detector. An infrared light source is mounted so that its light beam illuminates the infrared detector. An opaque flag is mounted concentrically to the camshaft and rotates with it between the infrared light source and detector. When the flag interrupts the light beam, the output of the detector is sensed by the microprocessor via an input port bit. Power to the infrared LED light source is controlled by the motor driver circuit and is off when the motor is not running to conserve battery life.

In the microprocessor software, multiple checks are made on motion of the camshaft. When the motor is commanded to start, the infrared sensor must show that half a revolution has occurred within five seconds and that the motor has stopped when half a rotation was completed. In addition, no camshaft rotation can take place when the motor has not been commanded to run.

Power Circuitry

Power for the pump is normally supplied by a 9-volt alkaline battery, 9-volt lithium battery, or AC adapter. These types of batteries have a fairly low internal resistance over their discharge range, which will keep power supply noise low. Other types of batteries, such as carbon-zinc, exhibit high internal resistance, especially near depletion. A voltage drop across the internal resistance occurs when current is drawn by the motor during pump activations. This current is demanded in short pulses when the motor is first turned on and generates large spikes in the battery voltage. This noise can cause the low battery detection circuit to shut down the pump.

The motor driver circuit power is taken directly from the battery, but the microprocessor and its associated circuitry requires closely regulated and filtered 5-volt power which is supplied from the micropower voltage regula- tor. This regulator will supply 5-volt power until its input voltage is approximately 5.3 volts. After that point, the output of the regulator will follow the input voltage down.

Voltage Reference Circuit

A voltage reference circuit provides a constant DC voltage to the microprocessor Analog to Digital Converter (ADC). By reading this input and comparing the value to a predetermined range, the microprocessor can validate the accuracy of the 5-volt power supply. Variations in the 5-volt supply left undetected can result in inaccuracy in the low battery alarm set points and variations in other calculated values.

VoltageCADD® Pump Status
Trip Point*

 

 

 

>7.0V

No alarm

6.4–7.0V*

Transition to low battery

 

condition; battery low

 

message appears; 3 beeps

 

every 5 min.†

6.0–6.6V*

Transition to depleted

 

battery condition; battery

 

depleted message appears;

 

continuous alarm††

5.25–5.95V

Hardware reset occurs.

 

Pump continues to indicate

 

depleted battery condition.

*Voltage ranges are due to component tolerances. Actual trip values are guaran- teed to be non-overlapping.

Table 12. CADD-Prizm®pump low battery conditions.

†The pump emits 3 beeps every 5 minutes, and the message “9 Volt Battery Low” appears on the pump’s display, indicating that the battery power is low, but the pump is operable.

††The pump emits a continuous, variable-tone alarm, and the message “9 Volt Battery Depleted” appears on the display, the battery power is too low to operate the pump, and pump operation has stopped.

Pumping Mechanism

The pumping mechanism is linear peristaltic with two active valves. Pumping occurs when the expulsor presses on the reservoir pump tubing in sequence with the inlet and outlet valves. At rest, the outlet valve is pressing down fully on the tubing and the expulsor and inlet valve are retracted. (See Figure 7.)

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