Need help on a timer circuit

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myp71
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Need help on a timer circuit

Post by myp71 » Thu Apr 08, 2004 12:46 pm

Hi <p>I need your help on a timer circuit this timer circuit will be used for controlling a 12vdc aprox 350 mA solar powered fan I would like the fan to come on when my truck's bumper presses up against the switch the timer starts counting a couple of hours then turns off the fan until the truck resets the switch again by leaving the garage and coming back.<p>Thanks for your help again :) <p>(Ryan)

perfectbite
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Re: Need help on a timer circuit

Post by perfectbite » Fri Apr 09, 2004 10:08 am

From my very limited understanding of all of the 555 chip's applications, if you mix and match the 555's timing cap and resistor you'd have a very low power timer device/switch that, using a transistor to allow power to the relay's coil, could trip either a mechanical relay (the relay contacts should easily handle the 300mA load) or solid state switch (a saturated TIP???) when the 555 switches to its alternate state. Another 555 chip (556) could shut everything down (saving power) after the fan has run long enough.

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haklesup
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Re: Need help on a timer circuit

Post by haklesup » Fri Apr 09, 2004 12:40 pm

Using a 555 for a period of hours can work but you need to use a very large R and C and the time period may not be exactly repeatable each time. Big value resistors are cheap but larger value caps can cost more than you would expect. <p>A better way is to set the 555 up for a 1 second period and use a few counters to divide that count to the number of minutes you really want. YOu will get much more precise and repeatable time periods this way.<p>Use CMOS chips (4018, 4029 etc) that accept variable supply voltages so you can power it off the same solar cell. If you use bipolar(74xx), you will need a 5V regulator to ensure proper operation. Look in the specs to know the typical supply current for the chips you plan to use, you don't want to rob too much current from what you need to power the fan.<p>There are also many other ways of making a 1 second time pulse, RC timing is inherently inaccurate (especially as temperature changes), but it dosen't sound like you need exact timing anyway.<p>Since the fan is solar powered there is no battery power to preserve or house current to conserve, and if it is relatively quiet, might as well let it run all day unless this promotes too much temperature change in your garage.

myp71
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Re: Need help on a timer circuit

Post by myp71 » Fri Apr 09, 2004 5:05 pm

Would anyone happen to have a circuit digram for this? I'm not that far along with electronics yet I know I said I need help but I guess I kinda mean all out full blown circuit digram I thought this would be a better way of wording it for a change.
But I do appreciate any and all help I get so thanks a lot!<p>Also I should have said that solar charges the battery then the fan runs off of battery power and that's why I would need that timer circuit.<p>I tried running the fan 24/7 but the battery couldn't keep up when the solar panels where dark so now I just run the fan for a couple of hours and that seems to be working great for now. <p>
Thanks again
(Ryan)

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Re: Need help on a timer circuit

Post by perfectbite » Sun Apr 11, 2004 3:59 pm

MyP71, here is very useful 555 tutorial page (at least I thought it was useful). http://www.uoguelph.ca/~antoon/gadgets/555/555.html
As far as conecting the variable voltage supply CMOS devices versus the bipolar devices (providing that reulated 5V for them would be a lot more work) for more accurate timing you'll have to ask haklesup. I am barely past comprehension of the 555's operation. I know what he intends but can't tell you how to do it.

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Edd
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Re: Need help on a timer circuit

Post by Edd » Sun Apr 11, 2004 11:45 pm

Since you potentially might be utilizing this basic 555 circuitry in a building block of a slightly more involved time scaling/divider circuit.
Why not initially try this circuit, since its nothing in complexity. Plus it sounds as if you are not requiring chronometer repeatable accuracy. Its using the flea power C-MOS version of the 555 with its resultant higher Z. Initially try a quality low leakage 100ufd, on upwards to 330ufd tantalum timing capacitor in place of the specified 33ufd unit for C1. Naturally, there will be a functioning point where the further increase of that cap value will not be tolerated, hopefully it will not be before your max timeout period has been established.<p>http://www.mitedu.freeserve.co.uk/Circu ... 0timer.htm<p>BTW, you have all of our curiosities piqued….whats the functionality of a (350 ma) small? solar powered…cooling?...fan… in a hot?...Vas Legas…garage?<p>73's de Edd
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myp71
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Re: Need help on a timer circuit

Post by myp71 » Mon Apr 12, 2004 12:14 am

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>BTW, you have all of our curiosities piqued….whats the functionality of a (350 ma) small? solar powered…cooling?...fan… in a hot?...Vas Legas…garage?<hr></blockquote><p>
First off thanks for the help for the circuit second this is a single car garage and this fan moves a lot of air for its size also I have been using it and it works great in cooling this area after 6-8 hours of use.Also it is something I can call that I use everyday that is powered by solar.
Now in the summer that's a different story the fan helps a little bit but not like in the winter
like now.<p>Thanks for the help.

