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timer with 555

Discussion in 'General Electronics Discussion' started by hossimo, Jun 2, 2010.

  1. hossimo

    hossimo

    1
    0
    Jun 2, 2010
    HI,
    I came in need of a timer circuit to operate a small car fan mtor 12v dc.I need that at connection of power to cigarette lighter socket,the timer turns On the fan motor for around 2 minutes and leave it off for 5 minutes.The repeats the process without sopping unless I disconnect the supply from the cigarette socket.Any idea of a circuit diagram that I may build ?
     
  2. pdjh

    pdjh

    4
    0
    Jun 28, 2010
    This is a crude, but simple circuit that should suffice. You can increase time by using larger values for R1 & R2. Both affect off time, only R2 affects on time. Choose a low leakage capacitor for C1. Note that the initial delay before the first on period will be longer than subsequent off periods. Transistor Q1 and resistor R3 can be omitted if a relay with a maximum 80mA coil is used. The relay is then connected directly to pin 3 of U1 .

    [​IMG]
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,211
    2,695
    Jan 21, 2010
    How you allow for the leakage current of the capacitor and the input current of the 555?

    Have you suggested a CMOS 555?

    Also have you done anything to suppress voltage spikes from reaching the 555 and destroying it?

    I think the answer is no to all of these, however even answering yes to the second one would not be a cure.

    As has been pointed out in other threads, the leakage of the capacitor is likely to exceed the current available to charge it. This will lead to the situation where the capacitor will never reach the voltage required to change the state of the 555.
     
  4. pdjh

    pdjh

    4
    0
    Jun 28, 2010
    As stated, this circuit is crude, it's a minimal component count to do the job, and it does just that. I have specified a low leakage capacitor for C1, and have built working 555 timers with delays up to 10 minutes using the standard 555 part and a tantalum electrolytic for the timing capacitor.

    BTW; Steve if you're going to be a moderator you need to polish your people skills, try to be less argumentative. The circuit above was hashed together in a couple of minutes, so Instead of carping on about the circuits obvious failings, how about demonstrating some useful improvements. I'm sure the OP and others would find that far more helpful.
     
    Last edited: Jun 29, 2010
  5. NickS

    NickS

    367
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    Apr 6, 2010
    Judging from your post count you are new to the forum. If steve sounds harsh its because the topic of long delay timers using the old 555 comes up frequently enough that it could almost be counted with a 555. Many people try and fail to make these times work for long time delays. That is why steve compiled an elaborate sticky thread detailing the pitfalls of exactly what you were suggesting.

    As for the circuit protection he makes a very important point. I am currently utilizing a coil that is saturated with at a few volts then starved to produce voltage spikes in excess of 1000V. A motor is a large coil and the rules of physics cannot be overlooked when actually building the circuit. It wouldn't take nearly 1000V to destroy the rest of your circuit. If that is not your thing its ok but don't get all defensive about your design, like you said you just threw it together in a few minutes.

    Which leads into my next point, you saying that it is something that you just threw together quickly does not lend much confidence in its functionality. I am not saying your circuit can't work but steve has identified pitfalls for good reason(to help the OP not to belittle you).

    Would you care to offer us a part number for that capacitor? Perhaps you have found an gem that belongs in the timers sticky thread(but we would like to see the numbers).

    no offence intended and welcome to the forum.
    -Nick-
     
  6. Mitchekj

    Mitchekj

    288
    0
    Jan 24, 2010
    Charging 100uF with 3uA... I'm having a hard time finding a tantalum (even a 3V one) with a leakage current less than 3uA. Even then, you'd want a rated leakage current down in the nanoAmps, if you hope to have any kind of accurate timing. I don't believe such things even exist.

    The reason why people are jumping all over this, pdjh, is because it's been brought up so many times, and it's been answered the same way by many, many people... that's why I posted the link to the sticky on timers for the OP. It explains all the pitfalls. HUGE resistance, LARGE capacitance: That's always the quick answer people offer to these problems, but no one takes into account the real-world problems with such solutions.

    No one means any personal offense to you. :)
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,211
    2,695
    Jan 21, 2010
    I have no doubt that you can make a 555 timer with time delays of 10 minutes, but you can't use the formulae that are used for short durations because they ignore leakage and input current. They will also be far less stable (with temperature, voltage, and over time).

    My reply to you should be read literally. I was asking you if you had taken the leakage into account. I didn't put the figures into a normal 555 timer "calculator" to see what it theorised. Perhaps you have tried and tested these values? However -- also reading literally, I assumed that you had not. Evidence of that was that you were not specifying a specific type of 555 (or more importantly, the capacitor). The input currents vary by a significant margin, and if your charge currents are designed to have a net 3uA over the leakage of a capacitor, then the input currents will be a significant part of your calculation.

    If you had given me a dressing down, backed up by calculations of terminal charge current, ranges of leakage and input currents expected, then we would all have learned something about long period 555 timer circuits. However, you didn't.

    I will try to find one of my earliest posts concerning protection of electronic circuits when powered from a car battery. The original poster may find it useful. If anyone beats me to it -- thanks :)
     
  8. pdjh

    pdjh

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    Jun 28, 2010
    Adios

    Hossimo asked for a circuit 3 months ago. Mitchekj at least posted a usful link, but where were the rest of you? No circuit in sight, and no wonder the poor guy never posted again. You lost a member there, and now two.

    I remember when I first got interested in electronics back in the mid eighties. One of the first 'projects' I built used a 555 timer, a coutesy light delay for my old Ford Zephyr. I knew nothing of leakage currents or voltage spikes back then, but with the help of Walter Jungs's excellent 'IC timer Cookbook' I was able to put together a simple circuit to do the job. It was a library book, but I seem to still have it(!) and still refer to it on occasion. Anyway I digress, the point is that circuit was very similar to the one here, and though the delay was only 3 minutes, it used a old (scavenged) aluminium electrolytic timing capacitor, and no regulator or over voltage protection. That little circuit continued to function flawlessly until I sold the car a few years later.

    Now having finally come out from hiding, you guys raise some valid and some not so valid points above, but at the same time you're missing the point. It's not a timebase for some precision instrument, it's to turn a fan on and off! But I have no wish to argue these points because this is not about electronics, it's about attitude. Read Steves original response again, can you really not see my point?

    I am not being defensive, neither am I offended, and if you want to run that kind of forum where everyone is spoiling for an argument, then good luck to you, many people enjoy that, and there's nothing wrong with it, but really it's not for me.

    I wish you all well.
     
    Last edited: Jun 30, 2010
  9. NickS

    NickS

    367
    0
    Apr 6, 2010
    I wouldn't say we like to argue so much as we dislike seeing the propagation or bad design principles.

    By the way where do you live that counts 28 days as 3 months? ...oops there I go clouding the issue again, never mind.
     
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