7555 timer - which is more 'stable' or better?

Discussion in 'General Electronics Chat' started by greg123, Mar 25, 2008.

  1. greg123

    Thread Starter Member

    Mar 23, 2008
    13
    0
    Hi,

    I'm getting some help on another forum but getting some conflicting advice.

    I need the attached circuit and have lined up an 18v 7555 and most of the bits for it. The circuit will constantly be supplied by car +12 (relatively unsmoothed DC from 12 to 14.4v, 16v max to be on safe side) and the trigger side will be supplied from the same source. I want the circuit to produce it's output once only, when I cut the trigger supply (circuit will remain powered throughout).

    But... someone else suggested I shouldn't use the values suggested in the attached and instead use a 10 meg pot + 1 meg fixed and a much smaller cap of 5uf.

    I don't know enough to work out if/why this is 'better' or more 'stable'

    I'm not worried about the out of the box time, that's what the trimmer is for. So long as it can go from 1 to 60 sec give or take 5 sec and once trimmed to the correct time can be left for a couple of years and not require a trimmer adjustment that's all I need. If it counts, the longest the circuit would be powered is a few hours, before being shutdown overnight (used in a car).

    Comments appreciated as I really need to fix on the componants and get them ordered, I'm buying good quality stuff too so best if I can just spend on what I need, though the resistors and caps are not expensive.

    Thanks, Greg.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Using a larger R and a smaller C allows the circuit to draw less current. Less current means less power, less waste heat, and more stability.
     
  3. greg123

    Thread Starter Member

    Mar 23, 2008
    13
    0
    Thanks, appreciate that.

    Okay I have re-done the circuit and chosen differnt C/R values and put the type of componants on the diagram - maybe you guys would be kind enough to look over for comments as to the circuit and my choice of componants.

    The only thing, if the circuit looks okay, is that I can't find tant caps for C1 & C3 (1 & 10nf), should I use different values in Tant or go for a different/inferior type of cap?

    Comments appreciated, Greg.

     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    There's not much point in using 1% tolerance resistors - unless you happen to have them lying around.

    The capacitor values you've chosen are standard values. You should have no trouble finding a 47uF tantalum cap.

    10 M-Ohm potentiometers are available new for less than five dollars (US) each. You could use a 4.7uF cap.
     
  5. greg123

    Thread Starter Member

    Mar 23, 2008
    13
    0
    Hi thing,

    Thanks for that. The resistors are 0.05 each so I just figured they would be pleanty good enough? I can save 2p getting cheaper ones if it mattered lol.

    The 47uF no problem - the tantalum ones I was struggling with was the C1 & C3, the 1nF and 10nF values - is it normaly to have such small values for those two caps? If yes, what can I use instead of tantalum which is going to be stable over time (values not degrade with age as the circuit will probably be in use for a few years).

    Regards and thanks,
    Greg.

     
  6. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    Electrolytes, especially the larger ones won't be very stable in an environment where the temperatures, depending on where you live/drive, can cycle from far-far below freezing to around +80°C when parked in strong sunlight.

    Another, just as important, fact about cars is, that they are electrically noisy, which makes it a bad idea to use too high resistor values.

    In a car, the difference in consumption between a 1k and a 10M resistor is nothing to worry about, when even the smallest lamp in the instrument cluster is less than 200 Ohm.


    If you need precision AND long time intervals, use the 555 as an AMV with low temperature coefficient components, driving a divider like eg. a 4040 for an easy output of 1 pulse each 2048 pulses input. Or better, use a 4060 and ditch the 555. Or best, use a µcontroller - even a tiny PIC10F2xxx, the size of an SMD transistor will outbest all of the above and be able to pull some other stunts for good measure.
     
  7. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    AMV equals Astable-MultiVibrator.

    hgmjr
     
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