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From: "Terry Moreau"
Subject: Re: what is this type of oscillator?
Date: Tue, 31 Dec 2002 21:29:18 -0800
Organization: Posted via Supernews, http://www.supernews.com
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"Dave" wrote in message
> I want to make an oscillator that has a frequency of 2500hz but the width
> the pulse should be 200 or 300 microseconds. what is this type of
> oscillator called and what is the simplest way to implement it? it's for
> muscle stimulator.
Sounds like you want a variable duty-cycle oscillator. A 555 IC circuit
example is given in with most 555 datasheets and/or app notes. You may also
need a power transistor driver before the step-up transformer.
I did a muscle stimulator cicuit one time and found that waveform envelope
control was also quite important for optimal patient comfort and muscle
response. A gentle attack and prompt decay seems to better "prepare" the
muscle to contract and reduce patient discomfort and stress. I recall
frequency of the AC was not a big issue but if too low (below 100Hz there
was more discomfort felt). Amplitude, envelope, and duration were all quite
critical. Also the contact area, placement, and dispersive conductivity of
the body pads make a huge difference. I considered having a special section
of the pads isolated and used as feedback of actual pad voltage, and to
monitor output current, thus having the circuit regulate actual pad voltage
and/or control the source impedance of the high voltage AC but I never got
around to putting the idea into practice.
Muscle stimulation requires a threshold level to be exceeded to create
actual contraction but also staying below patient discomfort levels. A
muscle contracted too strongly will cause pain and the high voltage AC
applied too quick or too slow or at the wrong level can also cause
complaints of too much electrical tingling feeling. An over the counter
pain medication such as Tylenol or Advil can help reduce discomfort during
treatments. A good stiff drink is another option!
You could vary the dutytime from 200-300us as you propose, however in my
circuit I left it fixed and varied the DC voltage to the final driver stage
instead. That was controlled by a second 555 circuit that let me adjust
repetition rate, duration, attack/decay times, and amplitude. In the end
the box looking like a multi-track mixing console, full of knobs!
Such circuits should have some sort of fail-safes. NEVER use these devices
where they can shoot a path directly through or near the heart or head or
internal vital organs!
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