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Using a 555 timer how do we output a biphasic square wave instead of a monophasic? Is there any way to make it asymmetrical? If not are there any other devices or circuits to use to make an asymmetric biphasic output waveforms?

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  • \$\begingroup\$ Have you tried dropping the ground? \$\endgroup\$ Feb 7, 2014 at 2:25
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    \$\begingroup\$ What, exactly, is a "biphasic" waveform? It sounds like a medical term, but google is just returning stuff about EEG/EKGs. Please define exactly what you want. \$\endgroup\$ Feb 7, 2014 at 12:13

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555 it is a legend bi or monostable oscillator with a quite sufficient accuracy (at around 1%~0,1%) and with stable operation. This general characteristics plus the low price mace this circuit to survive for decades. All this years of using, lot of “tricks” developed by the people they doing what they want it to.

But it is not a function generator and has it's limitations.

Maximum operating frequency is 300kHz - 500kHz

Duty cycle adjustment is not between 0% and 100% but it is within reasonable range (around 5% or higher and 95%). This can improve if you are use a 556: One to set frequency and the second as a one shot of variable width (duty cycle). But the same you can do with two opamp, at the same cost of 555!

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The output of the 555 can be 0.5v to 1.5v above ground, so a trick is how to get a bipahse signal is to use a capacitor network and some load.

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I can reccomend a good function generator that can build around an 8038 chip

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  • \$\begingroup\$ Please refer me to function generator built around 8083 chip please. I am trying to make a biphasic asymmetric waveform pulse for use in a muscle stimulation device. Thank you! \$\endgroup\$
    – user36793
    Feb 8, 2014 at 19:15
  • \$\begingroup\$ @user36793 I see...so my reccomendation it is too much for you since this dealing with a bench function generator for use by electronics enginears. In your application needs an adjustable high voltage/low current output. In this case a kind of output amplification and a transformer required. If the frequency you need is around 100Hz, then you can use an ordinary transformer i.e 200V/12V in reverse connection. Look here webcache.googleusercontent.com/… \$\endgroup\$
    – GR Tech
    Feb 8, 2014 at 20:23
  • \$\begingroup\$ ...or this one mediafire.com/download/qanaffac3qhbxmh/… \$\endgroup\$
    – GR Tech
    Feb 8, 2014 at 22:23
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The short answer is you can't. At least not with a single 555 alone .

The fundamental issue is that at the heart of a 555 is a flip flop which has two states, and you need three states for a biphasic square wave.

There are lots of ways to get another state by adding additional components to your 555. Probably the simplest is to just use a second 555 (or a 556).

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I would setup a 556 to provide two pulses. Setup one half as a the first pulse and have it be astable. Setup the other to be monostable triggering off the first. Then get a motor driver chip or build a motor bridge to control a step up transformer. The first pulse turns on the driver chip/motor bridge in one direction and the second pulse turns on driver chip/motor bridge in the other direction.

This might be simpler to use a small microcontroller for this as you can control timings and enable your transformer driving circuitry with more accuracy.

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