555 mono

Discussion in 'The Projects Forum' started by vinodquilon, Dec 24, 2009.

  1. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    Pls find the attachment. Our aim is to turn on relay when telephone detects ringing signal. 555 configured as monostable. T1 turns on by optocoupler. This applies a negative going trigger at pin2 of 555.
    Is there any issue of shorting both Vcc's when T1 on. Kindly reply......
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Well, your signal is going straight to Vcc, which might be a bit of a problem. The transistor T1 going between Vcc and ground will likely smoke on ring. This circuit needs a lot of work I think.

    [​IMG]

    JPG have to be the worst format to use for schematics, .gif or .png (preferred) are lots better.

    You need a collector resistor for T1, which feeds pin 2. Pin 4 and pin 8 should be Vcc as shown.

    You will need a surge suppression diode on the relay coil, similar to what is shown below.

    [​IMG]
     
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  3. Audioguru

    New Member

    Dec 20, 2007
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    This is the second thread about this horrible old circuit that is full of errors.
    It is obvious that the transistor T1 short circuits the power supply Vcc to ground when it turns on.
     
  4. Wendy

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    Mar 24, 2008
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    I think the schematic I showed by example should be easy to modify for what the OP wants.
     
  5. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    Sir,
    I have one doubt about the transistor. I think there is no need for Transistor, R1, R2, & R3. We can directly fed +5V pulse to C1 to trigger 555. C1 & R4 act as a differentiator to produce trigger spikes.We can place one diode across R4, facing anode down.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    No.
    The 555 is triggered when pin 2 goes low, not high.
     
  7. vinodquilon

    Thread Starter Member

    Dec 24, 2009
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    Pls find the attachment which will tell what I mean. Differentiator would produce negative going spikes on trailing edges of trigger pulses.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    Attach the photo-isolator to your new 555 circuit to see that the input pulse to pin 2 of the 555 does not go low enough to trigger the 555.
    The 1k resistor at pin 2 must have a much higher value.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    The phototransistor will basically replace the transistor I showed. I pulled this schematic from my cookbook, how you modify it is up to you. It works though, which the original schematic most definitely did not.

    You do not want to connect the last circuit directly to the phone signal though, it will smoke the 555. The typical ring for a phone can be around 20-60V P-P.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    90VAC at 20Hz in North America. 90VAC is 255V p-p plus the -50V battery.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    Typically the ring is a square wave (ideally), so the 255V is a pretty high number. It is generated with a relay. I have FIOS, which uses a 12V SLA battery and wall wart, so I have no idea what my current ring signal is.

    You have to remember the kind of insulation phone companies land lines have, they tend towards 48VP-P, and go down from there.

    Either way, that 555 isn't going to complain long.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Sorry Bill but the ringing voltage of a phone is a 90VAC sine-wave with a peak voltage of 127V plus the 50V battery makes it have a peak of -177V.
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    After doing a little research, I stand corrected.
     
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