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UM66 is not giving full tone

Discussion in 'Audio' started by rahulb, Apr 19, 2018.

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  1. rahulb

    rahulb

    45
    0
    Mar 14, 2018
    Hi,

    I made a UM66(BT66) door bell circuit. it is operated from mains by main door bell switch.

    It has a LM386 amplifier circuit also.

    My problem is BT66 is not giving full tone on pressing the switch. I mean whenever the switch is pressed and released, the bell circuit should ring until the tone is finished.

    But, as I press the switch, the bell is giving sound for only the time, the switch is pressed. when the switch is released the bell is turning off.

    I don't know what is going wrong. Below if the full schematic of the circuit.

    [​IMG]

    Please help me.

    thanks
     
  2. kellys_eye

    kellys_eye

    4,276
    1,146
    Jun 25, 2010
    Where is the bell push? Where does the tone come from?
     
  3. rahulb

    rahulb

    45
    0
    Mar 14, 2018
    Hi Kelly,

    Bell push is before TR1 Transformer, it is not shown in the schematic.

    I used BT66 (Q1) melody generator which is similar to UM66

    Output is speaker connection J1 at extreme right side of the schematic.
     
  4. kellys_eye

    kellys_eye

    4,276
    1,146
    Jun 25, 2010
    You're using a bell push on the MAINS side of the transformer?

    But how do you generate the tone? Where is the tone 'input' to the circuit?

    Post (or link to) the complete schematic - all we have here is the amplifier part.
     
    Last edited: Apr 19, 2018
  5. duke37

    duke37

    5,361
    767
    Jan 9, 2011
    Where does the collector current of Q1 come from?
     
  6. duke37

    duke37

    5,361
    767
    Jan 9, 2011
    I have had another look.
    The input transformer is shown as a centre tapped resistor. Presumably the push switch is in the mains feed.
    Q1 is not a transistor but an integrated circuit supplied with 3.3V.
    To get a output for longer, then the supply must be held on for the required time. This could perhaps be done by increasing the reservoir capacitor from 1000μF to 10,000μF.
    The amplifier could be powered continuously, if only the BT66 is timed, then a smaller capacitor may do.
     
    Last edited: Apr 19, 2018
    kellys_eye likes this.
  7. Bluejets

    Bluejets

    4,054
    847
    Oct 5, 2014
    Eeeeek.....Bad idea.
     
    duke37 likes this.
  8. rahulb

    rahulb

    45
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    Mar 14, 2018
    yes, the bell push is on the main side of the center tapped transformer.

    tone generator is IC BT66 ( Q1) which gets 3.9v through zener diode (Zn)

    sorry, I didn't got proper symbol on my schematic software, so I used a transistor symbol.
     
  9. Audioguru

    Audioguru

    2,823
    633
    Sep 24, 2016
    The transformer symbol is wrong and the tone generator symbol is wrong.
    It should be obvious that the transformer powers this circuit so the tune plays only while the pushbutton is pressed.
    The datasheet of the tone generator shows how they make it work and play the entire tune.

    The LM386 is missing the important RC to ground at its output as shown in its datasheet.
     
  10. kellys_eye

    kellys_eye

    4,276
    1,146
    Jun 25, 2010
    Not sure if using a mains voltage at a doorbell pushbutton is even LEGAL!!!! It sure wouldn't be allowed here in the UK.

    Based on the revelations in the above posts (thanks chaps!) then C5 would appear to be the deciding factor on how long the circuit operated for.

    Overall the design is 'poor' to say the least - 'dangerous' is more the word used to describe it. Put the pushbutton on the secondary side of the transformer before you kill yourself or some innocent caller.
     
  11. WHONOES

    WHONOES

    885
    218
    May 20, 2017
    You could use a 555 timer that is initiated by the bell push to supply the BT66 part of the circuit. The 555 period can then be set to play the entire repertoire.
     
    hevans1944 likes this.
  12. hevans1944

    hevans1944 Hop - AC8NS

    4,473
    2,084
    Jun 21, 2012
    Putting mains voltage on a doorbell push-button switch is insane. Doorbells are wired with low voltage AC, typically 12 to 24 VAC. The cheap push-button switches used for doorbells are not rated to switch mains voltage, which IIRC is 230 VAC, 50 Hz, in India.

    Not showing the complete schematic when asking for help on this forum is less than bright: it is downright negligent! You probably don't have a fuse in series with the transformer primary either.
     
  13. rahulb

    rahulb

    45
    0
    Mar 14, 2018
    I think there is some misunderstanding here.

    by bell push, I mean 230v 6A bell button. Such button is given below

    [​IMG]
     
  14. duke37

    duke37

    5,361
    767
    Jan 9, 2011
    That diagram does not tell us where the bell push is connected.
    Does the switch connect the transformer to the mains, giving the possibility of electric shock should the switch get wet.
    The transformer should be placed where it is shielded and never gets wet and is energised all the time. The bell push should then be used to switch the low voltage supply.
     
  15. rahulb

    rahulb

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    Mar 14, 2018
    the transformer is inside a box with the circuit. it is added inside drawing room. so there is no chance than it can get wet. the switch is on main gate outside.
     
  16. rahulb

    rahulb

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    0
    Mar 14, 2018
    I added two capacitors , you can see them below in schematic in red color. But, it is not working yet.

    [​IMG]
     
  17. duke37

    duke37

    5,361
    767
    Jan 9, 2011
    Are you being deliberately difficult?
    Where is the switch connected in the circuit. If it is connected to the primary of the transformer, then a shock is very possible.

    To add more safety, use a 'wall wart' to supply a low voltage, 12V 1A is probably enough.
    Connect the amplifier to the power supply.
    Put the switch between the 12V supply and R2 to turn on the sound when it is pressed.
    Put a large capacitor (1000μF?) top of R2 to ground to supply current for a little while until the tune is ended.
     
  18. hevans1944

    hevans1944 Hop - AC8NS

    4,473
    2,084
    Jun 21, 2012
    More like deliberately arrogant.

    His non-functional box is safely placed in a "drawing room," impervious to outside influences, and connected to a mains-capable switch "on the main gate outside." The do-dad musical melody generator appears to be like a gimmick from inside a cheesy greeting card that sounds off for a few seconds when you open the card.

    Since the doorbell switch is already in place and rated for six amperes at 230 V AC, the OP might as well scrap his electronics and wire the switch to activate a loud, conventional, electric bell... But Noooo... that would disturb the drawing room guests! Gotta have that sweet little melody!

    Now he just has to figger out how to make it play to the end and then shut off until the button is pressed again. Prob'ly a few more capacitors placed here and there will do the trick... or maybe even a 555 timer, as suggested by @WHONOES in his post #11, but then that would require some real electronic design, none of which has surfaced yet in this thread. FWOT.
     
  19. rahulb

    rahulb

    45
    0
    Mar 14, 2018
    ok. I got it. it needs a continuous supply to the amplifier. I will add a 12v adapter and switch on safe side.

    thanks a lot.
     
  20. rahulb

    rahulb

    45
    0
    Mar 14, 2018
    sorry if you think I am arrogant. I am just a beginner with little knowledge, I am no match for knowledge you have.

    I am only somewhat confused. I was just trying something on my own.
     
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