Problem using output from NE567 tone decoder

Discussion in 'The Projects Forum' started by numberdude, Dec 29, 2010.

  1. numberdude

    Thread Starter New Member

    Dec 29, 2010
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    I am trying to use the HI or LO output from pin 8 on two NE567 tone decoders to trigger an H-Bridge motor reverser. My H-Bridge is set up so that if both inputs are the same, the motor is off. It works fine when the two inputs are connected to HI or LO from the power supply, but when connected to the normally HI outputs of the NE567's the BJT's get hot. The HI output from the NE567's doesn't seem to be able to supply enough current to trigger the BJT's. It barely lights an LED. I tried using a FET to amplify the current from the 567's, but I couldn't get it to work. The H-Bridge is made of two NPN and two PNP bipolar transistors. Maybe I need to use MOSFETS in the H-Bridge instead of BJT's? Would they be simpler to trigger? Any help would be greatly appreciated. Thanks.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    We probably need a schematic to help.
     
  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you have a resistor to the powersupply on pin 8?
    This must be done because the output is open-collector.

    [​IMG]

    I also attached the complete datasheet.

    Bertus
     
  4. numberdude

    Thread Starter New Member

    Dec 29, 2010
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    First, thank you for your reply. Yes, I have a 20k resistor from pin 8 to hot. I tried using lower values to get more current, but still couldn't get it to trip my H-Bridge. The H-Bridge apparently needs a few milliamps to operate correctly. I tested the H-Bridge by connecting the two inputs directly to the power supply and also indirectly through various size resistors. I found that I couldn't use a resistor greater than about 100 ohms, so taking the HI output from pin 8 thru a 20k won't work. It seems that the 567 can sink current, but it can't source very much. I need an intermediate circuit between the 567 and my H-Bridge, but there I'm stuck. Or, suppose I made the H-Bridge with MOSFETS instead of BJT's? Don't MOSFETS require voltage as opposed to current? Would that work?
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Do you have the schematic of the H-bridge?
    Perhaps you need a buffer between the NE567 and the H-bridge.

    Bertus
     
  6. numberdude

    Thread Starter New Member

    Dec 29, 2010
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    Hi Bertus,

    Here's a schematic for the H-Bridge I'm using. It works great if it gets enough current, but overheats if it doesn't. Two NE567's (not shown) supply the trigger signals to the H-Bridge, but they don't seem to be able to source enough current to operate the H-Bridge. I don't know what kind of intermediate circuit I could use to boost the current. The NE567's work great and trigger as expected. Their output is normally HI and goes LO when triggered. I can light an LED brightly using the LO output, but the HI output just barely lights the LED, indicating the the NE567 can sink but not source current. (I think that's the right interpretation.)

    Here's the H-Bridge circuit:
    http://i1138.photobucket.com/albums/n525/numberdude/SDC13132.jpg

    Thanks again for your help. I've done several experiments and done a lot of searching without success. I suspect there's a straightforward solution. I just don't have that level of expertise.

    Allen
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The NE567 can drive loads uptp 100 mA.
    It is even possible to drive small relais with it.
    The minimum load at 12 Volts is 12 / 0.1 = 120 Ohms.

    Bertus
     
  8. numberdude

    Thread Starter New Member

    Dec 29, 2010
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    Yes, it can drive loads up to 100ma, but only if you SINK the 100ma. It can't source worth a darn. I need an output that can both source and sink (is that even possible?) so that it will operate my H-Bridge. I still don't know how to do this.

    BTW, the circuit in my last post shows TIP transistors. Actually, I'm using general purpose NPN and PNP transistors which probably require at least a couple milliamps to saturate. The NE567's do provide +6VDC (my supply voltage) at pin 8, but at an apparently high impedance that limits current too much to trigger the H-Bridge.

    Again, thank you for your thoughts.

    Allen
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
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    Your 'H' bridge employs two NPNs & two PNPs in an Emitter Follower configuration. This means that the Emiter voltage will be .7V less than your base voltage. When one NPN is on its complimentary PNP is also turned on. If the 567 is supplying +6V then the best you can feed to the + side of the motor will be 5.3V minus the Emitter/Collector voltage drop of the complimentary PNP. That would leave about 5V across the motor. Also, because the NPNs are Emitter Followers I don't think that you need 1K base resistors for them. I would expect a value about 1/10th that. The PNPs are a different story because they're wired as a 'Common Emitter' but even there 1K may be a bit high. What's the E/I rating of the motor?

    EDIT: Read post 11 for a clearer explanation and additional findings. Also After looking at your H-Bridge again I realize that all the transistors are wired Common Collector, not just the NPNs. It's impossible to saturate Common Collector amplifiers. This means that some of the expected voltage drops are going to be worse than stated above.
     
