Instead of a preface, we will give several excerpts from the author’s methodology [1-2]:

The device presented here differs from the author’s [1-2] in its more modern element base, which made it possible to expand some of its capabilities, make the device more economical and require fewer recharges. This was achieved using a microcontroller and a full bridge driver, replacing the unreliable mechanical relay. Instead of three 9V batteries, the device uses a Li-ion battery, which is recharged using a standard inexpensive board powered by a USB connector. This allows you to recharge the device from a standard 5V telephone adapter, or from any computer. Also, in this version of the device you can set the session time, receive sound and light notifications for some important events.

The device was developed by the author taking into account the requirements of the FDA TENS and CES, as well as the necessary conditions under which the device is not allowed to be powered from the mains when its contacts are connected to the human body. Therefore, the device is powered by a built-in battery. This mode also allows you to avoid exposure to harmful network interference on the body.

The main technical characteristics of the device are as follows:

• Voltage at the electrodes is 27 V;
• The frequency of polarity change on the electrodes is 3.8 Hz;
• Adjustable current on the electrodes - from 0.3 mA to 27 mA;
• Visual control of the current supplied to the electrodes using the blinking green LED - the higher the current, the higher the blinking frequency;
• Complete decoupling of the device from the network - the device is powered by a built-in battery;
• Battery charging from a standard USB connector (5V);
• Control of battery discharge and recharging using special LEDs;
• Set the duration of the treatment session: 15, 20, 45 or 60 minutes and control it using a blue LED;
• After the end of the session, the voltage on the electrodes is removed and a sound signal is given.

The schematic diagram of the device is shown in Figure 1. After turning on the device with a switch located in the same housing with resistor R1, power from the battery is supplied to U1 - a 5V step-down voltage converter, power supply microcontroller DD1, and to the boost converter circuit, consisting of switch Q1, inductor L1, and rectification circuit D1-D2, C3-C4. The latter, using the internal generator of the microcontroller DD1, feedback through R11-R12 and a software stabilization circuit, generates a voltage of 28V, which is supplied to the dual driver U1. The driver is controlled by the same microcontroller, and signals with an amplitude of 27V and a frequency of 3.8Hz appear at its outputs. Thanks to this, every 0.26 seconds the polarity at the outputs X1 and X2 changes, as required for the method [1-2]. After the end of the treatment program, these pins are switched DD1 to the off state, and the buzzer BUZZ1 emits a sound signal.

Also, a sound signal sounds when the device is turned on and when the SW1 button is pressed. By pressing this button you can set the time of one session: 15, 30, 45 or 60 minutes. This will be signaled by LED3, blinking in batches of 1, 2, 3 or 4 times, respectively. If you do not press the button, then after turning on the device, the treatment session is not limited in time, and LED3 lights up constantly.

LED2 starts flashing at a certain frequency when the electrodes are connected to X1-X2, and to the patient's body. The optimal current through the electrodes is set by variable resistor R1, and the greater this tok, the more often LED2 will flash. This point is needed for visual control of the current by the doctor conducting the session. The current is found by the microcontroller by calculating the voltage difference across R4 and R6. At the same time, the circuit of resistances R2-R6 and R13 represents a differential bridge for the possibility of such measurement. The balance of the bridge is adjusted by trimming resistor R13.

The microcontroller also monitors the voltage on the battery using the R7R8C7 chain, and, when it decreases to a certain value, it signals this by blinking LED1. After this, the operator needs to recharge the device by connecting an external power source (5V adapter or USB connector of a computer) to the U3 module. To control the charge, there is a fourth LED located on the module itself. A very important factor is the stability of all device parameters until LED1 starts blinking, despite the fact that the battery is discharged during the session and its voltage decreases.

The circuit diagram does not indicate a Li-ion battery, which must be connected to the GND and +ACC connectors, and consist of two cans, giving a total voltage of 8.2-8.4 volts, at fully charged. Its normal operating voltage is 7.4 volts: example .

Also, the diagram does not show the electrodes connected to the X1 and X2 connectors on the one hand, and to the human body on the other. The “ECG Clip Electrode” is very suitable for them, the contact pads of which are made of a special alloy: example.

The following is a list of remaining electronic parts for assembling the device:

• DD1 - microcontroller PIC16F684;
• U1 - two powerful drivers in one package IXDN602PI;
• U2 - 5 volt voltage converter L78L05;
• U3 - a module for charging two Li-ion batteries with a USB input, for example a module on a CN3303 chip Multi-Cell 2S, at 1A;
• Q1 - mosfet transistor with low-voltage control IRLZ44NPBF;
• D1 - Schottky diode SR506L;
• D2 - TVS diode (suppressor) P6KE51A;
• BUZZ1 - passive buzzer for 5 volts, for example like this;
• LED1-LED3 - any LEDs for voltage below 4 volts, 3 mm in diameter: LED1 - red, LED2 - green, LED3 - blue;
• SW1 - a button without fixation, for example TACT;
• L1 - 1.5 mH inductor with a current of at least 0.2 A, for example ;
• R1 - variable resistor with a switch, for example RV097NS;
• R2 - resistor 0.5 W;
• R3-R16 - resistors 0.125-0.25 W;
• C3 - electrolytic capacitor 100μF x 63V.

The remaining capacitors can be used here at your discretion.

Setting up the circuit comes down to balancing the bridge using trimming resistor R13. First you need to install the variable resistor R1 approximately halfway, and connect a resistor of about 1 kOhm (load) to X1-X2. By rotating R13, it is necessary to ensure that LED2 blinks at the same frequency with one and the other polarity of the output pulses. It is also necessary that this LED does not blink at all if the load is disconnected.

If the microcontroller is correctly flashed, the rest of the configuration circuit does not require.

Instructions for flashing the pic controller firmware can be found here.