The electrical system within this project consists of main supplies:
12 volts to drive a number of windshield washer pumps, and the 5 volts
necessary for all the control logic used that enables a computer to control
these pumps. In addition to the control of the pumps it was also required
to be able to control a solenoid, used within the system for the carbonator
that supplies carbonated water to the system.
The AC requirements consisted of supplying power at various voltages.
The initial supply to the system was a standard 110V - 60Hz wall outlet.
From this supply we needed to make 25 volts AC, and both 12 and 5 volts
The 110 volts AC power was used to supply power to various devices
in the system: the refrigerator, the LCD, the PC the carbonator,
and the two-25 volt AC Transformers. The transformers were also needed
to supply the twenty-five volts AC to switch the solenoid on and
off. The output of the transformer was also used to create the DC
voltages needed for the pumps and the CMOS logic.
There were two DC voltages required in the system: twelve volts
for the pumps, and five volts for the CMOS logic. The twenty-five
volts AC from the secondary side of the center-tapped transformers
was rectified through a full wave bridge rectifier to obtain 18 volts
DC.In addition to acquiring the 18 volts DC the voltage needed to
be regulated down further to acquire both the 12 and 5 volts DC required.
Switching was also appended after the bridge rectifier to add the
capability to disable power from the pumps.The switching was implemented,
by using a relay that was turned on and off by a push button. The
coil of the relay was connected to the 18 volts DC from the full
wave rectifier output through the push-button switch. In the off
state of the relay, 18 volts DC was connected to a yellow LED through
a resistor, turning it on. The LED indicates that the power to all
of the pumps is off. When the relay is turned on, The LED turns off
and the 18 volts is connected to the pumps and control logic.
The twelve-volt DC supply was required to handle the necessary
power required to operate up to two windshield washer pumps simultaneously.
Each pump requires 3A of continuous current. In addition to the
continuous current, the initial startup current of each pump requires
a peak current of 4.5A. It was determined that when two pumps were
run at the same time, a peak supply current of 9A was required
at the startup, and a steady state requirement of 5.5A. With the
large variation in the current supply, a change in the output voltage
The voltage supplied to the pumps during single and double pump
operation required as little variance in supply voltage as possible.
The reason for this requirement was to ensure consistent flow rates
between one-pump operation and two-pump operation. It was noted
through testing that the marginal difference in flow rates of the
pumps varied more significantly at 12 volts DC supply then at 13.5
volt supply. Because of this difference the 12volt supply to the
pumps was increased to 13.5 volts DC. The addition of capacitors
were to the 13.5 Volt output were included to help supply the surge
current at the start up of the motors.
The 13.5 Volts DC was supplied to the pumps from the output
of a NPN power BJT. The BJT (2N3055) was set up in an emitter
configuration, with a 1K to ground on the emitter. The other
connection to the BJT was 18 Volts DC to the collector and
14.1 volts to the
base.The reason for the 14.1 Volts required at the base was
to ensure the correct output voltage of 13.5 volts was available
the emitter of the BJT due to the voltage drop across the base
emitter junction. A 1000F capacitor was also added to the emitter
of the power BJT so that the voltage would not vary significantly
during the start up of the pumps. The supply of the 14.1 Volts
DC to the base was delivered via a three terminal 12 Volt regulator
(MC7812).The regulator is capable of supplying twelve volts
DC with a 1 amp supply current. To increase the output voltage
the regulator three diodes where attached to the ground reference.The
voltage drops introduced as a result of adding the diodes increased
the ground reference by 2.1 Volts resulting in an output of
This completed the power requirements for the pump operation.
The 5 volt DC supply was regulated using a MC7805. The input voltage
to the five-volt regulator was 12 volts DC that came from a MC7812
that was connected to the 18 volt supply. The 5 volts DC was used
to supply VCC power to all of the CMOS circuitry, including
the de-multiplexer and the tri-state buffers.
Using eight output pins (D0:D7) from the parallel
port of a computer, all of the pumps and the solenoid were controlled.
Two of the three address lines of the de-multiplexer are driven
from the first two data pins (D0:D1) of the
parallel port via the PC. The third address pin was grounded. This
configuration allowed us to set one of four output lines of the de-multiplexer
to a logic low state. The three enable lines were wired in a configuration
such that the de-multiplexer was continuously enabled, meaning that
one of the tri-state buffers were always enabled.
The other 6 available data pins (D2:D7) of the parallel port were
wired to each of the four tri-state buffers. The four outputs of
the de-multiplexer were wired to the active low enables of each tri-state
buffer. This configuration restricted the tri-state buffers to have
one continuously enabled at a time. Using D2:D7 of the parallel port,
up to six pumps could be controlled. This gave us the ability to
control up to 24 different solenoids and pumps, having the capability
to turn on up to all six, connected to one tri-state buffer, at a
The final design had 20 pumps and 1 solenoid connected to the system.
