ME 218C

Electrical Description

 

Watercraft

Power Board

Description

The purpose of this circuit board was to provide the following power outputs from two rechargeable NiCad Battery packs, each supplying a nominal voltage of 7.2V:

Power Level

Destination

14.4 Volts

Water Pump

7.2 Volts

Drive Motors

5 Volts

Communications Board, XBee Board

 

Schematic

 

Design Calculations

LM7805

This component is rated to supply up to 1 Amp of current and will take an input voltage of up to 20 Volts.  A conservative estimate for the amount of current being drawn by the 5 volt components on the watercraft are:

 

These numbers are very conservative (for instance the XBee PIC will be drawing less than 5mA because it is on 3.3Volts power and the two LEDs will never be on at the same time) and the amount of current drawn is still much less than the maximum allowable value of 1amp.

 

IRLZ34N

This component was selected because it has a continuous drain current up to 21 Amps.  For this application the following equations was used for calculating the drain current:

 

As can be seen from this calculation the IRLZ34N is a suitable component for the amount of current.

 

The other consideration when using the component as it was intended was the voltage drop across the component due to the RDS on value of 0.035 (with Vgs of 10 V).  Even with the maximum current draw from above of 16Amps the Voltage drop across this transistor will be less around 0.56Volts, which is not high enough for the components to register false high values.  If the value were around 1.75V then there would be a cause for concern.

 

10K resistor

This is a pull down resistor on the IRLZ34N.  The very low leakage currents of this component makes a 10K resistor suitable for this application.

 

300 Ohm Resistor

This is a current limiting resistor on the LED.  The voltage drop across the LED was assumed to be around 2 volts leaving 3 volts to be dropped across the resistor.  The following calculations were made:

 

 

2 pin headers

The headers available in the TA area are rated up to 4amps.  From ME218B it was known that they could handle higher current values without getting failure.  It was assumed they would be okay for this application, since the motors will rarely see stall conditions when turning the propellers in the water.

Communications Board

Description

This circuit board houses the two motor drive PICs and acts as a junction point for all the ancillary electrical components and circuit boards on the watercraft. 

 

Schematic

 

Design Calculations

243904

There were many design considerations when selecting the 2N3904 transistor.  In this application the transistor was being used to toggle to the slave select lines on the two PIC microcontrollers.  The signal from the XBee board was first put through the LM399 comparator so the input was at 0 and 5 volts.

 

From the specifications for this component the collector emitter voltage could be as high as 40V while the emitter base voltage could be as high as 6 volts.  In this application neither of these two voltages would be much above 5 volts nominal.

 

The next calculation was to consider using this component as a switch, which is dependant on the ration of collector-emitter current to base-emitter current.  Ideally, when using this component as a switch the ration would be between 10 and 20. 

 

The following equations were use used for finalizing on this component:

 

As can be seen from the design calculations the ratio is a little lower than desired.  However, there were no problems with the component in this application as verified with voltage measurements so the resistance values were not changed.  The power dissipation of less than 1 milliwatt is well below the rated value of 625 milliwatts. 

 

LM324

The LM324 was used to source current for the display LEDs on the watercraft.  The component can source up to 40mA of current which is well below the current that was to be drawn by the LEDs.  Not shown on the schematic diagram are the 300 Ohm resistors used to limit the current to the LEDs which would guarantee they are drawing less than 40mA. 

 

Headers

The headers available in the TA area are rated up to 4amps. 

 

Dip Sockets

The Dip sockets house the PICs and the LM324.  They will not be seeing currents greater than 20mA which is well within their rated specifications.

 

300 Ohm Resistor

This is a current limiting resistor on the iButton LED.  The voltage drop across the LED was assumed to be around 2 volts leaving 3 volts to be dropped across the resistor.  The following calculations were made:

XBee Additions

Description

The XBee Additions are the additional components that were added to the XBee boards which were issued at the beginning of the project. 

 

Schematic

 

Design Calculations

Headers

The headers available in the TA area are rated up to 4amps. 

 

5K Pull Up Resistor for iButton

Specified in the iButton documentation.

 

LM339

This component was selected to act as a level shifter.  The intention was to shift the 3.3V signal from the XBee pic to 5 volts for the PICs on the Communications Board.  The nice teaching assistant recommended that we use this approach as the level shifter provided on the XBee boards were slow and susceptible to noise. 

 

The LM339 is the comparator of choice in the ME218C series so it was implemented into the watercraft and its addition seemed to resolve the issues that were being seen when using the XBee level shifters.

 

10K Resistors for Voltage Divider into LM339

Voltage divider to set comparator voltage value.  10K Ohm is within range of values that is typically used in this type of application. 

 

3.3K Pull up resistors for output of LM339

Recommended “rule of thumb” value for use as a pull up resistor on an open collector component. 

