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Switching leds

Discussion in 'LEDs and Optoelectronics' started by Nickb1969, Sep 15, 2016.

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  1. Nickb1969

    Nickb1969

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    Aug 28, 2016
    hi guys, I am sure this will be an easy one for you.

    I am a circuit that uses a zvn transistor to light an led when a contact is made and led is out when contact is broken. I want to replace the led with 2 bi colour LEDs, red and green 3 pin. Green when contact made, red when broken. My question is, how can I do this simply, I assume that I could add another transistor but not sure how to do this. Any help would be great.
     
  2. davenn

    davenn Moderator

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    Sep 5, 2009
    hi there

    what is a zvn transistor ? ... in 45 yrs in electronics, I have never heard of one

    maybe is would be a good idea to post your circuit



    Dave
     
  3. Nickb1969

    Nickb1969

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    Aug 28, 2016
    Hi, it's a zvn2110a transistor. I will have photo take a photo of the circuit and post it
     
  4. Nickb1969

    Nickb1969

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    Aug 28, 2016
    [​IMG]
     
  5. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I didn't have a common anode (3 pin) Bi-Color LED in my library so when viewing this schematic assume the LED anodes are tied together within the oval. I've labeled the new parts with an asterisk (*). Parts that don't contain an asterisk already exist.

    Chris
    upload_2016-9-19_19-45-33.png
     
  6. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    That looks good if the LED is actually common anode. I can't find one in my library either. All are common cathode.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Dave jones of eevblog did something very similar to change a LED on a soldering iron. If you can find that video, he goes into some detail about various methods.

    Here it is:
     
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    My Tina library has 3 pin common cathode models and a 4 pin (independent Anodes & Cathodes) dual LED. I used it and tied the Anodes together. The rest of my circuit just echos the existing circuit he posted. Including the oddball 4.7KΩ current limiting resistor in the Drain. He can change the value if he wants a brighter LED but that's true for the other 4.7K that already exists on the board.

    Chris
     
  9. Nickb1969

    Nickb1969

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    Aug 28, 2016
    Hi there, really sorry for the very late reply. I have not been able to get back here for a while. I have decided to stick to a red and a green led rather than bi colour. Your solution has been used for this. I was waiting for prototype boards to arrive and then start experimenting. The boards only had the one red led and no green. I have added the extra transistor and 1m resistor and wiring as per your diagram and it works well so far, no blown transistors and the LEDs switch properly with the LEDs.

    Thankyou for the post it is greatly appreciated and sorry again for the late reply.
     
  10. Nickb1969

    Nickb1969

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    Aug 28, 2016
    I did change the 4.7k resistor to 1k. The LEDs and resistors are all on a piggyback display board.
     
  11. Nickb1969

    Nickb1969

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    Aug 28, 2016
    image.jpeg image.jpeg image.jpeg

    This is the project so far. It is a modular level switch for power stations etc. Each daughter board is identical and takes input from a conductivity probe. The display board plugs into each daughter board to give a local display. The is a DPDT relay on each board.

    I am not an electronics engineer. The circuit has been lifted from an old circuit and modified.

    I only have to channel boards made so far for testing, hence the missing ones.
     
  12. Nickb1969

    Nickb1969

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    Aug 28, 2016
    i do have a couple of other questions, yes they are newbie one so please bear with me.

    On the original circuit there is a 1N3595 diode across the 12v+ and the 0v tracks. Why is this done?

    Secondly, the original unit used a onboard transformer, capacitor, resistor setup to for the power supplies. I am using a SMPS mean well caged unit. On a similar unit that uses SMPS there is a diode on the +v track, is this a good idea? image.jpeg image.jpeg

    You can see the diode across the relay terminals in one picture and the D25 diode on the +v track in the other picture. It connects to the + pin of the connector on the reverse side
     
  13. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    The diode across the relay coil is there to absorb back EMF. The other one you mentioned I don't know without seeing a schematic. It'll make it much easier than figuring that out from a PCB photo which I'm too lazy to do.

    Chris
     
  14. AnalogKid

    AnalogKid

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    Because R15 is on the high side of LR1, you're in luck. Add a second LED and a 1N4148 diode in series, connected from LR1 anode (the low end of R15) to GND. This string is in parallel with LR1 and TR2. When TR2 is off, R15 supplies current through the new LED and diode. When TR2 is on, the forward voltage of LR1 is less than the combined voltages of the new LED and diode, so LR1 comes on and the new LED goes off. No second switching transistor, no additional current limiting resistor.

    ak
     
  15. Nickb1969

    Nickb1969

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    Aug 28, 2016
    Thanks for all the replies. I will get round to replying better in a week or so, as I have to head off abroad today.

    I am getting there now. I have one issue that I can not sort out. On the initial schematic posted R14 has in input of 12V. On the original piece of equipment the output from this resistor is 12V also. On my circuit which is pretty much a copy of the circuit R14 has a 12V input and approx 9.5V output. I just can not figure out why there is a different between the two.

    I know it is difficult for everyone without seeing the equipment but any ideas would be great why of the voltage change in the two circuits.
     
  16. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    It's good news that you're still young enough to do that. I vaguely remember those days! I couldn't resist it. It only required a space between "a" and "broad".:)
    R14 feeds the input side (LED) of an OptoIsolator through (D8) Diode. While TR1 is off the voltage at the junction of R14 and D8 can never be equal to the supply voltage because the Opto LED is drawing current which in turn creates a voltage drop across R14 and D8.

    Chris
     
  17. Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    What is OP1 like? Is it a typical IR LED in the optoisolator? I would expect about 1.2 V across it and 0.6 V across the diode, making the business end of R14 around 2 Volts at about 2 mA (??!!). With TR1 on it would be even less.
     
  18. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    I drew this up to help clarify my last reply. The Gate circuitry of TR1 does not follow your schematic exactly though. Instead I've placed a timed switch (SW1) on the gate to turn TR1 On and OFF. As you can see from the current and voltage plots, the voltage in question (VM1) can never reach 12V whether TR1 is On or OFF.

    Chris
    upload_2016-10-1_11-4-49.png
     
  19. Nickb1969

    Nickb1969

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    Aug 28, 2016
    Thankyou for all your efforts on this.

    I have removed the op amp from the circuit. There a four sets of the circuit in one unit. The op amps are used to drive a validation logic of the channels. I have lifted the circuit to make single channel unit that can be combined on a motherboard. I removed the validation logic and therefore the op amps.

    There is one led on the original but I have added the second to give a green for on and red for off. The basics are that it is a specialist water level switch with different settings for different water purities. The relay can be set to be normally energised in steam or normally energised in water. This is a failsafe feature in case of power loss. So if you set the channel to be normally in steam, when water is detected the relay will de-energise to complete the relay output.

    There does not seem to be much of an issue will normally is steam but there is a floating 0.4v across the relay coil when de-energised. When normally in water there is only 8.8v to the positive of the relay coil and when de-energised there is a floating 3.3v across the coil, there should be no voltage across the coil when de-energised. I really can not figure out why this is happening. It means that sometimes the relay is only clicking very slightly and not switching.

    I really wish I knew more and I am more into the mechanical side of my job which is making conductivity based level indicators and switches.
     
  20. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Talk about loosing me? At this point I'm so damned confused I don't know what to reply. My problem is I can't make a connection from the schematic you posted, the photos you posted and the replies you posted. For instance my last post and schematic makes sense as a reply to the question you asked but your last post...eh not so much? o_O Where's the connection?? o_O:confused:

    I don't even see a relay in that schematic. Is that what the Opto (not shown) is driving?
    Chris
     
    Last edited: Oct 1, 2016
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