This was done with 5 pumps attached to each of the tri-state buffer
and the solenoid connected to the last bus.
The twenty output lines for the pumps from the four tri-state buffers
were connected to each gate pin of twenty power MOSFETS. These power
MOSFETS were used to switch the power on and off to the pumps. An
N-channel MOSFET was used since we wanted to use a logic high level
to turn on the pumps.
A 10K pull-down resistor was required on each of the gates of the
MOSFETS to ensure the pumps would turn off when the tri-state drivers
where disabled. This was due to the relatively low turn on voltage
of 0.6 volts of each MOSFET.
The 13.5 volts was connected from the BJT to the Pumps. The pumps
were then connected to the drain of the MOSFETS. The source of the
MOSFETS was then connected to ground so that when the MOSFET was
turned on it completed the circuit turning on the pump.
The solenoid was used to control the output of the carbonator. The
solenoid required 25 volts AC to operate. The 25 volts AC was switched
using a relay. The coil in this relay was switched with 12 volts
DC.The 12 volts to the coil was turned on and off in the same configuration
as that of the pumps.
+12 Volt Power Supply
The Voltage out of the BJT that supplies 13.5 volts varies dependent
on the current into the pumps.
# of Pumps
I Steady State
Table: Voltage regulation and Steady State Current
It can be noted that as you go from using no pumps to one pump the
supply voltage drops 1 volt. The supply voltage also drops by one volt
when two pumps are switched on from one pump operation.
Two Transformer Implementation
Initially our design used one transformer.When two pumps were running
the current rating of the transformer was being exceeded. One transformer
was rated 25 volts AC @ 5A. The current measured at the output was
5.5A.To ensure the necessary current could be supplied safely another
transformer was added in parallel to increase the output current rating
from five to ten amps.
In our initial design approach to create 12 volts DC, we were going
to use two LM338 voltage resistor set voltage regulators. Each regulator
was rated to supply 5 amps with a peak startup current of 12 amps each.
After implementing and testing this configuration it was determined
that the output voltage from the regulators was very dependent on the
load current. Even with a minimum load of two hundred milliamps. The
current draw of two pumps lowered the output voltage to 10.2 V. This
drastically reduced the flow rate of the two pumps relative to the
flow rate of one pump operation
We also attempted an alternate configuration using 3 regulators with
a LM741 operational amplifier similar to the circuit in the figure
bellow. This configuration is capable of supplying up to 15 amps; however,
after implementing this configuration it was also determined to rely
too heavily on the load current and the results were similar to that
of the configuration above.
Implementing the power BJT (2N3055) required a worst case of nine
amps current when two pumps are running. This current initially overheated
the first BJT used, as there was no heat sink to cool the BJT. After
utilizing some thermal compound and an old CPU heat sink and an alternate
BJT this issue was resolved.
The first idea for implementing the control logic was to split the
8 data bits from the parallel port into two sets of four. This would
use one of the bits to enable the chip and then the other 3 pins would
be de-multiplexed. This would allow for the capability to pump 16 different
pumps, with at most two pumps pumping at the same time. Another variation
on this was to use 2 data pins to pick one of four de-multiplexers
to turn on. This would let us control 32 pumps but only one pump at
a time. These two solutions weren't exactly what we wanted for the
control. The design criterion was that we wanted the ability to turn
on more than one pump at a time and we wanted to have a reasonably
large number of pumps hooked up to the system.
The Final solution to this used part of the second idea of using to
data bits from the parallel port to enable tri-state buffers. This
gave us 24 different control signals with the ability to turn on up
to six pumps at a time. This was limited to 2 pumps only due power
After initial software integration it was noticed that pumps were
not turning off after the tri-state buffers were disabled. It was determined
that this was due to the capacitance of the MOSFET gate storing a charge.
This charge was larger that then 0.6 volts required turning on the
MOSFET, this was causing the MOSFET to not turn off properly. Adding
a pull-down resistor to the gate of the MOSFET dissipated the stored
charge easily solving this issue.
It was determined that by using 2 bits to select one of four 6 bit
buses, we where able to control up to 24 pumps with up to 6 pumps pumping
at the same time. The two lowest bits on the printer port (D0,
D1) were used through a de-multiplexer to select one of four
tri-state buffers. The first three tri-state buffer where connected to
5 pumps each, where the last tri-state buffer was connected to 5 pumps
and the solenoid for the carbonator. They system is limited to running
only 2 pumps simultaneously from any one of the tri-state buffers due
to the large current requirements of the pumps.
SWITCHED POWER TO SOLENOID -- FOR SODA
POWER FOR PUMPS
ONE PUMP CONTROL
PARRALEL PORT PINOUT