Electrostimulator

Electrostimulator

Is a dispositive that apply a stimulus on a nerve motor to generate the action potencial to contract and excite muscles, lettting treat with pain, strengthening of muscles, supply medicines, healing of injuries and relaxation of muscles.

In this design is created a continuos stimulus with bipolar current, low current and frecuency and a output between 0-70V AC.

Image 1. Diagram of how work the electroestimation in muscles.

Through differents types of electrostimulation, here the circuit is developed according to TENS mode (Transcutaneous Electrostical Nerve Stimulation) which consist on generate a square wave (bipolar) in a range in time of 40-200us (50-150Hz).  In this case, we think on treat only with pain and therapy consist on session of 5 minutes caused tingling or high intensity sensations in muscles and a variable of fast analgesia.

Image 2. Kinds of signals used in electrostimulation.

The design of circuit to generated that wave (square) is developed with the model of a astable multivibrator timer (normally constructed with a 555 integred timer) with the next way:

Image 3. Schematic circuit of 555 astable multivibrator timer.

Image 4. Waveform generated

So, putting that Vcc=10V, Frecuency=150Hz, C=0.01uF and RA=10k Ohms, RB is:

The same equation is used to calculate with 50Hz and RB is 1,4MΩ aproximately, so RB is a potenciometer of 1MΩ that generate a wave between 72Hz- to 14kHz but this frecuency is limited with a alarms in the final dispositive.

The next stage is the amplification of signal which is make with another potentiometer and a little stage of power how you can see in the next image: 

So to the output of 555 timer (pin 3) is connected a 47uF capacitor with a potentiometer of 10kΩ and this amplify the signal since 0.75 V to 2-4 V.  After with a power transistor as TIP41C the current is amplified too (because of 55 is approximately 200mA) and with a transformer of 6V to 110-120V AC signal is amplified a lot by inductance.  The final signal have 0-80 VAC and the current in patient (according to galvanic resistance of human body and class of skin) is about 100-500uA.