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NA555 Timers

Jeffery Mize
Prodigy 120 points


I am trying to cascade two timers to produce a 33KHZ burst. 1ms pulse width 10ms rep. The application is for an IR emitter and a proximity sensor with an Automatic Gain Control circuit. The first timer is designed for Astable operation to produce the 33KHZ signal that's done the problem is feeding that signal in to the 2nd cascaded timer. What I have done to date is to design using Monostable operation, and according to the data sheet I selected the RA to be 1M Ohm and C is .01uf and I also used C= .1uf. My output rep rate is 10ms which is what I am looking for however when I expand the pulse its a 12us pulse, I need that pulse to be 1ms. If I change RA to give me a 1ms pulse width, I loose the 10ms rep rate. I may not have this configured correctly, can you help me with this problem. All I am trying to do is to cascade of two 555 timers to generate a burst pattern at 33kHz signal for 1ms, an OFF period for 10ms. Thank you.

  • 0
    TI__Guru**** 168766 points
    Jeffery,

    How to connect depends on whether the 1mS and 33kHz need to be (phase) synchronized in any way.
    Do you prefer a dead time output low of high? Does the 33kHz need a specific duty cycle?
    You can add a steering diode if you want a low time to be greater than high output time.
  • 0 in reply to Ron Michallick
    Prodigy 120 points

    Hey Ronald,
    This pulse is going to an IR emitter. (TSAL6200) The carrier frequency needs to be contained within the 1mS pulse. At the end of the pulse there should be a 10mS dead time before the next pulse in order for the AGC circuit inside the the TSOP4833 to settle. The AGC of the TSOP4833 is evaluating any continuous signals like noise and will reduce the gain which in turn affects that output. The 33KHZ frequency does not need any specific duty cycle, would that steering diode need to be connected between the DISCH (7) and TRIG (2) or between the DISCH (7) and THRES (6). The first timer (Astable operation) to get the 33KHZ frequency 61% duty cycle, that's already designed. Thanks.

  • 0 in reply to Jeffery Mize
    TI__Guru**** 168766 points
    Jeffery,

    For the gating oscillator, the diode can be anode to pin 7 and cathode to pin 6. Charge time (output high) will be set by just RA and discharge time will be set by RB (as always). The output of this oscillator can drive the reset pin of the 33KHz oscillator.
  • 0 in reply to Ron Michallick
    Prodigy 120 points
    Hey Ronald,
    So am I cascading 2 555 timers in Astablle operation? Right now I have it configured the first timer 33KHZ Astable operation feeding the second timer Monostable operation, that's how I got the 10mS burst.
  • 0 in reply to Jeffery Mize
    TI__Guru**** 168766 points
    Jeffery,

    Interesting. I wouldn't think it work work that way. If it does work then go ahead and use it.

    Otherwise try my suggestion, 11mS (9.1% DC) first oscillator turning on and off the second oscillator 33KHz.
  • 0 in reply to Ron Michallick
    Prodigy 120 points
    Hey Ronald,
    No it doesn't work that's why I requested help. I could set up the 2nd timer (Monostable operation) for the 1mS burst but when I expanded the pulse it was 15us and I was not able to adjust it. Adjusting RA would control the burst rate only and I could not widen the pulse width to 1mS. So I think your method will work so I will follow your advise and use 2 555 timers and configure them for Astable operation. Thank you for all your efforts it is appreciated, and If I have any other questions I will let you know.

    Regards,

    Jeffery Mize