I'm trying two design a bionic prosthetic hand adding control and movement to an open-source Thingiverse mechanical hand.
The main is to move two MG995 when a muscular contraction is performed (reading using myoware adafruit sensor.) The proposed circuit has two 18650 batteries of 2200 mAh each of it, two charged modules TP4056 (one of it for each battery), a voltage step-up MT3608, a Bluetooth module, and an Arduino Nano for control.
The development was proposed using two PCB circuits: one for control (Arduino, myoware, and Bluetooth) and the other for power (2-TP4056 and the MT3608.) Two 18650 batteries are connected in parallel.
The output of the two batteries (3.7 V) after passing through the TP4056 is connected directly to the two servomotors power and to the MT3608 (which boosts the voltage to 10V and feeds the Arduino board through Vin, Arduino regulates the voltages and power up the Myoware and the Bluetooth board through the 5V pin.)
Between the two positives outputs of the TP4056, there is a switch that should be turned off when the batteries are charging (the batteries stop being in parallel mode, and each TP4056 charges each battery, but the input of the TP4056 is always in parallel.)
There is another switch that turns off / on all the circuits after the TP4056 modules.
I attach the schematics of the circuits.
To power 1 servo everything works perfectly, the voltage goes down 1 volt when its moves. The problem is that when I connected the two servos (connected, not even moving) both start jittering and no moving.
Measuring the voltage in the servo input when two servos are connected, I realize that the voltage goes down from 3.7V to 2 volts when they jitter.
I decided then to add another MT3608 between the two TP4056 and the servos boost the voltage from 3.7 volts to 7 volts. The problem still continued, moving perfectly with 1 servo but jittering when both servos were connected.
I decided to increase the voltage from 3.7 to 12 volts and decrease it with a voltage regulator to 7 volts again. It was worse, even not 1 servo has enough power to move.
I thought that the problem was related to the TP4056 module, so I connected the output of the two batteries directly after the TP4056, bridging it, and the problem continues.
Hope you help me guys, have no ideas left.
If you have any other recommendation, all corrections are recived.
UPDATE: I connected 4 18650 batteries in parallel directly in the power input servos. The 4 batteries feed directly the servos (with 4.2 v when charged) and power the MT3608 (which elevates the voltage to 5 v to Arduino, instead of 12v like the first). There is not any moving, even when just 1 servo is connected!!