Converting a variable frequency to TTL HIGH and LOW levels, based on a fixed (possible non-fixed?) frequencySwitching relay with a PNP and TTL levelsMC34063A: Why am I overclocking this chip?Changing Low Frequency to High FrequencyDebouncing a TTL Photodiode DetectorHow to make a AC current source of variable low frequency and variable low amplitudeOp-amp for pulse width modulation, 5v single rail supplyConverting varying frequency and duty cycle to varying frequency fixed duty cycleState Variable Filter - Unexpected High Frequency BehaviorMulti PWM switch with mosfetAmplification of sinusoidal signal from filter circuit

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Converting a variable frequency to TTL HIGH and LOW levels, based on a fixed (possible non-fixed?) frequency


Switching relay with a PNP and TTL levelsMC34063A: Why am I overclocking this chip?Changing Low Frequency to High FrequencyDebouncing a TTL Photodiode DetectorHow to make a AC current source of variable low frequency and variable low amplitudeOp-amp for pulse width modulation, 5v single rail supplyConverting varying frequency and duty cycle to varying frequency fixed duty cycleState Variable Filter - Unexpected High Frequency BehaviorMulti PWM switch with mosfetAmplification of sinusoidal signal from filter circuit













1












$begingroup$


I have a square output from an LM324 op-amp (0 and 5 volts) which has a frequency between 1 to 500 Hz (yes, low frequencies). I would like to find a simple way to have HIGH TTL signal when it is over 50 Hz and a LOW when it is below that. It's not necessary to be so precise, but around 50 Hz will be fine. I know that I can do it by uCs, and I did it. However I have 1500 outputs which I should make a network from, and I would like to make it cheap and fast.



More info about the signal: The frequency is almost fixed at something above 50, say 150Hz or may be 250Hz or so, and the frequency variation is less than 10 Hz, then suddenly it will drop to less than 50, stays for a minute and again back up.



Finally: Is there a way I can control this 50Hz threshold by uC?










share|improve this question











$endgroup$











  • $begingroup$
    High pass filter?
    $endgroup$
    – Linkyyy
    Mar 16 at 20:14










  • $begingroup$
    nope! it is still a frequency over 50 Hz, I need a near-flat voltage at different levels below and over the treshold.
    $endgroup$
    – sina rahbari
    Mar 16 at 20:21










  • $begingroup$
    on in another word, simply: how to know if a frequency is higher or lower than a value? represent in 0 or 1, 0 or 5 volts?
    $endgroup$
    – sina rahbari
    Mar 16 at 20:24










  • $begingroup$
    A high pass filter sounds like a good start, since it turns any frequency lower than your value to (close to) zero.
    $endgroup$
    – Hearth
    Mar 16 at 20:29






  • 1




    $begingroup$
    Consider the 74LS123 retriggerable one-shot.
    $endgroup$
    – analogsystemsrf
    Mar 17 at 1:52















1












$begingroup$


I have a square output from an LM324 op-amp (0 and 5 volts) which has a frequency between 1 to 500 Hz (yes, low frequencies). I would like to find a simple way to have HIGH TTL signal when it is over 50 Hz and a LOW when it is below that. It's not necessary to be so precise, but around 50 Hz will be fine. I know that I can do it by uCs, and I did it. However I have 1500 outputs which I should make a network from, and I would like to make it cheap and fast.



More info about the signal: The frequency is almost fixed at something above 50, say 150Hz or may be 250Hz or so, and the frequency variation is less than 10 Hz, then suddenly it will drop to less than 50, stays for a minute and again back up.