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haklesup
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Re: Need help on a timer circuit

Post by haklesup » Tue Apr 13, 2004 12:58 pm

If you did not have the battery, changing light conditions which result in changing operating voltage that occur during the long period time cycle would have the effect of streaching and squeezing the pulse. The end of the pulse is triggered when output reaches a certain threshold, but if that is changing randomly, each pulse will trigger at a different point making the timing very unpredictable. A shorter time interval will reduce the error since the lighting conditions cannot change that fast. A battery eliminates this error by stabalixing the supply voltage. <p>I suggest you have a series diode too to prevent reverse leakage through the solar panel at night or whenever the cell voltage drops below the battery voltage (this will further preserve battery charge)<p>I do agree with Edd though that starting out with a single stage 555 circuit with a long delay is worth a try since the required time period is probably not precise. Here is a link I found with the search term 555 Calculator you can plug in values for components you have and see the timing results.<p>You can also get a time delay relay and do it all off the shelf with one part. Here is a partial listing from the Tycho Website<p>My preference for the CMOS 555 was for the low static operating current not necessarily the wide operating voltage range.<p>One last thought, the relay used to switch the fan is probably consuming more current than is necessary, consider replacing with a FET, transistor or other solid state switch to further reduce load on the battery/solar cell. The less power you waste in the circuit, the more you have to drive the fan longer.

myp71
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Re: Need help on a timer circuit

Post by myp71 » Tue Apr 13, 2004 1:39 pm

All right gonna make it hard for me huh? No its ok :D <p>I was wondering is this circuit going to be able to do a trigger pulse because when the truck is in the garage the switch will be active all of the time so the circuit will never count down unless the switch is off and that won't happen unless the truck is gone you know what I mean ? <p>Thanks for the help

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haklesup
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Re: Need help on a timer circuit

Post by haklesup » Tue Apr 13, 2004 4:46 pm

Ahhh, the thing I hate most about 555 chips. The trigger input is level sensitive not edge triggered.<p>If you want to stick with the 555, the traditional solution is a circuit sometimes called a "glitcher" because it creates a glitch (duh). It is basically a n.o. pushbutton switch with one pin tied to ground and the other side of the switch has a capacitor (~.001uF) in series to the trigger pin which is also pulled up through a resistor (~10k) to Vdd. The trigger is normally held high but when the button is pushed, it is momentarily pulled low until the cap can recharge. (someone correct me if I got it a little wrong)<p>Or you can get yet another chip like a flip flop that IS edge triggerable and make the 1 shot out of that. This needs to be done with CMOS (long explanation but it has to do with the VIL/VIH and the way the output can drive current in both directions).Im a little rough on this one but I'll give it a try so Ill leave it to this link.D flip flop One shot What I had in mind is the right half of these two circuits. Both these circuits feature a positive going version of the glitch circuit I described above (cap switch to Vcc and input pulled down thru R)<p>The FF based one shot circuits should do the same thing as the 555 based circuit with the exception that timing will not be 1.1RC but some other constant times RC but who cares for a rough delay.

myp71
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Re: Need help on a timer circuit

Post by myp71 » Thu Apr 15, 2004 11:15 pm

Thanks for the info.<p>Would it be better to just have push button switch that i can hit to turn on the fan with the timer instead of trying to get a switch to work on the truck? If I just use a switch I would still use the same timer right? <p>Thanks again for the help

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haklesup
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Re: Need help on a timer circuit

Post by haklesup » Fri Apr 16, 2004 2:35 pm

I see no difference between a switch pushed by your bumber or your finger. <p>Is this because the bumper option does not seem to be a momentary switch and as a doorbell button would be? A little mechanical design creativity can fix that.<p>For a 555 circuit, as long as your button push (trigger pulse) is shorter than the period of the one shot, it will not retrigger and you will only get the one shot delay.<p>Instead of a plain n.o. switch with some sort of rod your bumber pushes consider a PIR or a pneumatic switch, like those black rubber hoses you drive over at gas stations to make the bell ring. Here is a small switch designed for toys but should work well. pnuematic switch others are available,just search the web<p>Another switch. Two tall floppy springs stick up from the floor each is one pole of the switch, when the metal bumper touches them as it drives by, it completes the circuit. Position them so that they are under the truck and not touching when you are at your usual parking position.<p>Another switch. A tennis ball hangs from the rafters by a metal cable which passes through a metal ring near the top. When you drive in, the ball is moved so that the cable touches the ring and the circuit is completed. Position the ball so that it hangs free again once you reach the normal parking position.

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