDs lit when counting for the two minutes and red LEDs when counting for one minute.

I know I haven't explained this very clear so something like:

switch closed.
2min count GREEN
Beep
1min count RED
beep
2min count GREEN
beep
1 min count RED
beep
2min count GREEN
beep
then turns off.

Any help from someone with more electronics experience than myself would be a great help.

This is an example of a state machine. If I was building it, I'd use:

• A clock generator (e.g. 555-based)
  
• A clock divider (to get a 1 minute clock; it's tricky to build accurate, slow clocks with 555-based parts)
  
• Some J/K flip-flops to implement the state machine
  
• A monostable multivibrator to generate the beeps.

Your university will have books which discuss implementing state machines in hardware.

Alternatively, just use a microcontroller (e.g. a Microchip PIC part). This will have the advantages of lower cost (a microcontroller capable of doing what you need can be obtained for pennies), lower circuit complexity, and lower part count.

Hope that helps.

-------------------------
Satellite Remote Sensing Research Group, Surrey Space Centre
Open Source EDA with gEDA

I would recommend Microchip PIC's as the best solution.
They have a huge range of potetially suitable components and a large amount of support data on their website.
Check out www.microship.com.
There is information on LED lighting, I/O control with switches, sample code and other support available. One of their low-end development kits would be ideal as a base for your design - in the past I have donated dev kits that are no longer required to my local college, see if any of the companies in your area do the same thing, womeone is bound to have a kit they could donate.
The PIC as good as most newer ones have LED current spource / sink capability which will simplify the circuit required.

-------------------------
Rory Speirs

I agree with the previous post. I would use a PIC. They come in all shapes and sizes e.g 6-pin SOT23 SMT. The benefit of using the midrange PIC's (18F) is the free C18 compiler with libraries for delay's already for you! If power consumption is a problem, these PIC's can run off a coin cell and have sleep functions.

I agree, a PIC, or a PAL (GAL, PLA), is the best solution for you. Any of these solutions will enable you to modify your code without having to rebuild the electronics. MicroChip sell a PIC Start kit and software. Worth the money from RS, or Farnell.

Steve Clemmet

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Stephen Clemmet CEng MIET

Using the 555 approach, it might be better linking up the 555 astable circuit to a 4017 decade counter. If the astable sends a pulse signal to the 4017 every minute, it is fairly simple to link up the LEDs and speakers to the different outputs, so that they light up at the right time. Then at the final output, a thyristor latch can be opened to bypass the two chips and effectively "turn off" the circuit. The PIC microcontroller will work as well and is probably a better solution.

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First year undergraduate studying Electronic and Electical Engineering (MEng)

The 555 has three operating modes:

* Monostable mode: in this mode, the 555 functions as a "one-shot". Applications include timers, missing pulse detection, bouncefree switches, touch switches, frequency divider, capacitance measurement, pulse-width modulation (PWM) etc
* Astable - free running mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation, etc.
* Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc.

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You will initially find the PIC route is harder. but even if you only get part way through it, your project should get better marks.

No one in industry would use a 555 and board full of logic to realise your project. Use this to justify your approach. - but be careful lecturers are often out of date.

I don't know how much you already know but Nigel Godwins PIC tutorials is a good start - google it.

Hi,

I know it's a little late - but you may want to investigate the 14541 timer IC.

I can think of a few safety critical applications that this device is used in, where as I can think of none that use a 555. Maybe that says something about the quality?

But judging by what you need, a PIC or AVR would be much easier. Take something like a PIC509 or Tiny AVR with 8 pins and use internal clock. A few lines of C code and you will have a one chip solution.

Cheers,