Repetitive capacitor charge/discharge circuit...

Discussion in 'General Electronics Chat' started by Externet, Jun 8, 2018.

  1. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    What considerations to observe ? Need 1 Hz, sharkfin waveform charging/ discharging oscillator.
    3.3V supply, around 100mA load solenoid to move a lever...

    A 7555 timer could work on low voltage, and has a pin with the proper waveform; perhaps driving a transistor to feed the solenoid.
    Anything I should be aware of ? Needs to be highly reliable.
     
  2. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    What are the required voltages for the upper and lower peaks of the waveform across the solenoid? Anything that drives an R-C network with a square wave will yield the "shark fin" (exponential triangle wave) waveform.

    Are you saying the solenoid also is running on 3.3 V and needs 100 mA, or 100 mA from some other (higher) supply voltage?

    ak
     
  3. Externet

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    Nov 29, 2005
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    Thanks.
    Yes, the 100mA draw tiny solenoid to be driven by 'sharkfin' waveform at around 3V peak, 0.3V valley, obtained from the threshold pin of a 7555. The overall supply is 3.3V for everything.

    +3.3V-------------astable7555-------------thresholdpin------------buffertransistor on 3.3V-----------solenoid

    The motion of the lever attracted by the solenoid to mechanically follow (move, deflect) proportionally to a 'sharkfin' waveform
     
    Last edited: Jun 8, 2018
  4. Tonyr1084

    Distinguished Member

    Sep 24, 2015
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    Just to be sure you understand that the shark fin wave will not be rail to rail. Typically (and not specifically) the voltage will swing between 1/3 and 2/3 of the rail. So on a 3.3 volt system, the peak of the wave will be 2.2 volts and the valley of the wave will be 1.1 volts. Something LIKE this. Likely not exactly, but similar in nature:

    Shark Wave.jpg
     
  5. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    Thank you.
    Yes, those are the limits the 7555 has on the threshold. That pin will not be rail-to-rail. A buffer transistor fed by 3.3V may get closer to rail when 'high' threshold is 2.2V, minus CE saturation voltage. I think.
     
  6. AnalogKid

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    Sounds like what you need is a rail-to-rail output stage opamp with a little non-inverting gain, that also can make 100 mA of output current, all running on +3.3 V and GND. That could be difficult.

    but ...

    If you are driving the solenoid single-ended (one end to the circuit output, the other end to GND), AND (big and) you don't need a true totem-pole output stage that pulls the solenoid toward GND on the decreasing half-cycles, then things get a bit more simple. I'm thinking of a R-R opamp driving a PNP transistor with the solenoid from the collector to GND and negative feedback taken from the collector. I'll try to draw a schematic tomorrow. Hmmm ...

    ak
     
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