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  1. #11

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    Looks like an RC timing circuit to me...

    Do you see any numbers on those two IC chips? I'm guessing one of them is 555 or 556.... the other one may be 723. I'd really have to trace the circuit and draw it out before I can be 100% certain.
    Develop, stop, fix.... wait.... where's my film?

  2. #12
    ic-racer's Avatar
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    Thanks for the pictures. I see two ICs. Perhaps IC2 is a 555 chip?

    WILD, PARTIALLY EDUCATED GUESS: one trimpot each for high and low ranges and a second trimpot to affect linearity.

    If it were mine, I'd mark each trimpot as to where it is now and then see what each one does.

    Since it is probably not quartz regulated, I'd be happy to be within 5 seconds at one minute.

  3. #13

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    I was guessing, one is for 10x multiplier switch, one is for 0.1 second min, the other is for 11.0 sec max.... As ic-racer says, linearity may be an issue... in which case, it will never be right at all setting.
    Develop, stop, fix.... wait.... where's my film?

  4. #14

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    The IC is labled NE556N so you guys know what you are speaking about.

    If I trace the circuit board, the one wire from the 10x switch is directly connected to R7, one prong of R20 is directly wired to the 1 second switch, and R10 is well, not connected directly to the .1 second switch, but to some other resistors and capacitors. Would you start with R20 that is connected to the 1 second switch? I am assuming these things rotate without harm, correct?

  5. #15

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    Oh, the smaller IC is NE555P.

  6. #16

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    Huh? Both are 555 derivatives? Both 555 and 556 are RC based timer circuit chips. The difference is, 555 has one timer, 556 has two timers both of which are identical to one 555. This is weird.

    You *can* rotate them, but it will change the timing and probably for worse. Don't do that yet.

    IC-RACER, can you think of a reason why a design like this would use THREE 555s? What the heck?
    Develop, stop, fix.... wait.... where's my film?

  7. #17

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    Quote Originally Posted by tkamiya View Post
    IC-RACER, can you think of a reason why a design like this would use THREE 555s? What the heck?
    How about this:
    One is for counting off the value of the first knob.
    The second is for the counting off the value of the second knob.
    The third is for the 1x /10x scale switch.

    I would guess the scale switch controls the speed of the signal that feeds the other two knobs with a faster or slower count (X/10 speed). That one would be the "scale" that IC Racer is talking about.

    Adjust the rate that that signal is clicking away at and you could change the linearity of the timer.
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

  8. #18

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    If you have a volt meter with a freqency counter or an oscilloscope (who has those sitting around - I wish I did) you could use them to see the rate the clock signal is running at. Or you could make a probe out of a mini-phono (1/8th in.) cable and then use a sound card on a computer and a program like Audacity to see the clock signal.

    If using your sound card, keep away from 120V/line voltage with it...
    Kirk

    For up from the ashes, up from the ashes, grow the roses of success!

  9. #19
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    The two timing adjustments are R7 and R10: one is for the x1 range and the other for the x10. I can't tell which is which from the photos supplied, but you will see one pot is connected to each side of the range switch. I take it it works OK on x1 but not on x10.

    The remaining pot is probably for adjusting the 1 second beep period.

    The reason for 3 timers: one is for timing exposure; the second for timing the one second beep period; and the third for generating the beep tone.

    The circuit design is a train-wreck. There is no transformer to generate the 12 volts (or so) that the timer circuitry works on. The 120VAC is connected to a large power dropping resistor (you can see where the circuit board has been discolored by heat from the heat of the transistor. As the circuit is directly connected to the AC line you can get a 120V shock from contact with the circuit parts. When you get shocked there is also a very good chance of blowing out the 555/6 timers. If you are going to adjust timing with the timer working then adjust the trimpots with a plastic or wooden tool - something like a Popsicle stick that has been whittled down to a square point.

    555's are very linear - adjusting the basic timing will have little to no effect on linearity (assuming the circuitry has been designed properly [possibly a bad assumption]). Linearity is set by the accuracy of the resistors connected to the two time adjustment switches.
    Last edited by Nicholas Lindan; 06-27-2010 at 04:05 PM. Click to view previous post history.
    DARKROOM AUTOMATION
    f-Stop Timers - Enlarging Meters
    http://www.darkroomautomation.com/da-main.htm

  10. #20

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    Thanks everyone for your help. To complete this thread I can confirm the following:

    R10 adjusts the 1x time (clockwise increases time)
    R7 adjusts 10x time (clockwise increases time)
    R20 adjusts audio beep increments (clockwise increases time in between the beeps)

    You have to be careful, but I am quite satisfied with the accuracy. I just used a phillips head screwdriver for adjustments.

    Again, gotta love Apug.

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