help with simple capacitor timer

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  #1  
Old 04-09-06, 08:46 PM
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help with simple capacitor timer

is it project I'm working on. and I need to make a small simple timer with a resistor (possible potentiometer) and capacitor.
is it possible to make one with out using a circuit(555 circuit)?

the project is basically to have a led turn on after "x" time has pass since the switch was closed.

i haven't messed witch electronics for years, i forgotten so much.
 
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  #2  
Old 04-09-06, 10:10 PM
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If the circuit is DC, you can use the switch to charge the capacitor via the resisistor and when charged, the voltage across the resistor goes to 0 and the voltage across the capacitor equals the supply voltage. You can use this state to trigger your LED. But you'll need more components including a reset to discharge the capacitor.
 
  #3  
Old 04-09-06, 10:29 PM
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So this well give the effect of a time delay?
what would less would i need?
thanks for the help

I also plan to use this idea to make a sonic grenade.
http://imakeprojects.com/projects/sonic-grenades/
just seems like a circuit isn't needed..
 
  #4  
Old 04-09-06, 11:36 PM
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> So this will give the effect of a time delay?

Of course. How long it takes for the capacitor to charge is roughly R1 times C1.

> what else would i need?
A transistor biased by the wire between R1 and C1.
When that voltage reaches a certain point, Q1 will begin conducting.

> I also plan to use this idea to make a sonic grenade.
> just seems like a circuit isn't needed.

You need to provide a way to shut it, to have it stop automatically, and to reset it.
High-frequency, programmable counters are much more consistent than analog circuitry.
 
  #5  
Old 04-10-06, 03:22 AM
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Originally Posted by bolide
> So this will give the effect of a time delay?

Of course. How long it takes for the capacitor to charge is roughly R1 times C1.

> what else would i need?
A transistor biased by the wire between R1 and C1.
When that voltage reaches a certain point, Q1 will begin conducting.

> I also plan to use this idea to make a sonic grenade.
> just seems like a circuit isn't needed.

You need to provide a way to shut it, to have it stop automatically, and to reset it.
High-frequency, programmable counters are much more consistent than analog circuitry.
I'm trying to avoid programmable circuitry.
why would i want it to stop? i want the thing to keep going till the battery dies or till the switch is open

thanks for the help tho. now i just gotta remember how "transistors" work.
 
  #6  
Old 04-10-06, 05:30 AM
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What you are looking for is called the "R/C time constant". It's the core circuit concept that all electronic timer circuits are based on. It can be found in most basic electronics books. It's been awhile since I built one from components.
 
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Old 04-10-06, 10:16 AM
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icechrono,

The problem that you have is that you will need _some_ circuit.

A resistor and a capacitor taken together will have a 'time constant'. You can certainly use this to time an event. However the resistor and capacitor time constant is expressed as a change in voltage versus time. To actually _use_ this time constant, you need to _measure_ the voltage somehow.

There are a zillion ways to do this. For example, if you select appropriate components, you could simply place your sound source in parallel with the capacitor, and then connect the capacitor to your battery via a resistor. When you first apply power to the circuit, then all of the current that flows through the resistor will go to the capacitor, and the noise source won't get any juice. As the capacitor charges up, the noise source will get more and more juice. Very simple. But all of the noise source current flows through the resistor, so you need a low value resistor. Then to get a long enough time you will need a very large capacitor.

A far better approach is to use a large value resistor and a small capacitor, and have the voltage in this 'timer' trigger some sort of switch. bolide presented one approach, using a small transistor as the switch.

Still, by far, the best approach is to use a programmable microcontroller. Something like the ATTINY15 costs a buck or two, and can be programmed to act as your initial timer, and then to modulate your noise source in an interesting fashion. You could probably learn such programming in a couple of days if you have any experience with computer languages.

-Jon
 
  #8  
Old 04-10-06, 10:22 AM
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i have no means to program the circuit.
 
  #9  
Old 04-10-06, 10:45 AM
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Try this site for the basic timer circuit, and the explaination of how it works.
 
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Old 04-10-06, 10:47 AM
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Originally Posted by MrRonFL
What you are looking for is called the "R/C time constant". It's the core circuit concept that all electronic timer circuits are based on.
What I was proposing is a simpler one shot timer, not an oscillator.
 
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Old 04-10-06, 11:01 AM
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Ujt

This is a job for the previously mentioned RC and a unijunction transistor. I would not feed the base of a "standard" bipolar transistor with the RC junction. Instead, feed the UJT emitter. It presents a high impedance until the "trip point". This makes the trip, consistent and predictable.
http://www.talkingelectronics.com/Pr.../UJT_Page.html

The link "time delay to SCR" shows what I'm thinking. That author claims UJT may be hard to find; not sure, but the last time I used one was 20 years ago.
 
  #12  
Old 04-10-06, 02:35 PM
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so this will work?

I havint put in any values yet.
 
  #13  
Old 04-10-06, 03:31 PM
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Why would you NOT want to use a 555 for a timing circuit? There is a bunch of literature out on the device, including the values of cap. and res. needed for specific time periods so guessing isn't involved. No programming is involved save picking the correct cap. and res. values. The device will operate in monostable or astable configurations and has a fairly robust output for driving devices. Additionally. an open collector circuit is easily added for enhanced output capacity. The newer varieties will also operate on a fairly wide range of voltages. The device tales care of all the "housekeeping" an RC circuit needs. And, last I checked they were around a buck. They are actually a pretty simple solution to your needs.
 
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Old 04-10-06, 03:49 PM
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The 555 was my first thought, but he seems adamant against an IC chip, so I tried to find a link to the kind of discrete component circuit he seems to be looking for.
 
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