# Need help with muscle stimulator project

Discussion in 'The Projects Forum' started by Armita, Sep 4, 2016.

1. ### Armita Thread Starter New Member

Sep 4, 2016
2
0
hello evryone
I want to build a muscle stimulator for a school project, ( I attached the source circuits )
but i decided to change it a little, instead of a signal generator part i want to be able to connect it to an audio source ( like an mp3 player or a computer ) ,
but the output signal become very weak for frequencies under 1000hz and very strong for freqs over 2k,3khz.
tens unit frequencies ranges between 20 to 500hz, i don't understand what the problem is, any idea what should i change/add to make it work for 100 to 1000hz maybe .
thanks

thanks.

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2. ### jpanhalt Expert

Jan 18, 2008
6,314
1,189
You don't need the resistor in the source lead of the mosfet. That could affect mosfet behavior more at low frequencies.

John

3. ### Armita Thread Starter New Member

Sep 4, 2016
2
0
thanks John for quick reply, sorry i forgot to mention that i already did remove the resistor, didn't make that much difference.

4. ### DickCappels Moderator

Aug 21, 2008
4,918
1,493
the drain circuit is a high pass filter. To get better low frequency response you will need to increase the inductance of your primary and/or drive it with negative resistance to compensate for the wining's resistance thereby increasing L/R to get a lower cutoff frequency.

Do you realize that your circiut is extremely non-linear?

5. ### wayneh Expert

Sep 9, 2010
14,786
5,276
In the BJT circuit, what's the base resistor value? You may be overloading the output of the audio amp.

One way to do this circuit would be to use a comparator to square the incoming waveform. The intensity of the output pulse depends on the rate of change of current in the coil, so at low frequency you are getting very low dI/dt in the coil and therefore very little pulse. If every wave is square, the intensity should be more-or-less independent of frequency.

Have you tried a square wave out of your signal generator? Many free apps can give you square, triangle or sine waves at a selectable frequency.