Finally: Is there a way I can control this 50Hz threshold by uC?










share|improve this question











$endgroup$











  • $begingroup$
    High pass filter?
    $endgroup$
    – Linkyyy
    Mar 16 at 20:14










  • $begingroup$
    nope! it is still a frequency over 50 Hz, I need a near-flat voltage at different levels below and over the treshold.
    $endgroup$
    – sina rahbari
    Mar 16 at 20:21










  • $begingroup$
    on in another word, simply: how to know if a frequency is higher or lower than a value? represent in 0 or 1, 0 or 5 volts?
    $endgroup$
    – sina rahbari
    Mar 16 at 20:24










  • $begingroup$
    A high pass filter sounds like a good start, since it turns any frequency lower than your value to (close to) zero.
    $endgroup$
    – Hearth
    Mar 16 at 20:29






  • 1




    $begingroup$
    Consider the 74LS123 retriggerable one-shot.
    $endgroup$
    – analogsystemsrf
    Mar 17 at 1:52













1












1








1





$begingroup$


I have a square output from an LM324 op-amp (0 and 5 volts) which has a frequency between 1 to 500 Hz (yes, low frequencies). I would like to find a simple way to have HIGH TTL signal when it is over 50 Hz and a LOW when it is below that. It's not necessary to be so precise, but around 50 Hz will be fine. I know that I can do it by uCs, and I did it. However I have 1500 outputs which I should make a network from, and I would like to make it cheap and fast.



More info about the signal: The frequency is almost fixed at something above 50, say 150Hz or may be 250Hz or so, and the frequency variation is less than 10 Hz, then suddenly it will drop to less than 50, stays for a minute and again back up.



Finally: Is there a way I can control this 50Hz threshold by uC?










share|improve this question











$endgroup$




I have a square output from an LM324 op-amp (0 and 5 volts) which has a frequency between 1 to 500 Hz (yes, low frequencies). I would like to find a simple way to have HIGH TTL signal when it is over 50 Hz and a LOW when it is below that. It's not necessary to be so precise, but around 50 Hz will be fine. I know that I can do it by uCs, and I did it. However I have 1500 outputs which I should make a network from, and I would like to make it cheap and fast.



More info about the signal: The frequency is almost fixed at something above 50, say 150Hz or may be 250Hz or so, and the frequency variation is less than 10 Hz, then suddenly it will drop to less than 50, stays for a minute and again back up.



Finally: Is there a way I can control this 50Hz threshold by uC?







arduino operational-amplifier frequency ttl conversion






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Mar 16 at 22:07









SamGibson

11.5k41739




11.5k41739










asked Mar 16 at 20:09









sina rahbarisina rahbari

83




83











  • $begingroup$
    High pass filter?
    $endgroup$
    – Linkyyy
    Mar 16 at 20:14










  • $begingroup$
    nope! it is still a frequency over 50 Hz, I need a near-flat voltage at different levels below and over the treshold.
    $endgroup$
    – sina rahbari
    Mar 16 at 20:21










  • $begingroup$
    on in another word, simply: how to know if a frequency is higher or lower than a value? represent in 0 or 1, 0 or 5 volts?
    $endgroup$
    – sina rahbari
    Mar 16 at 20:24










  • $begingroup$
    A high pass filter sounds like a good start, since it turns any frequency lower than your value to (close to) zero.
    $endgroup$
    – Hearth
    Mar 16 at 20:29






  • 1




    $begingroup$
    Consider the 74LS123 retriggerable one-shot.
    $endgroup$
    – analogsystemsrf
    Mar 17 at 1:52
















  • $begingroup$
    High pass filter?
    $endgroup$
    – Linkyyy
    Mar 16 at 20:14










  • $begingroup$
    nope! it is still a frequency over 50 Hz, I need a near-flat voltage at different levels below and over the treshold.
    $endgroup$
    – sina rahbari
    Mar 16 at 20:21










  • $begingroup$
    on in another word, simply: how to know if a frequency is higher or lower than a value? represent in 0 or 1, 0 or 5 volts?
    $endgroup$
    – sina rahbari
    Mar 16 at 20:24










  • $begingroup$
    A high pass filter sounds like a good start, since it turns any frequency lower than your value to (close to) zero.
    $endgroup$
    – Hearth
    Mar 16 at 20:29






