Trip 5 IC anomaly

Discussion in 'The Projects Forum' started by rc3po, Feb 12, 2014.

  1. rc3po

    Thread Starter Member

    Feb 12, 2014
    56
    1
    I have found an interesting anomaly with the 555 IC. Everywhere I’ve read and looked, the #4 (Reset) Pin has voltage held high to allow the IC to conduct, and dropped low to turn off (Reset) the IC.
    I have 8.58V at 39.5mA at Vcc ( #8) Pin, when the IC conducts. As you can see in the schematic, I am controlling the IC through the Reset Pin ( #4), with a CdS Photocell Photoresistor. When light is removed from the CdS Photocell and the IC conducts, it sends 6.5V at 18.2mA to the Output ( #3) Pin & LED’s. Apply light to the CdS Photocell and the IC stops conducting and you get -0.2V at 0.0mA at the Output ( #3) Pin. Since I added a 4.7uF electrolytic capacitor to the Output Pin, the IC now conducts 7V to the LED’s.
    I am an Electronics Hobbyist who has been learning electronics for 1 year. I welcome any comments and ideas anyone has about what is going on with the IC. Thanks.
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
    2,339
    1,022
    With no light on the CDS its resistance is high enough so that it does not pull the required 100ua Typ. from the RESET terminal to reset the 555, so it runs in the astable mode. Light on the CDS drops its resistance enough to pull current out of RESET and it stops the oscillation with the output low. If you look at the schematic and Fig. 1 of the datasheet, it shows RESET as the base of a PNP transistor which is current operated.

    The cap on the output is integrating the oscillating output to give you a net higher voltage.

    Welcome to AAC!
     
  3. rc3po

    Thread Starter Member

    Feb 12, 2014
    56
    1
    Thanks JohnInTx,
    I'm also from Tx.
    Well if it's so simple, why have I not seen this in any books? Also, one of the biggest gripes I've read about the Trip5 is it's current loss when "off", which makes it a poor choice in battery operated circuits. The way I'm controlling it, it has zero current loss when off. I was thinking about devising a way to control the chip with an opto-coupler. lol, I guess I should just move onto micro-controllers...
     
  4. BC107C

    New Member

    Apr 3, 2011
    14
    1
    To me the schematic is weird not only in the resetting part, but also in how the control, discharge and LEDs are hooked up.

    Capacitor on the output -not advised unless you really need to achieve something specific with that cap hooked up there.

    On the reset you need a different circuit to give you the needed voltage and current levels required for the function.

    Can you post the original schematic that led you to this one?
     
  5. JohnInTX

    Moderator

    Jun 26, 2012
    2,339
    1,022
    Don't know why. I just read the datasheets. Keep in mind that the 555 is an OLD design. Its really an analog circuit that happens to have a flip flop in it. Maybe a CMOS 555 would be better. As for using uControllers instead of finicky 555s , that's what I do.

    Gotta go, there's a huge, terrifying ball of light and fire over Dallas.. its ITS - whew, its only the sun. Ready for spring.
     
  6. rc3po

    Thread Starter Member

    Feb 12, 2014
    56
    1
    There isn't one - I wrote this schematic representing the circuit on my breadboard.
    I only put the e-cap in the circuit to see what it would do. I've been using the circuit as a night-lite for several months, without the e-cap on the output.
    To be honest, I haven't studied circuit design and stuff much. I've spent most of this year learning about TV, Computer, and Electronics repair because I love to fix stuff.
    I just couldn't find anything like this anywhere - books, u-tube, etc...
    I was planning to get into micro-controllers in a month or so anyway - maybe Arduino.
     
  7. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,281
    1,232
    Just a couple of observations:

    The 4.7Nf from threshold and the 100Nf on control should go to ground not the 180 ohm resistor.
    It's hard to say without the specs on your ldr but you might need a pull up to 9 volts on the reset pin. You would have to experiment.... Maybe 220k.
    It's not usually a good idea to run LEDs in parallel. Especially 2 different colors. You might want to at least give the red ones and yellow ones their own resistors.
    Moving the cap from the output and putting it from +9 to ground might be a better use for it.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,515
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    If your intent is a standard 555 oscillator, your schematic deviates from the data sheet in several important ways, not the least of which is that the capacitors on pins 5 and 6 should be connected directly to the circuit ground.

    ak
     
  9. rc3po

    Thread Starter Member

    Feb 12, 2014
    56
    1
    Thanks ronv,
    The caps are going straight to ground. The 180 ohm resistor is grounding the LED's.
    The fact that I'm controlling the chip with a Photoresistor through the reset pin is the whole point. I've never read or seen anywhere that it can be done - but I'm doing it.
    Yes, I thought about giving each LED their own resistor, but it works fine like I have it setup, so I left it alone. I've been using it as a night-light for 11 months so far - no problems.
     
  10. rc3po

    Thread Starter Member

    Feb 12, 2014
    56
    1
    Thanks AnalogKid,
    They are straight to ground. Sorry - I made a mistake and drew the schematic showing them going to ground together. On my B/B, they are not. Lol, I did admit to being a newbie ...
     
  11. rc3po

    Thread Starter Member

    Feb 12, 2014
    56
    1
    Sorry,
    I made a mistake on the schematic and combined the grounds of the ceramic caps with the cathodes of the LED's. My bad.
     
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