The PWM based on the popular 555 timer architecture was chosen for this design. One 555 timer was used in an astable mode as a multivibrator. This produces the pulse train which triggers the input of a second 555 timer. There are some practical limitations of the 555 timer as outlined below.
1) Capacitance should be greater than 500pF to swamp stray capacitance.
2) Resistance should be less than 3.3M ohms but greater than 1k ohms
to limit the current.
The circuit schematic is shown in Figure 1 below. This drawing shows both the astable and monostable circuit timers as well as all resistors, capacitors, output transistors, and wiring. The circuit also shows the pin layout of each timer.
where C the capacitance and f is the frequency.
Equation 1.
I wanted a duty cycle equal to about 30 percent, typical for a pulse train circuit. Using Equation 2, R1 was calculated.
where T is the period.
Equation 2.
The second 555 timer was configured as a monostable circuit, commonly referred to as a one shot since an output pulse only occurs if there is a trigger on the input. When this timer is triggered, the potentiometer in the RC timing network allows the output duty cycle to vary, which determines how long each output pulse is high. As the output high pulse gets shorter, less current will flow in the motor causing the motor to slow down. Figures 2 and 3 show a high and low duty cycle pulse for the circuit, respectively, captured on an oscilloscope. The top graph of each figure is the output pulse, while the bottom graph is the input into the monostable timer.
Figure 2.
Figure 3.
If the duty cycle is brought down to zero percent, there will no longer be a high part of each pulse, therefore no current will flow. The capacitor value was chosen to be 0.039uF. I wanted a minimum RPM of 500 for the motor, so the potentiometer was chosen accordingly. Equation 3 shows how the value of the potentiometer was chosen.
where f was chosen as 500 minimum.
Equation 3.
The maximum resistance was calculated as 46k ohms, therefore, I used a standard value 50k precision audio potentiometer. This would assure that I could achieve a low RPM on the motor. Position of the potentiometer can be adjusted using the rotary to linear action of the servo system of the radio. For this design, a linear slide potentiometer was chosen to accommodate this rotary to linear action of the servo system.
As mentioned, the motor can draw several amps, requiring a robust transistor to provide the current control in the circuit. Based on their current handling capabilities, TIP120 BJT transistors were chosen as the output stage to drive the motor. Two TIP120’s were wired in parallel to distribute the current load and keep either transistor from running too hot. Also, space is at a premium on the boat, therefore the transistor had to operate with little or no heatsinking. After testing showed that the transistors were pulling too much current directly from the monostable 555 timer output and causing premature failures, an additional TIP120 was incorporated in a cascade arrangement with the output transistors to produce a larger gain. The output of the 555 timer has a maximum current output of 200 mA and Equation 4 shows how there could be an overcurrent condition, based on a start up current of 20 amps and a typical gain of approximately 100.
where 
is the gain of the
transistor, 
is the base
current, and 
is the current
through the motor.
Equation 4.
If the gain is 100 and the motor requires 20 amps, the 555 timer would have to supply about 200 mA, demanding nearly the maximum current from the timer and causing it to overheat and burn up. By saturating the first stage of the cascade with the output from the monostable 555 timer and running the emitter of the first transistor into the bases of the output transistors, the gain on the output stage was increased by approximately 100, lowering the current demand of the timer to 2 mA.
The operating voltage for the system is 7.2 volts. This voltage was chosen because this is the voltage of the motor battery pack. The 555 timers can run on +5 to +18 volts so the battery pack can be used to run the speed controller as well with no voltage regulation. The circuit schematic is shown below in Figure 3 as designed and built.
The circuit is working correctly, the way it was designed, but the speed controlling potentiometer is slightly nonlinear. The output pulse falls low and the transistors turn off completely when the slide is all the way down so the motor will not draw current when idle. However, the very top end of the slide does not increase the speed past a certain point. For this design and motor, though, the controller works fine.
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