Help with a push button timing system - The Planted Tank Forum
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post #1 of 26 (permalink) Old 01-02-2008, 03:52 AM Thread Starter
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Help with a push button timing system

Kind of a weird electrical engineering kind of question here. I'm looking to have a switch that when pushed, counts a timer off for 30 seconds, then triggers an LED. Each subsequent push of the button would increment an additional 30 seconds. I've been thinking of using the NE555 timer chip, but didn't know if cheaper solutions exist, catch is, it needs to be tiny. Like AA batter tiny in size. Thanks in advance.

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post #2 of 26 (permalink) Old 01-02-2008, 06:54 AM
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What do you mean by "triggers an LED"? So the LED is off for 30 seconds, then it turns on? For how long? What happens, exactly, with subsequent pushes? What's the supply voltage?
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post #3 of 26 (permalink) Old 01-02-2008, 07:24 AM
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may i know what this is for ?
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post #4 of 26 (permalink) Old 01-02-2008, 01:14 PM Thread Starter
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By triggers an LED, you are correct in what i was asking you. It's off after the switch is pushed initially, then 30 seconds later, it comes on. Subsequent pushes add 30 seconds to whatever the current timer count is. Ideally, they would all be pushed at the beginning of the sequence. I.e. Push, push, push = 1 minute 30 seconds. As far as the voltage goes, I haven't decided yet, it will be low though, not wall powered if that's where you're going with that.

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post #5 of 26 (permalink) Old 01-02-2008, 05:03 PM
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Subsequent pushes add 30 seconds to whatever the current timer count is.
To what maximum? You'll need to specify this - some storage element will need to store it, so it can't be infinite; i.e., by necessity, you'll get to some point where subsequent button pushes just reset the delay to this maximum, but doesn't go beyond it.
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post #6 of 26 (permalink) Old 01-02-2008, 05:29 PM Thread Starter
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ah gotcha. 5 is max. are we talking storing some amount of charge in a capacitor? If thats the case, i could always scale that up or down.

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post #7 of 26 (permalink) Old 01-03-2008, 01:16 AM
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Quote:
Originally Posted by bgoodwins View Post
By triggers an LED, you are correct in what i was asking you. It's off after the switch is pushed initially, then 30 seconds later, it comes on. Subsequent pushes add 30 seconds to whatever the current timer count is. Ideally, they would all be pushed at the beginning of the sequence. I.e. Push, push, push = 1 minute 30 seconds. As far as the voltage goes, I haven't decided yet, it will be low though, not wall powered if that's where you're going with that.
Sounds like a good job for a PIC microprocessor, although it may be more involved than what you are hoping for.

Do a web search on 'Microchip' or PIC.

I can't think right off how you would do it with just capacitors. You are going to need some type of digital circuitry, and as I mentally guestimate what it would take the PIC looks more inviting and more compact!
[Edit] the 16F84 PIC is available in an 8 pin DIP package---about 1/4 inch square. Total parts count: 4, not counting battery (or programming circuit)

Jim

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post #8 of 26 (permalink) Old 01-03-2008, 01:53 AM Thread Starter
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I've got a minor in CS, so I'm not afraid to delve into a bit of programming. However, I'd like to keep it as simple as possible as far as parts cost. I think the capacitor option sounds inviting. For instance: Push a switch, X amount is stored in a cap, push it again, you get X amount again, and so on. It will take a measured amount of time for the energy to dissipate and as soon as the energy is gone, that then could trigger the LED. It seems as though i could almost do away with the NE555. Thoughts?

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post #9 of 26 (permalink) Old 01-03-2008, 01:36 PM
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Push a switch, X amount is stored in a cap, push it again, you get X amount again, and so on.
The problem is that the voltage on the capacitor is an expontential function (v = e^(t/rc) where r is the circuit resistance, c is the capacitance, and t is time). Each time you push the button you will have to dump a different value of charge into the cap in order to get the equal length time periods.

Interesting problem, though. I will have to think about this one. Its just that you wanted something compact. The most compact thing I can think of is a 16F84 PIC, a push button, an LED with resistor, and a couple of AA batteries (and maybe a couple of small caps for the 16F84 oscillator).

My major, by the way is EE. Tried CS but kept getting closed out of required classes because I wasn't a CS student!

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post #10 of 26 (permalink) Old 01-03-2008, 02:33 PM
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Would it be possible to use a digital kitchen timer instead? The button presses would be different, and you'd have to figure out whether it was possible to tap into the circuitry to get a signal to trigger the led.
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post #11 of 26 (permalink) Old 01-03-2008, 03:42 PM Thread Starter
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The controlling factor on just using something larger is the size this thing needs to be. Small. Think the length and size of a single AA or AAA battery.

Jbolinger - It's been a while since i had physics II, i totally forgot that cap charge isn't linear :/ Any non PIC solutions come to mind?

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post #12 of 26 (permalink) Old 01-03-2008, 03:57 PM
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The controlling factor on just using something larger is the size this thing needs to be. Small. Think the length and size of a single AA or AAA battery.
Hope you're good with a soldering iron. Damn, that's some tight space requirements.
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post #13 of 26 (permalink) Old 01-03-2008, 04:02 PM Thread Starter
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I'm alright, first step is finding a circuit that works though, then minimizing it is step 2.

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post #14 of 26 (permalink) Old 01-03-2008, 07:50 PM
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Originally Posted by bgoodwins View Post
The controlling factor on just using something larger is the size this thing needs to be. Small. Think the length and size of a single AA or AAA battery.

Jbolinger - It's been a while since i had physics II, i totally forgot that cap charge isn't linear :/ Any non PIC solutions come to mind?
Well, yes. You could probably do it with 6 or 7 14pin or 16pin integrated circuits, but it would be much larger than what you want. Unless, of course, you want to try soldering some surface mount parts .

What's this for anyway? How does it apply to a planted aquarium?

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post #15 of 26 (permalink) Old 01-03-2008, 08:56 PM Thread Starter
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its going to be used for part of a manual fert dosing out of a peristaltic pump. I would like to be able to do my dosing based off of 30 second increments, and space under my stand is at a premium. I'm basically trying to get it all into a single package instead of exploding all over the stand. So you feel like the PIC is the only way to go here?

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