  • 1




    $begingroup$
    Consider the 74LS123 retriggerable one-shot.
    $endgroup$
    – analogsystemsrf
    Mar 17 at 1:52















$begingroup$
High pass filter?
$endgroup$
– Linkyyy
Mar 16 at 20:14




$begingroup$
High pass filter?
$endgroup$
– Linkyyy
Mar 16 at 20:14












$begingroup$
nope! it is still a frequency over 50 Hz, I need a near-flat voltage at different levels below and over the treshold.
$endgroup$
– sina rahbari
Mar 16 at 20:21




$begingroup$
nope! it is still a frequency over 50 Hz, I need a near-flat voltage at different levels below and over the treshold.
$endgroup$
– sina rahbari
Mar 16 at 20:21












$begingroup$
on in another word, simply: how to know if a frequency is higher or lower than a value? represent in 0 or 1, 0 or 5 volts?
$endgroup$
– sina rahbari
Mar 16 at 20:24




$begingroup$
on in another word, simply: how to know if a frequency is higher or lower than a value? represent in 0 or 1, 0 or 5 volts?
$endgroup$
– sina rahbari
Mar 16 at 20:24












$begingroup$
A high pass filter sounds like a good start, since it turns any frequency lower than your value to (close to) zero.
$endgroup$
– Hearth
Mar 16 at 20:29




$begingroup$
A high pass filter sounds like a good start, since it turns any frequency lower than your value to (close to) zero.
$endgroup$
– Hearth
Mar 16 at 20:29




1




1




$begingroup$
Consider the 74LS123 retriggerable one-shot.
$endgroup$
– analogsystemsrf
Mar 17 at 1:52




$begingroup$
Consider the 74LS123 retriggerable one-shot.
$endgroup$
– analogsystemsrf
Mar 17 at 1:52










2 Answers
2






active

oldest

votes


















1












$begingroup$

You have multiple alternatives, each one of them with different trade offs.



  • The conceptually simplest and most precise one, would be to use a pulse counter.

This is probably what you implemented with your micro controller. Gate a known fast clock with each cycle of the input signal and compare that count to a reference.



  • Translate that same idea to the analog domain. Using a ramp as the “counter”.

Start a known slew-rate ramp with an edge of the signal and capture the value on the next edge. Compare this value to a reference. Monolithic latching comparators make this simpler than it sounds.



  • Use a retriggerable monostable. With the duration being the comparison value.

A retriggerable monostable with a period of 20ms will remain triggered as long as the signal is above 50Hz, and will generate pulses when it’s below that. A second retriggerable monostable can “filter” these pulses into a continuous signal.



  • use a filter slope and a peak detector (just as some cheap FM radios)

A filter slope (high-pass or low-pass will do) attenuates different frequencies differently. By detecting the amplitude of the output signal you can compare to a threshold. The higher the filter slope the higher the frequency gain.



  • use a known pulse size to convert to PWM. (Frequency to voltage conversion).

By triggering a known-size pulse with each signal edge (with a monostable or just an edge detector) you convert the signal to PWM whose DC average will be proportional to the frequency. You can then use an analog comparator to detect the transition.



Given that you are talking of thousands of signals, any analog solution might be difficult to use reliably with a desired tolerance. Besides, you will need multiple components for each channel; at least 1 IC and multiple passives.



If you simply program a 14-pin microcontroller to implement just this function on several channels, you can use 1 IC for 5 channels or so. With much better precision, reliability, and repeatability. Even a 6-pin micro at 1 IC per channel would be smaller and simpler.






share|improve this answer









$endgroup$




















    1












    $begingroup$

    One way to solve this problem is to use a re-triggerable one-shot component. You said that absolute tolerance is not super critical and so the typical tolerances of a one-shot may be a suitable and simple means to detect the frequency shift of your signal.