 

 

Helm

Power Board

Description

The purpose of this circuit board was to provide the following power outputs from one rechargeable NiCad Battery pack, supplying a nominal voltage of 7.2V:

Power Level

Destination

7.2 Volts

E128 Microcontroller

5 Volts

Communication Boards 1 and 2, Ship Number Display Board

 

Schematic

 

Design Calculations

LM7805

This component is rated to supply up to 1 Amp of current and will take an input voltage of up to 20 Volts.  A conservative estimate for the amount of current being drawn by the 5 volt components on the helm are:

 

This is a very conservative estimate because it assumes that all the LEDs will be on at one time.  Even so, the expected worse case current draw is much less than the 1 amp the LM7805 is capable of supplying. 

 

IRLZ34N

For this application the following equations was used for calculating the drain current:

 

As can be seen from this calculation the IRLZ34N is a suitable component for the amount of current (which is rated up to 21 amps)

 

Headers

The headers available in the TA area are rated up to 4amps. 

 

Battery Life Calculation

It was required that a calculation of battery life was completed.  The selected battery was a Tower Hobbies 1500 Ultra Sport with a nominal voltage of 7.2Volts and a capacity of 1500mAh.  The requirement was that the battery be able to last at least 8 hours. 

 

For this calculation the current draw from above may be used as a starting spot but it is assumed that this maximum current draw will be done for very short durations (5 minute games, conservative 6 games).  This means that the full 510mA will be drawn for only ˝ hour of the eight total hours.  For the rest of the time the system will be in a much less taxing mode where:

 

This reduces the current draw to around 150mA.  The following equations were used to verify the system would stay on for eight hours:

 

 

Since there was a considerable amount of assumptions made to come to this value the helm was left on overnight to verify. 

 

Communications Board 1

Description

The purpose of the Communications Board 1 was to provide input / output connections for the following inputs and outputs that were associated with the 24 pin connector on the E128:

 

Schematic

 

 

Design Calculations

Encoders – Interrupt driven

The encoders are a stand alone, two channel encoder unit that is power with 5 volts.  Before integration with the system was completed the unit was measured using and oscilloscope to ensure the voltage level outputs were correct.

 

iButton

A 5K Ohm pull up resistor was used on the data line per the iButton reading specifications. 

 

The iButton LED, a purely cosmetic, no functional LED, was powered with 5 volts and a 300 Ohm current limiting resistor. 

 

LED display (non ship numbers)

The LEDs were powered with an MM74HC244N buffer IC.  They were wired in series with a 300 Ohm current limiting resistor.  See the design calculations / considerations for the 244 in the Ship Number Display Board section. 

 

Communication with the XBee Pic for Okay to Send and Message Waiting

This was a data line only and did not require any special considerations.  The only issue was that the signal from the XBee was a 3.3 Volt signal and the E128 was at 5 volts.  From the specifications for the E128 it was found that a voltage level over 3 would register as a high so it was thought that the signal would not need to be level shifted.  After some initial testing the assumptions were proven true. 

 

Throttle, Special 1 and Special 2 Buttons

These buttons were all push type buttons that needed some form of debouncing.  Instead of trying to accomplish the debouncing in software it was decided to do it with hardware.  Using one 74HC14 inverter, a resistor and a capacitor it was possible to debounce the signal to an acceptable level.  The circuit used was the same switch debouncing circuit that our group had used in the other two projects, as well as the same circuit that was presented in class.  Because of the previous experience, and the trusted source of information, the circuit was used with no additional design considerations. 

 

Communications Board 2

Description

The purpose of the Communications Board 2 was to provide input / output connections for the following inputs and outputs that were associated with the 20 pin connector on the E128:

 

Schematic

 

 

Design Calculations

The only component that had design considerations on this board was the tape sensor.

 

Tape Sensor

 For the Tape Sensor input an LM324 was used to amplify the signal for reading by the E128.  Per the instructions given last quarter a virtual ground of 2.5 volts was created using on of the 4 op amps on the LM324 and a simple voltage divider of two 10K Ohm resistors. 

 

The gain resistor for the detector half of the tape sensor and the current limiting resistor for the emitter portion of the tape sensor were dialed in empirically to achieve the proper performance from the water delivery button.  

Ship Number Display Board

Description

The Ship Number Display Board was built to drive the 2 digit 7 segment LED displays for showing the Craft Number currently linked to the helm.  The seven segment LEDs require a considerable amount of overhead to make them work so the circuitry was moved to its own circuit board. 

 

Schematic

Design Calculations

The main design considerations for this board were the interactions between components. 

 

MM74HC244N

The main consideration for the MM74HC244N was making sure the unit could provide enough current to drive the LEDs.  The LEDs had a current limiting resistor in series to keep the current for each LED at 10 milliamps.  Each pin on the 244 can source or sink up to 35 milliamps.  In the intended application they will actually be in a sinking configuration.

 

When sourcing on seven different pins the power dissipation in the 244 becomes an issue.  The power dissipation is rated at 600mW.  The following equation was used to figure out the power:

 

 

SN74HC595N

The main consideration with the shift registers is ensuring that they will be able to turn to the LED buffers (the 244) on and off.   The ability to meet the requirement is dependant on the amount of current that the shift register outputs can source and sink.   From the spec sheet the 595 can source or sink up to 35 milliamps per pin.  From the spec sheet for the 244 the maximum current on the input pin is +/- 1 microamp.