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LM555: 5 minute equal duration timer switch.

Christopher James
Intellectual 880 points
Part Number: LM555
Other Parts Discussed in Thread: LM3886, TMUX6119, , CD4024B, TPA3116D2, UAF42

I am designing a circuit as part of my research on alternative strategies for overcoming cancer. The design consists of two oscillating circuits (already designed using programmable crystals and ripple-counters) producing the main frequency and a lower harmonic at typically 1.58 volts DC output. These are then passed through UAF42: Band Pass Filters to convert into sine-waves, the design of which has been so kindly created for me by kai klaas69. The outputs would then pass through an LM3886 amplifier to an impedance of 175 to 200 ohms.

 

The aim is to switch from one frequency to another at an interval of 5 minutes continuously. I am proposing that the Band Pass Filters are powered continuously from a parallel +/- 15volt supply. The two oscillator circuits are switched on at 5-minute intervals using a combination of a TMUX6119 switch controlled via an Astable LM555 circuit. The output from LM555 would go to SEL Pin8 on the TMUX6119, the 15 volts + supply would go to D Pin6, the 15 volts + output to the low-frequency circuit would be connected to SB Pin5 and the 15 volts + output to the high-frequency circuit would go to SA Pin 8.

 

As my knowledge of electronics is very limited, I would be extremely grateful if it was possible to evaluate the values of Ra Rb and Rl for the LM555, please? I also assume that the ouput from LM555 is at Pin3?

 

Thank you so very, very much.

 

CJJ

 

  • 0
    TI__Guru**** 156616 points

    Hello CCJ,

    It that 5 minutes of one oscillator then 5 minutes of the other and repeat that indefinitely?

    5 minutes = 300 seconds; so 300 = 0.693 * RB * C ;  RB =921k, C =470uF (is one option),  RA just needs to be small compared to RB so it can be 4.7k for example. RL is optional and should not be needed in this case.

    I assume the 5 minute period selection is somewhat arbitrary, so the tolerance of the capacitor won't be critical. 

  • 0 in reply to Ron Michallick
    Intellectual 880 points

    Dear Ronald,

    I cannot thank you enough!

    When you say the two resistors Rl are optional, what happens about the link between Pin8, Pin3 and Grnd, please?

    Once again, I am most sincerely grateful,

    Christopher

    PS Yes the five minute timing is not critical. If I wished to make the timing adjustable, could I replace RA with a multiturn Potentiometer of say 1k to 2k Ohms?

  • 0 in reply to Christopher James
    TI__Guru**** 156616 points

    Christopher,

    Pin 3 can drive low and high without an RL resistor. See the data sheet VOL and VOH specifications.

    For multi turn potentiometer tuning, put pot in series with RB; You could make the POT 1M ohms and RB 422k. This will allow a wide range of time periods. However, tuning very slow clock will be a tedious and time consuming task.

    The basic design has one interesting side effect. On initial power up, the first value for (output pin 3) is high and this high time will be 58% longer (7.9 minutes) than subsequent low and high times that are 5 minutes. The reason is that timing capacitor starts at 0V for first high time and starts at 1/3 VCC for every other high time.

    If this first longer high time is an issue or you need a way to reset the timer, then I can provide a more complicated timer circuit to address that. Another option is to use a counter like CD4024B to divide the LM555 output by 128. This way the timer could run at 4.69Hz which would be way easier to tune than a 10 minute clock (5 min per output state). The first LM555 longer high output effect would be greatly reduced (by 64:1)

     

  • 0 in reply to Ron Michallick
    Intellectual 880 points

    Dear Ronald,


    You will no doubt have gathered that this complete circuit is to assist cancer sufferers to self-heal, and thus once switched on, is most likely to be left-on.

    Whilst, I have suggested that I might wish to adjust the time interval of 5 minutes, at this moment in time, that is an unlikely scenario.

    The most important criteria are reliability, and if you believe that you have a more stable, accurate and reliable circuit that would achieve these, I would be most sincerely grateful for your kind guidance.


    Thank you, once again.



    Christopher

  • 0 in reply to Christopher James
    TI__Guru**** 156616 points

    Christopher,

    The simple timer circuit will work to run the switch because the timer should be left on and there is no known significance to 5 minute period value. The output will change periodically so both modes can be used alternately.

  • 0 in reply to Ron Michallick
    Intellectual 880 points

    Dear Ronald,

    I am certain I have queried this previously but do I still need to interconnect between Pins 8, 3 and 1 to Ground, if RL is not required?

    Thank you so much.

    Christopher

  • 0 in reply to Christopher James
    TI__Guru**** 156616 points

    No interconnects needed. Just connect pin 3 to the select pin on the analog switch IC. Ground goes to ground. VCC+ can use same positive supply that the analog switch uses. 

  • 0 in reply to Ron Michallick
    Intellectual 880 points

    Dear Ronald,


    Thank you so much for the clarification.


    I am very aware that so often the advice given, albeit with good intention, lacks the attention to detail that you have kindly afforded me.
    I would not want you to think therefore, that I am not truly grateful, for the attention you have generously given me. Thank you so much!


    My frequency generating circuit is very simple, utilising a voltage reduction to 3.3 volts, a programmable Epson Crystal - Q33519E000002 SG-8003CE and a Nexperia 74HC4060D,652 14-stage Binary Counter, outputting +1.58 volts DC at the required frequency.


    Originally this triggered an N-Channel MOSFET, 9.7 A, 100 V, 3-Pin D2PAK Infineon IRF520NSTRLPBF, enabling the 1.58-volt frequency pulse to be reconstructed back to a pulsing 0-volt ground.


    However, if my assumption that a higher voltage output would ripple down the line, is correct, can you suggest better than the alternative Panasonic 250 mA SPNO Solid State Relay, PCB Mount, PhotoMOS, that I currently use to trigger the +15volt supply. I have found that it is very susceptible to short-circuiting etc. Until HM the Queen signs off on my equipment funding, my prototypes are built by me by hand.


    Then there is the question of the LM4766T amplifier, as this is an old device, would I be better using one of the modern amplifiers from the TPA3116D2 range?


    And finally, as I start to prepare circuits for the other components similar to that which I have for the UAF42, would this connection be correct for me of the TMUX6119, as appended below?


    Once again, I cannot thank you enough for your kind, detailed advice.


    Christopher