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Need help testing this circuit

Discussion in 'General Electronics Discussion' started by Mark34, Dec 3, 2017.

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  1. Mark34

    Mark34

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    Sep 23, 2016
    Hello ! I build this Transistor-triggered photogate circuit and it doesn't seem to work. When the light between the emitter and detector diodes is interrupted it should trigger something, can someone tell where to connect a light led in order to test it ? I tried to connect a light led like in the second picture but it didn't turn on.
    http://hiviz.com/tools/triggers/triggers3.htm

    [​IMG]

    [​IMG]
     
  2. Bluejets

    Bluejets

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    This is a good example of what happens when circuits are drawn unconventionally. Try connecting the led load to the positive and include a current limiting resistor.
     
  3. Mark34

    Mark34

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    Sep 23, 2016
    What do you mean it's drawn unconventionally ?
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Signal flow conventionally proceeds from left to right, top to bottom.

    Here's one that is even worse:

    [​IMG]
    Is it obvious to you how to use "SCR OUTPUT" in the above schematic diagram?
     
  5. Mark34

    Mark34

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    Sep 23, 2016
    I guess use the arrow terminals, I am noobish in electronics but I can build a circuit if I have a correct and complete schematic, so I am here because I need a straight answer for my schematic. I am not even sure if the SCR triggers in a 9V circuit.
     
  6. duke37

    duke37

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    Jan 9, 2011
    You have a +9V line - good.
    You have a 0V line - good
    You have the SCR below the ground line - bad
    You have the SCR connecting the ground to the ground, Why should it pass any current? See #2.

    There are two versions of SCR triggers. Look up the specification of the device you are using, it could need 0.5V or 3V or so. 9V is plenty but a big SCR may need more than the current through a led to hold it on.
     
  7. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Turn over the SCR so the cathode still is connected to GND but the anode is upward. Now a DC load positions naturally between the anode and the positive rail (+9 V). The 10K resistor should go out horizontally to the SCR gate.

    Strongly suggested guidelines for schematic information:

    Power flow - top to bottom
    Signal flow - left to right.

    You got this part right but I'll reinforce it anyway:

    Positive power symbols point up.
    Negative power symbols point down.
    GND symbols always Always ALWAYS point down.

    ak
     
    Last edited: Dec 4, 2017
  8. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Okay. In your first schematic (reproduced below) the 470 Ω resistor provides current-limiting for the series-connected LED between +9 V and GND. The LED illuminates the Photo-Transistor (PT) which then conducts current from +9 V through the variable 100 kΩ resistor from the PT collector to the PT emitter to GND. This causes the collector voltage of PT to vary from +9 V when the PT is NOT illuminated down to almost +0 V when PT is fully illuminated by the LED.

    A sufficiently large positive voltage on the collector of PT (greater than about +2 V), which will occur when the light from the LED is blocked or interrupted, will forward-bias the base-emitter junction of the 2N2222 transistor, causing it to conduct from +9 V through its collector to its emitter and through the 10 kΩ resistor in its emitter then through the gate-to-cathode junction to GND of the SCR, which should trigger the SCR to conduct.

    If there is a steady positive voltage on the anode of the SCR with respect to GND, the SCR will latch on and stay on if there was sufficient gate current supplied by conduction of the 2N2222 to trigger it. This is NOT a good way to operate this circuit. Presumably you want the SCR to turn ON when the the light path between the LED and the PT is blocked, and to turn OFF when the PT is illuminated by the LED. This won't happen with DC applied to the anode of the SCR. You need a low-voltage power-line operated AC voltage source, typically provided by a small step-down transformer, to power your indicator light and to commutate the SCR, causing it to cease conduction when its anode goes negative with respect to GND each half-cycle of the AC power source.

    [​IMG]
    So, get yourself what is commonly called a "filament transformer" providing about 6.3 VAC with a few hundred milliamperes current capability. Current rating is not critical since all you want to drive is a small LED indicator light for now. Voltage rating isn't critical as long as it is large enough to turn on your LED. Next from the following description, draw up a proper schematic that you can construct from: Connect one wire from the transformer low-voltage secondary winding to GND. Connect the other wire from the low-voltage secondary winding to a 470 Ω, 1/4-watt, current-limiting resistor. Connect the other end of the 470 Ω resistor to the anode of an LED of your choosing. Connect the cathode of the LED to the arrow labeled OUTPUT in the schematic above, which is the anode of the SCR. Plug in the transformer to the power line, connect the 9 V power source, and you should be good to go.