    The one-shot needs to be setup with a R/C timing delay just at the timing period of your signal at 50Hz. This would be 20msec. As long as the input trigger signal at the one-shot stays above the 50Hz rate the output of the one-shot will stay re-triggered at a high level. When the input frequency drops below 50Hz the one-shot will start to timeout on each input pulse and its output will start to pulse with each cycle of the lower frequency input.



    You could possibly use this as is depending upon what you are using to detect the frequency changes or you could add some additional simple logic to convert the lower frequency pulsing output of the one-shot to a static signal envelope. Below I show an example of using a second re-triggerable one-shot to convert the output pulses of the first one shot to the envelope pulse. In this example the venerable 555 chip is shown being used for the re-triggerable one-shots.



    enter image description here



    enter image description here



    In this simulation the first 555 has its timing components (R3 and C4) set to detect just at 50 Hz. An input below 50Hz causes the output to start pulsing. The input shown here has switched to 25Hz. The second 555 chip has to have its timing components (R1 and C6) set for a delay time a little longer than the period of the lowest frequency expected for the input signal.



    If you decide to use this circuit concept it would be a good application of the NE556 dual 555 chip. The NE556 is only marginally more expensive than the NE555 so would be more economical to use in the build out of this circuit.






    share|improve this answer











    $endgroup$












    • $begingroup$
      Thanks, it looks great from the simulation. most probably i will use this idea.
      $endgroup$
      – sina rahbari
      Mar 17 at 13:54










    Your Answer





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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    1












    $begingroup$

    You have multiple alternatives, each one of them with different trade offs.



    • The conceptually simplest and most precise one, would be to use a pulse counter.

    This is probably what you implemented with your micro controller. Gate a known fast clock with each cycle of the input signal and compare that count to a reference.



    • Translate that same idea to the analog domain. Using a ramp as the “counter”.

    Start a known slew-rate ramp with an edge of the signal and capture the value on the next edge. Compare this value to a reference. Monolithic latching comparators make this simpler than it sounds.



    • Use a retriggerable monostable. With the duration being the comparison value.

    A retriggerable monostable with a period of 20ms will remain triggered as long as the signal is above 50Hz, and will generate pulses when it’s below that. A second retriggerable monostable can “filter” these pulses into a continuous signal.



    • use a filter slope and a peak detector (just as some cheap FM radios)

    A filter slope (high-pass or low-pass will do) attenuates different frequencies differently. By detecting the amplitude of the output signal you can compare to a threshold. The higher the filter slope the higher the frequency gain.



    • use a known pulse size to convert to PWM. (Frequency to voltage conversion).

    By triggering a known-size pulse with each signal edge (with a monostable or just an edge detector) you convert the signal to PWM whose DC average will be proportional to the frequency. You can then use an analog comparator to detect the transition.



    Given that you are talking of thousands of signals, any analog solution might be difficult to use reliably with a desired tolerance. Besides, you will need multiple components for each channel; at least 1 IC and multiple passives.



    If you simply program a 14-pin microcontroller to implement just this function on several channels, you can use 1 IC for 5 channels or so. With much better precision, reliability, and repeatability. Even a 6-pin micro at 1 IC per channel would be smaller and simpler.






    share|improve this answer









    $endgroup$

















      1












      $begingroup$

      You have multiple alternatives, each one of them with different trade offs.



      • The conceptually simplest and most precise one, would be to use a pulse counter.

      This is probably what you implemented with your micro controller. Gate a known fast clock with each cycle of the input signal and compare that count to a reference.



      • Translate that same idea to the analog domain. Using a ramp as the “counter”.

      Start a known slew-rate ramp with an edge of the signal and capture the value on the next edge. Compare this value to a reference. Monolithic latching comparators make this simpler than it sounds.



      • Use a retriggerable monostable. With the duration being the comparison value.

      A retriggerable monostable with a period of 20ms will remain triggered as long as the signal is above 50Hz, and will generate pulses when it’s below that. A second retriggerable monostable can “filter” these pulses into a continuous signal.



