primary components needed:
Before you plug the circuit in figure 1 into a MIDI synthesizer, you should check the voltage range of the output signal with a voltage meter to make sure that it is in the range from 0 to 5 volts. If not, you may damage the synthesizer! The range from -0.5 to 5.5 volts should be safe enough for most cases. A protection diode placed on the signal output is a good idea since it will prevent negative voltages on the signal wire.
Figure 1 shows a two-battery circuit for use with the flex sensor. The circuit shown in figure 1 is good for use with a MIDI synthesizer as a continuous control voltage input, such as a volume controller or a pan controller. The input control voltage to a MIDI synthesizer is usually from 0 to 5 volts.
In the circuit shown in figure one, R0 is 2 kOhm. Resistors R1 and R2 should be 13 kOhms and 90 kOhms respectively, but need to be tuned for each specific built circuit to get the out signal voltage range to go between 0 and 5 volts. 5 volts should be output to the signal wire when the flex sensor is unbent, and as the sensor is bent, the voltage should drop to zero at a point determined by the resistances of R1 and R2. If you don't use the full range of flexing for the range from 0 to 5, you should put a protection diode on the signal wire to prevent any significant negative voltages from occuring on the signal wire.
The circuit in Figure 1 does not need to ues the +5 volts of power from the control-voltage input plug on a MIDI synthesizer since it is already using two batteries, so only the ground and signal wire need to be connected via a male 1/4" phono plug as shown in figure 2.
The circuit in figure three is a simple way to get a signal voltage from a flex sensor. The voltages won't be an ideal full range of 0-5 volts, but this circuit is easy to put together.
Figure 4 shows where the op-amp parts are located on the quad op-amp component in the diagram above: