Photocell and 1458 circuit

Discussion in 'The Projects Forum' started by bigt78130, May 18, 2009.

  1. bigt78130

    Thread Starter New Member

    Apr 15, 2009
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    Hey guys,

    I'm new to electronics and im basically trying to teach myself. I have a LM339 voltage comparator hooked up to a photocell and when it is hooked up to and LED for the output, The LED works fine. However, when i take the output and hook it up to a 1458 IC chip (this makes the LED fade on and off) the LED just rapidly blinks. What would my problem be?

    The 1458 Chip works fine when hooked up to a 9v battery.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The 1458 is a dual op amp - and a rather old design. Perhaps you could post up the circuits so we had something to look at.
     
  3. bigt78130

    Thread Starter New Member

    Apr 15, 2009
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  4. DonQ

    Active Member

    May 6, 2009
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    A couple of things that might have an effect:

    22μF is almost certainly a polarized capacitor (it is only meant to be used with the voltage polarity as marked on the package). Not the best choice for the location in the circuit where you are using it.

    Especially if you are using a 9V battery (and old technology amps, and saturating one of the outputs), you want to use a capacitor on the power supply. Something like a polarized 22μF would be a good choice. Got any of those?
     
  5. bigt78130

    Thread Starter New Member

    Apr 15, 2009
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    yeah i have a couple of polarized 22uf capacitors. but the problem is do i need to hook it up before or after the voltage comparator?

    On the LM339, the Cathode and anode of the LED are where i am hooking up the positive and negative of the 1458 circuit. I know i probably have all of you confused but im trying to figure this out
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    I'm a bit confused because these seem to be two totally unrelated circuits. The 1458 appears to be an Astable Flip-Flop, which is used to drive the 2N3904 and flash those two LEDs.
     
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I think the OP is wrongly trying to use the photo-transistor and comparator circuit to turn the oscillator/fader circuit on and off.
     
  8. DonQ

    Active Member

    May 6, 2009
    320
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    The added capacitor that I mean will be in parallel with the battery, with the + end of the cap hooked to the + end of the battery, and the same with the - side. It can be placed almost anywhere on the board.

    It could also help to hook something about 0.01μF to 0.1μF the same way. These capacitors are because a 9V battery is a fairly high resistance power source. This can make any switching circuit not work correctly unless the power supply has bypass capacitors like I am describing.

    The other capacitor that is already between the 1 and 2 pins can not supply 22μF in both the positive and negative directions. A polarized capacitor means that it only works in one direction. Some just act like a resistor in the reverse direction. Others may be different. You might be able to fix this problem by hooking 2 caps together, back to back. This means... hook the positive ends together and use the two negative ends like it was only one capacitor. Some polarized caps will let you do this and will still have 22μF, but now it will be in both directions.

    Otherwise, you need to redesign the circuit to use a non-polarized capacitor. Non-polarized with more than about 1μF are rare, so you would have to change the time-constant resistor also.

    Only after you do this can you tell if there is something else wrong with the circuit, but it looks like a standard circuit.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    This is how Bill Bowden connects two polarized capacitors back-to-back to make a non-polarized capacitor. The 47k resistor in the previous circuit is increased to 100k because the series-connected capacitors are half the value of one.

    I don't know why he uses half of an LM324 quad opamp instead of the MC1458 dual opamp.
    There are two sets of fading LEDs, one dims while the other brightens.
     
  10. DonQ

    Active Member

    May 6, 2009
    320
    11

    Ouch... I knew that! But I guess I don't get any points if I don't say it!:D
     
  11. Darren Holdstock

    Active Member

    Feb 10, 2009
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    Are they always though? I'm not sure. This would certainly be true for ideal capacitors, but aluminium electrolytics go a bit short-circuit when reverse biased, which is how I thought back-to-back electrolytics worked. There is only a small voltage drop on the reverse biased electrolytic (the cap is not destroyed as long as the current is not too high), so surely the equivalent capacitance of two identical back-to-back electrolytics is the value of one cap? I should do some experiments one day and find out.

    Here's another curious one - there are plenty of working split-rail-supply amplifier circuits out there with a single polarised electrolytic coupling the feedback network to ground. With any AC output centred around 0V, the cap is going to be reverse-biased half the time. What are the consequences of this for the amplifier performance?
     
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