      • use a filter slope and a peak detector (just as some cheap FM radios)

      A filter slope (high-pass or low-pass will do) attenuates different frequencies differently. By detecting the amplitude of the output signal you can compare to a threshold. The higher the filter slope the higher the frequency gain.



      • use a known pulse size to convert to PWM. (Frequency to voltage conversion).

      By triggering a known-size pulse with each signal edge (with a monostable or just an edge detector) you convert the signal to PWM whose DC average will be proportional to the frequency. You can then use an analog comparator to detect the transition.



      Given that you are talking of thousands of signals, any analog solution might be difficult to use reliably with a desired tolerance. Besides, you will need multiple components for each channel; at least 1 IC and multiple passives.



      If you simply program a 14-pin microcontroller to implement just this function on several channels, you can use 1 IC for 5 channels or so. With much better precision, reliability, and repeatability. Even a 6-pin micro at 1 IC per channel would be smaller and simpler.






      share|improve this answer









      $endgroup$















        1












        1








        1





        $begingroup$

        You have multiple alternatives, each one of them with different trade offs.



        • The conceptually simplest and most precise one, would be to use a pulse counter.

        This is probably what you implemented with your micro controller. Gate a known fast clock with each cycle of the input signal and compare that count to a reference.



        • Translate that same idea to the analog domain. Using a ramp as the “counter”.

        Start a known slew-rate ramp with an edge of the signal and capture the value on the next edge. Compare this value to a reference. Monolithic latching comparators make this simpler than it sounds.



        • Use a retriggerable monostable. With the duration being the comparison value.

        A retriggerable monostable with a period of 20ms will remain triggered as long as the signal is above 50Hz, and will generate pulses when it’s below that. A second retriggerable monostable can “filter” these pulses into a continuous signal.



        • use a filter slope and a peak detector (just as some cheap FM radios)

        A filter slope (high-pass or low-pass will do) attenuates different frequencies differently. By detecting the amplitude of the output signal you can compare to a threshold. The higher the filter slope the higher the frequency gain.



        • use a known pulse size to convert to PWM. (Frequency to voltage conversion).

        By triggering a known-size pulse with each signal edge (with a monostable or just an edge detector) you convert the signal to PWM whose DC average will be proportional to the frequency. You can then use an analog comparator to detect the transition.



        Given that you are talking of thousands of signals, any analog solution might be difficult to use reliably with a desired tolerance. Besides, you will need multiple components for each channel; at least 1 IC and multiple passives.



        If you simply program a 14-pin microcontroller to implement just this function on several channels, you can use 1 IC for 5 channels or so. With much better precision, reliability, and repeatability. Even a 6-pin micro at 1 IC per channel would be smaller and simpler.






        share|improve this answer









        $endgroup$



        You have multiple alternatives, each one of them with different trade offs.



        • The conceptually simplest and most precise one, would be to use a pulse counter.

        This is probably what you implemented with your micro controller. Gate a known fast clock with each cycle of the input signal and compare that count to a reference.



        • Translate that same idea to the analog domain. Using a ramp as the “counter”.

        Start a known slew-rate ramp with an edge of the signal and capture the value on the next edge. Compare this value to a reference. Monolithic latching comparators make this simpler than it sounds.



        • Use a retriggerable monostable. With the duration being the comparison value.

        A retriggerable monostable with a period of 20ms will remain triggered as long as the signal is above 50Hz, and will generate pulses when it’s below that. A second retriggerable monostable can “filter” these pulses into a continuous signal.



        • use a filter slope and a peak detector (just as some cheap FM radios)

        A filter slope (high-pass or low-pass will do) attenuates different frequencies differently. By detecting the amplitude of the output signal you can compare to a threshold. The higher the filter slope the higher the frequency gain.



        • use a known pulse size to convert to PWM. (Frequency to voltage conversion).

        By triggering a known-size pulse with each signal edge (with a monostable or just an edge detector) you convert the signal to PWM whose DC average will be proportional to the frequency. You can then use an analog comparator to detect the transition.