    Last edited: Jan 2, 2011
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    One more note: The 'H' Bridge works by virtue of when 'Control-1' is positive control-2 must be at GND. To reverse; 'Control-2 must be positive and 'Control-1' must be at GND. How are you doing this from only one 567? Is the motor supposed to run in one direction upon detection of one freq and reverse upon lock of a different freq, or is it supposed to run in one direction with no lock and then reverse upon lock? :confused:
     
    Last edited: Dec 31, 2010
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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    I just noticed something else about your H-Bridge besides the problems that Emitter Followers will cause, as stated in my earlier post. The other problem is that your control voltage can't be lower than the supply voltage (12V on your print) to the motor. It will cause the PNPs to partially conduct when they should be completely off. This, in turn will over work the NPN that on at that moment. For instance: In your H- Bridge circuit; If Q1 is on Q4 should also be on. Q2 and Q3 should be off. Because your Control voltage is much lower than the 12V motor supply When Q1 is on Q3 will conducting when it should be totally off.

    The circuit below doesn't eliminate the issue above (control voltage must not be lower than motor V+) but since it employs Common Emitter configurations only they will fully saturate and the control voltage can be much lower than an Emitter Follower, as long as the motor V is = or less than the Control voltage. Note that I chose 560 Ohm base resistors because I don't know how much current your motor draws and I wanted to insure that all the transistors would fully saturate when on.

    It's very late but tomorrow I'll see if I can work something out that would provide for a 12V motor supply while utilizing a much lower control voltage.
     
  12. numberdude

    Thread Starter New Member

    Dec 29, 2010
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    CDrive,

    Unless I'm seeing double, your H-Bridge circuit is exactly the same (except for the 560 ohm resistors) as the one I posted. Anyway, I thank you for your thoughts on this. I did come up with a solution (not very elegant) that works with my H-Bridge and NE567's. I used relays! It works great! My biggest problem was that I didn't really understand what the 567 does at pin 8. It might have a HI voltage at idle, but that's only because it's connected to +6v through a 22k resistor. Not nearly enough current to trigger the BJT's. You can only use pin 8 to SINK current to ground. I'd eventually like to try MOSFETS in place of the relays. Do you know how I could do that?

    Here's the (hand drawn) circuit I came up with:

    http://i1138.photobucket.com/albums/n525/numberdude/SDC13141.jpg

    Allen
     
  13. CDRIVE

    Senior Member

    Jul 1, 2008
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    They're definitely not the same. In my circuit the motor is in Collector circuit of all four Transistors. I explained it thoroughly. I suggest you read those two posts again. My New Year resolution is not to repeat myself.
     
  14. CDRIVE

    Senior Member

    Jul 1, 2008
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    The relays would work but only connect the coils to the 6V buss. The contacts have to connect to the +12V motor supply for reasons I have already stated. You don't need the 22K resistors either.

    EDIT: Your 100uF coupling cap is a bit large!
     
  15. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You can keep it much simpler.
    The NE567 can work with voltages upto 15 Volts on the output transistor.
    A pull up resistor will probably do.

    [​IMG]

    Bertus
     
  16. CDRIVE

    Senior Member

    Jul 1, 2008
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    Bertus, your solution should work fine but I have a question... How did you morph numberdude's hand drawn schematic into your schematic editor? It appears that the mods you made to his schematic are computer generated.... I like it but clueless of how you did it!
     
  17. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I do not have a schematic editor.
    I use (Koulour)paint to create the shematics.
    I loaded the schematic into the paintprogram, changed it to grayscale,
    raised the contrast and draw the things needed in.
    I created a folder with a lot of schematic symbols to use,
    and use insert image from file to put them into the schematic.

    Bertus
     
  18. CDRIVE

    Senior Member

    Jul 1, 2008
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    Most expedient and wonderfully clever!
     
  19. numberdude

    Thread Starter New Member

    Dec 29, 2010
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    Hi CDrive,

    I still can't see the difference between the two circuits. I have my motor hooked to the collector circuit too. I did make one mistake in my drawing, which you pointed out. The H-Bridge and motor are actually connected to 6volts, same as the rest of the circuit - not 12v as shown. That's probably why it works. Yes, the cap is probably too big - the signal frequency is only about 1 KHz. Thanks again for your help.
     
  20. numberdude

    Thread Starter New Member

    Dec 29, 2010
    19
    0
    Wow! That's what I wanted to do in the first place - connect the 567's directly to the H-Bridge. You suggest using 220 ohm resistors and running everything at 12 volts. That will provide a lot more current to the H-Bridge. I'm gonna try that right away. I didn't like the idea of relays, but the thing does work great. (But I'll miss that nice "click" sound).

    For the H-Bridge: Do you think it would be a good idea to run 10k resistors from base to ground on the NPN's, and base to hot for the PNP's? Wouldn't this insure that the transistors stay completely off when they're supposed to be off? They wouldn't drain much current when the transistors are on.

    Thank you for your help,

    Allen
     
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