    The above circuit is a very poor design. If the 100 kΩ variable resistor is set to too low a value, it may be possible to burn out the PT under strong illumination by the LED. This variable resistor limits PT collector current under strong illumination. Setting it to zero ohms value is not a good idea.

    Good luck with your "monkey see, monkey do" approach to electronics. Perhaps you would be more comfortable at instructables.com?
     
    Last edited: Dec 4, 2017
  9. Mark34

    Mark34

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    Sep 23, 2016
    Like this ?
    [​IMG]
     
  10. AnalogKid

    AnalogKid

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    Both the anode and cathode of the SCR are tied to Ground. You tell me.

    Don't guess; think.

    ak
     
  11. Mark34

    Mark34

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    Sep 23, 2016
    Thanks for the indications, I saw this schematic on a web page that seemed legit, i guess making a circuit on a diagram you don't understand makes you a moneky...
     
  12. duke37

    duke37

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    Jan 9, 2011
    I started off monkey see, monkey do by assembling Heathkit equipment. I learned what components looked like and how to solder but I learned more if the circuit did not work or failed and I had to find the fault. Some people change every component in this situation and never learn.

    I think I could now be described as a silver back.:D
     
  13. hevans1944

    hevans1944 Hop - AC8NS

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    Well, let's not denigrate the efforts of monkeys. Monkeys are quite intelligent, especially at copying things that they observe to work. Monkeys, for some reason, simply lack the capability for much of the creativity enjoyed by us humans.

    "Monkey see, monkey do" is the way we all learn how to do things at first. Nothing wrong with that, but it is rote learning (like learning the multiplication table) and by itself doesn't lead to much else. The web page you visited is a hobbyist website devoted to high-speed photography. Folks who contributed circuits there may or may not have been electronics savvy. Sometimes we just "futz around" with stuff until it "sort of works okay" and then make known the results of our efforts for others of like mind and interest to try. The Instructables website is famous for doing this, which is why I suggested you might be more comfortable there.

    Like @duke37, when I could eventually afford it, I started putting together Heathkits following their excellent "monkey see, monkey do" instructions. But even before that I wanted to know the "why" of how things worked. But most of all, in those early years, I wanted my work to look "just as good" as the factory assembled stuff. That usually isn't possible for the home hobbyist, but it's getting easier now than it was then to turn out professional-looking projects.

    I hope your current efforts ignite a spark of curiosity and further interest in electronics, but even if you just have a short-term goal of triggering an electronic flash by interrupting a beam of light, finding a circuit that works and that you can assemble with your own hands will bring you much satisfaction.
     
  14. Mark34

    Mark34

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    Sep 23, 2016
    I can't believe this, all I need is that schematic modified with the addition of a testing light led and the schematic posted here and you guys make long posts about electronics theory that's not helping me too much.
     
  15. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Believe whatever you want. Try to find what you need somewhere else if our answers are unsatisfactory. We try to help those who help themselves. Yada, yada, yada.

    BTW, I don't personally do schematics much. Maybe someone here will jump in and do that for you.:rolleyes:
     
  16. sashijoseph

    sashijoseph

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    Dec 3, 2017
    @Mark34
    You have your answer right in the 2nd post.
    You have connected the LED between the SCR's anode and Gnd...connect it instead,between the SCR's anode and the supply's +9V along with a 470 Ohms series resistor(much like the LED triggering the phototransistor,in your schematic)
     
  17. Mark34

    Mark34

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    Sep 23, 2016
    Thanks, is there any change to the circuit if instead the phototransistor I use IR receiver diode ?
     
  18. Audioguru

    Audioguru

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    Sep 24, 2016
    The output level from a photodiode is too small to turn off the 2N2222 transistor. A phototransistor amplifies the tiny voltage or tiny current at its base when light shines on it. Usually an opamp with high gain is used with a photodiode.
     
  19. Mark34

    Mark34

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    Sep 23, 2016
    So the base of the phototransistor is not connected to anything ?
     
  20. BobK

    BobK

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    The phototransistor will not have a base connection. The light input acts like the base.

    Bob
     
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