Need some help creating a timer circuit

[TBennettcc]

I need some help building a timer circuit. I've tried to design one using at least three 555 timers, but I just can't get my head wrapped around what the circuit needs to do.

Here are the functional requirements:

I have a NO SPST switch (+12VDC closed, 0V open) acting as the input. As soon as the switch closes, I need the timer to start. Timer needs to be approximately 3 minutes, but length of time is not critical. After the three minutes is up, I need the circuit to momentarily open an electrical equivalent of a NC momentary switch (10 seconds or so). The circuit should not reset with any further input from the NO switch, and the countdown should not alter once it has started, other than an input from a reset switch. This reset switch should be able to cancel the countdown without opening the (electrical replacement of the) NC momentary switch. Would it be possible to make this 'reset' switch a reed switch?

I tried to have a 555 timer do this, with two 555 timers (one on either side) to invert the input and output (as a 555 needs a low pulse for a trigger, and I have a high pulse of +12VDC). I tried to invert the output because I figured you would need to hold the base of (an NPN) transistor high to make it normally closed, and then lower the base voltage to 'open' the transistor. (If I understand transistors correctly, you could use a PNP transistor without inverting the input, because PNP transistors need the base low to complete the circuit, and high to open? Is that right?)

So far, I think this is my best bet, but I'm not sure how to translate it into what I need:

https://www.electronicspoint.com/555-used-delayed-pulse-generator-t7899.html

and

http://www.terrypin.dial.pipex.com/Images/10sMono+Pulse.gif

If there are any other options out there that are better, please don't hesitate to mention them. However, I think I may stick with analog circuitry, if it's possible to do so. I don't have any experience with microcontrollers, but I'm sure one would work fine in this scenario.

Any and all help is much appreciated.

Thank you very much for your time.

 

[daddles]

If I was faced with this, I'd head immediately to using a microcontroller because a) someone gave me some development tools and b) the whole thing could be done would a 50 cent part with some batteries and resistors (besides the other stuff). Most importantly, the design's objectives almost always change when one gets into the project; these changes are usually pretty trivial to handle when you use a microcontroller with extra pins that can be used as digital I/O or analog inputs.

If you're interested in this route, look up a post I made for poor mystic a while back about doing development work with PIC tools, as I had a friend visit last February and he bought a bunch of PIC development stuff for us to play with and he left it all here.

 

[TBennettcc]

I did happen to see that post as I was perusing the forums. IIRC,

The two main kits that I recommend from Microchip are: 1) the Low Parts Count Demo board and 2) the PICDEM Lab.

I don't know anything about microcontrollers. How do they work? I think you have to have some kind of a special jig to program them? But then they are non-volatile, correct? They don't need to be powered to keep whatever was programmed on them?

What kind of programming experience do you have? I spent most of my time in college programming in Java, with one or two semesters in C++, and one semester of Assembly. How difficult was it for you to program? Any difficulties / hiccups / things about doing it that particularly stood out to you? I might order one of those kits soon to mess around, as I'm sure it would be a good learning experience, not to mention fun. (First, I've gotta finish saving up for my new ham radio antenna, though. Then I get to contest on the weekends.)

Let's say this is a friend's project, and he's not really interested in 'learning' about microcontrollers, but just wanted to get this project done. Would something like the PicAxe kit be more what's recommended? Or maybe even some of the cheap Atmel chips? I know that some microcontrollers are dirt cheap. Are they easy to program? Do they require 'special' programming tools, jigs, etc., or could anybody make something to program them?

Thank you very much for your time.

 

[poor mystic]

I thoroughly agree with daddles and while my Arduino hasn't yet arrived I expect it to be easy to use and to be tidy and professional in appearance. These are important considerations for me since I hope to have some robotics out there earning money one day soon and I plan use a PIC to power it.

 

[TBennettcc]

You say 'PIC'. Do you actually mean a 'PIC' brand from Microchip, or do you use the term loosely, to refer to any microcontroller?

 

[poor mystic]

I used the term very loosely, not realising that it was a brand name. From now on I will use the term microcontroller or uC, which is easier on the typist.

 

[(*steve*)]

Also take a look at PICaxe. Easier for people who are perhaps less involved in writing software.

 

[MagicMatt]

Once I started using PICs (so far only the PIC16F690) I've all but deserted trying to do things like TBennett mentioned using seperate logic/555 etc. The flexibility you get opens up massively, and the cost of a uC means it's often cheaper in the process.

Never seen the arduino - my kit is a PICKit2 that was cheap and runs over the USB. I'm using the free MPLAB alongside it. You can play with MPLAB etc. to simulate running the program without even getting a PIC, should you want to experiment first. I can't compare that to any of the other systems as I've only used this one.

 

[donkey]

hey tbennet from what i read on microcontrollers(i did some research for a robot i am making for fun) they run on c. there may be one that use BASIC too. it is a side hobby, and as yet i haven't played too much with them. thought i would offer some info rather than asking for it lol

 

[(*steve*)]

there may be one that use BASIC too

Picaxe is one. There are also a plethora of compilers out there if you go searching.

It is probably best to stick to mainstream products (especially if they are free and supported by the chip manufacturer)