Tone Decoder LM567 not working

Discussion in 'The Projects Forum' started by prkandel, Sep 3, 2015.

  1. prkandel

    Thread Starter New Member

    Sep 3, 2015
    8
    0
    I bought an IC named LM567 from the market and wanted to use this an a bandpass filter which would give a low output when the input signal to it was 40kHz. In all other conditions and no input signal condition, as per the datasheet states, the output at pin 8 should be a high.

    But while connecting the circuit as per the datasheet, the output pin 8 does not give a logic high while there is no input to it. Should not it give a high output as soon as the IC gets the supply Vcc and there is no input to it?

    And one more question is that how much input voltage can I give to this IC ? In one equation in the datasheet, I read that the input should not exceed 200mV RMS. However, in maximum input column there was Vcc+0.5 volts. So does this mean that the input can be a 200mV signal riding on Vcc ? Or can I feed pure AC signal with RMS value nearly equal to Vcc ?

    Waiting for replies. Thanks in advance.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    Do you have a schematic you can post here?

    Bertus
     
  3. prkandel

    Thread Starter New Member

    Sep 3, 2015
    8
    0
    Yes. I can
    This is the circuit I am using with no input to it. The output should be high but it is not.
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    The output of the LM567 at pin 8 is open collector:

    LM567_functional_diagram.png

    You will need a resistor from the pin 8 to the powersupply to see level changes.
    Try a 4K7 resistor for a start.

    The given diagram can be found on page 8 of the attached datasheet.

    Bertus
     
  5. prkandel

    Thread Starter New Member

    Sep 3, 2015
    8
    0
    Okay, I will try this tomorrow.
    But can't I just measure the output from Pin 8 with respect to ground and know whether the output is high or low?
    I had read that the output of the decoder is low when the input signal is of the same frequency as the internal oscillator, which indeed is controlled by the resistor between pin 5 and 6 and the capacitor at pin 6. In other situations including when there's no signal, the output should have been high. However, for me, the output is not high when there is no input.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,516
    1,246
    You are assuming that any output pin can swing both high and low, and this is not correct. Some output pins do not have the ability to produce a positive output voltage. In this case, you complete the output circuit by adding a resistor from the output pin to the positive power supply at pin 4.

    ak
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    What values do you use for the timing resistor and capacitor?

    LM567_timing_circuit.png

    Bertus
     
  8. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    AFAICR: the lower the input voltage swing - the sharper the bandpass detection. The Vcc+0.5V should be among the absolute maximum ratings, that is to exceed that limit will likely damage the chip.
     
  9. prkandel

    Thread Starter New Member

    Sep 3, 2015
    8
    0
    I will try this too. Well then, if I add a resistor from output pin to power supply, the output at no input will probably be high. Will this also make sure that the output at required frequency be low?
     
  10. prkandel

    Thread Starter New Member

    Sep 3, 2015
    8
    0
    I understand this. I just wanted to know if I could provide this chip the input of some 3V RMS(peak to peak voltage 8.48V) while the Vcc is 5V ?
     
  11. prkandel

    Thread Starter New Member

    Sep 3, 2015
    8
    0
    Currently I am using a capacitor of 0.01μF and resistor of 2.2kΩ
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,986
    3,730
    The datasheet is your friend.

    Yes, to your question. The required tone causes the transistor to saturate and output will fall to ground (~0.2V or so).
    image.jpg
     
    prkandel likes this.
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,516
    1,246
    Yes, if the resistor value is not too small. The output can sink 30 mA, but there is no reason to drive it that hard. Start with something between 1.0K and 4.7K.

    ak
     
  14. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,773
    1,103
    Should be ok. Max input on pin 3 is 5.5V.
     
    Last edited: Sep 4, 2015
  15. prkandel

    Thread Starter New Member

    Sep 3, 2015
    8
    0
    Thank you for the solution. It is working now.
     
  16. prkandel

    Thread Starter New Member

    Sep 3, 2015
    8
    0
    Okay. Thank you.
     
Loading...