        Given that you are talking of thousands of signals, any analog solution might be difficult to use reliably with a desired tolerance. Besides, you will need multiple components for each channel; at least 1 IC and multiple passives.



        If you simply program a 14-pin microcontroller to implement just this function on several channels, you can use 1 IC for 5 channels or so. With much better precision, reliability, and repeatability. Even a 6-pin micro at 1 IC per channel would be smaller and simpler.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Mar 16 at 21:31









        Edgar BrownEdgar Brown

        6,8692837




        6,8692837























            1












            $begingroup$

            One way to solve this problem is to use a re-triggerable one-shot component. You said that absolute tolerance is not super critical and so the typical tolerances of a one-shot may be a suitable and simple means to detect the frequency shift of your signal.



            The one-shot needs to be setup with a R/C timing delay just at the timing period of your signal at 50Hz. This would be 20msec. As long as the input trigger signal at the one-shot stays above the 50Hz rate the output of the one-shot will stay re-triggered at a high level. When the input frequency drops below 50Hz the one-shot will start to timeout on each input pulse and its output will start to pulse with each cycle of the lower frequency input.



            You could possibly use this as is depending upon what you are using to detect the frequency changes or you could add some additional simple logic to convert the lower frequency pulsing output of the one-shot to a static signal envelope. Below I show an example of using a second re-triggerable one-shot to convert the output pulses of the first one shot to the envelope pulse. In this example the venerable 555 chip is shown being used for the re-triggerable one-shots.



            enter image description here



            enter image description here



            In this simulation the first 555 has its timing components (R3 and C4) set to detect just at 50 Hz. An input below 50Hz causes the output to start pulsing. The input shown here has switched to 25Hz. The second 555 chip has to have its timing components (R1 and C6) set for a delay time a little longer than the period of the lowest frequency expected for the input signal.



            If you decide to use this circuit concept it would be a good application of the NE556 dual 555 chip. The NE556 is only marginally more expensive than the NE555 so would be more economical to use in the build out of this circuit.






            share|improve this answer











            $endgroup$












            • $begingroup$
              Thanks, it looks great from the simulation. most probably i will use this idea.
              $endgroup$
              – sina rahbari
              Mar 17 at 13:54















            1












            $begingroup$

            One way to solve this problem is to use a re-triggerable one-shot component. You said that absolute tolerance is not super critical and so the typical tolerances of a one-shot may be a suitable and simple means to detect the frequency shift of your signal.



            The one-shot needs to be setup with a R/C timing delay just at the timing period of your signal at 50Hz. This would be 20msec. As long as the input trigger signal at the one-shot stays above the 50Hz rate the output of the one-shot will stay re-triggered at a high level. When the input frequency drops below 50Hz the one-shot will start to timeout on each input pulse and its output will start to pulse with each cycle of the lower frequency input.



            You could possibly use this as is depending upon what you are using to detect the frequency changes or you could add some additional simple logic to convert the lower frequency pulsing output of the one-shot to a static signal envelope. Below I show an example of using a second re-triggerable one-shot to convert the output pulses of the first one shot to the envelope pulse. In this example the venerable 555 chip is shown being used for the re-triggerable one-shots.



            enter image description here



            enter image description here



            In this simulation the first 555 has its timing components (R3 and C4) set to detect just at 50 Hz. An input below 50Hz causes the output to start pulsing. The input shown here has switched to 25Hz. The second 555 chip has to have its timing components (R1 and C6) set for a delay time a little longer than the period of the lowest frequency expected for the input signal.



            If you decide to use this circuit concept it would be a good application of the NE556 dual 555 chip. The NE556 is only marginally more expensive than the NE555 so would be more economical to use in the build out of this circuit.






            share|improve this answer











            $endgroup$












            • $begingroup$
              Thanks, it looks great from the simulation. most probably i will use this idea.
              $endgroup$
              – sina rahbari
              Mar 17 at 13:54













            1












            1








            1





            $begingroup$

            One way to solve this problem is to use a re-triggerable one-shot component. You said that absolute tolerance is not super critical and so the typical tolerances of a one-shot may be a suitable and simple means to detect the frequency shift of your signal.



            The one-shot needs to be setup with a R/C timing delay just at the timing period of your signal at 50Hz. This would be 20msec. As long as the input trigger signal at the one-shot stays above the 50Hz rate the output of the one-shot will stay re-triggered at a high level. When the input frequency drops below 50Hz the one-shot will start to timeout on each input pulse and its output will start to pulse with each cycle of the lower frequency input.



            You could possibly use this as is depending upon what you are using to detect the frequency changes or you could add some additional simple logic to convert the lower frequency pulsing output of the one-shot to a static signal envelope. Below I show an example of using a second re-triggerable one-shot to convert the output pulses of the first one shot to the envelope pulse. In this example the venerable 555 chip is shown being used for the re-triggerable one-shots.



            enter image description here



            enter image description here



            In this simulation the first 555 has its timing components (R3 and C4) set to detect just at 50 Hz. An input below 50Hz causes the output to start pulsing. The input shown here has switched to 25Hz. The second 555 chip has to have its timing components (R1 and C6) set for a delay time a little longer than the period of the lowest frequency expected for the input signal.



            If you decide to use this circuit concept it would be a good application of the NE556 dual 555 chip. The NE556 is only marginally more expensive than the NE555 so would be more economical to use in the build out of this circuit.






            share|improve this answer











            $endgroup$



            One way to solve this problem is to use a re-triggerable one-shot component. You said that absolute tolerance is not super critical and so the typical tolerances of a one-shot may be a suitable and simple means to detect the frequency shift of your signal.



            The one-shot needs to be setup with a R/C timing delay just at the timing period of your signal at 50Hz. This would be 20msec. As long as the input trigger signal at the one-shot stays above the 50Hz rate the output of the one-shot will stay re-triggered at a high level. When the input frequency drops below 50Hz the one-shot will start to timeout on each input pulse and its output will start to pulse with each cycle of the lower frequency input.



            You could possibly use this as is depending upon what you are using to detect the frequency changes or you could add some additional simple logic to convert the lower frequency pulsing output of the one-shot to a static signal envelope. Below I show an example of using a second re-triggerable one-shot to convert the output pulses of the first one shot to the envelope pulse. In this example the venerable 555 chip is shown being used for the re-triggerable one-shots.



            enter image description here



            enter image description here



            In this simulation the first 555 has its timing components (R3 and C4) set to detect just at 50 Hz. An input below 50Hz causes the output to start pulsing. The input shown here has switched to 25Hz. The second 555 chip has to have its timing components (R1 and C6) set for a delay time a little longer than the period of the lowest frequency expected for the input signal.



            If you decide to use this circuit concept it would be a good application of the NE556 dual 555 chip. The NE556 is only marginally more expensive than the NE555 so would be more economical to use in the build out of this circuit.







            share|improve this answer














            share|improve this answer



            share|improve this answer








            edited Mar 17 at 3:12

























            answered Mar 16 at 20:49









            Michael KarasMichael Karas

            44.9k348104




            44.9k348104











            • $begingroup$
              Thanks, it looks great from the simulation. most probably i will use this idea.
              $endgroup$
              – sina rahbari
              Mar 17 at 13:54
















            • $begingroup$
              Thanks, it looks great from the simulation. most probably i will use this idea.
              $endgroup$
              – sina rahbari
              Mar 17 at 13:54















            $begingroup$
            Thanks, it looks great from the simulation. most probably i will use this idea.
            $endgroup$
            – sina rahbari
            Mar 17 at 13:54




            $begingroup$
            Thanks, it looks great from the simulation. most probably i will use this idea.
            $endgroup$
            – sina rahbari
            Mar 17 at 13:54

















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