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Home Brew Compass Sensor CIRCUIT (LM317)

2014-11-22 20:48  
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Background

The goal was to make a compass sensor compatible with the RCX
and have it only take one sensor input port to take a compass
reading. We decided to use a Dinsmore analog compass part that
has tw1o outputs that need to be combined to determine the
direction the compass is pointing.

With our design, it takes tw1o readings by the RCX to get one
complete reading from the compass sensor. Softw1are then must
combine those readings together to determine the angle the
compass is pointing. We had to do it this way because of power
demands by the Dinsmore part and power limitations of the RCX.

We wanted to add hardware around the Dinsmore sensor to
combine the tw1o outputs into one RCX input. We first tried to
put all the “inside/outside crossings” logic into hardware, but
we found out the hard way that the RCX can only provide about
10mA of current. It turns out we can only power 1/2 the Dinsmore
compass sensor at a time.

Rick Sammartino (RickSam from the Mindstorms website) who has
been helping me all along, had and idea of reading one curve then
the other. This fit well with powering one compass sensor then
the other.

Dinsmore Compass

This article describes the Home Brew Compass Sensor CIRCUIT (LM317). The content is very simple, very helpful. Components in this article can help you understand better understanding of this article. For example, in this article, you can go to find and buy these components:LM317.

The Dinsmore compass sensor has tw1o outputs that need to be combined to know the direction you are pointing.

To determine the outputs` values when pointed a particular
direction, draw a verticle line at the angle you want the compass
to point. The vertical line intersects with the tw1o output
curves only once per vertical line. These intersections are the
tw1o output values you can expect when the compass is pointing in
that direction. As you rotate the compass, you are
basically sliding your vertical line left or right on the graph.

As you rotate the Dinsmore compass, there are tw1o angles
where the output values are the same. One is above the 2.5V
center line, and the other is below the 2.5V center line. We`ll
call these the upper crossing and the lower crossing. Knowing
these voltages are important to understanding the compass
output.

At any given orientation, one of the Dinsmore compass outputs
is either above the upper crossing, or below the lower crossing
while the other compass output is betw1een the crossings (unless
you are at one of the tw1o crossover points). The output that is
betw1een the crossings is follows a fairly straight line,
while the output outside the crossings does not.

The trick is to determining the direction the compass is
pointing is to determine which output is outside the crossings,
and then use the output that is inside the crossings to know the
angle.

Schematic

We designed and debugged a “tw1o reading” circuit shown in the
schematic. The circuit has four major parts: RCX power supply,
compass output select state machine, compass power and output
select, and RCX sensor readings hardware.

RCX Power Supply

We get the power we need to run the electronics in our sensor
by setting the RCX sensor port type to light sensor. This makes
the RCX output 7 volts at a maximum of about 10 mA of current.
Every 3 milliseconds the RCX drops the power for about 1/10 of a
millisecond to take a sensor reading.

The cables that connect sensors to RCX sensor inputs can be
oriented one of tw1o ways: cable pointing towards the IR port, or
away from the RCX port. There are tw1o terminals on an RCX
sensor port. The RCX always sends power out one terminal and back
in on the other terminal. When we connect a sensor with the
cable pointing toward the RCX`s IR port, the power travels
down one wire and comes back the other. When we orient the cable
the other way, the power flow is reversed. Reversing the power
can be bad for electronics, and they certainly won`t work as
planned.

To allow the user to connect their sensor cable with either
orientation we use something called a diode bridge to make sure
the power always flows the correct direction. A diode bridge is
make up of 4 diodes.

When the sensor cable is hooked to the sensor input with one
orientation, power travels into our circuit through D3, and back
to the RCX through D6. With the cable the other way, power
travels in through D4, and back through D5. The ends of D3 and D4
that are hooked together always provide the power, and the ends
of D5 and D6 that are hooked together always return the
power.

As mentioned before, every 3 milliseconds the RCX turns off
the 7 volt power to the sensor to take a reading for 1/10th of a
millisecond. This means that our electronics are unpowered during
that 1/10th of a second time period. 1/10th of a
millisecond (1/1000th of a second) may seem fast to us humans,
but this is pretty slow in electronics terms, so we need a way to
save up some power to run the electronics while the power is
off. We use a capacitor C1 to store up power for these power out
periods.

A capacitor acts like a water tower. It has pumps that fill it
up, but it takes time. As the tower fills up with water it gains
energy, because gravity pulls down on the water. As people use
the water, the gravity pulls the water down and into their
houses.

Capacitors work in much the same way. While the RCX is sending
power to our sensor, the other electronics in the sensor use the
power, and capacitor C1 charges up until it is full. When the
RCX shuts off the power, the rest of the electronics draws its
power from capacitor C1. The electronics can function normally
until capacitor C1 runs out of power.

The size of the capacitor (measured in farads) you use depends
on how much power you need and for how long a time. The larger
the capacitor, the longer it can survive a power outage. We need
our electronics to survive longer than the 1/10th of a
millisecond power outages.

Our sensor needs to provide tw1o different readings to
softw1are in the RCX to know what direction we`re pointing. This
means the softw1are needs to tell our sensor to stop reading one
compass output, and start reading the other compass output. The
softw1are signals this to our sensor by turning off the power for
a longer period of time. We need the electronics to stay
alive over this longer period of time so it can remember which
compass output it is currently on.

These power outages can last in the 10 to 20 millisecond
range. Capacitor C3 is large enough to provide power to the other
electronics for this long a power outage.

The 7V power provided by our power supply is called VCC (everything has to have a name;-)
. The place where the power returns to the RCX is called
ground. In electronics ground has its own special symbol that
looks like a triangle pointing down mad up of horizontal lines.
Since our sensor is a rather complicated RCX sensor we do not run
lines all over the page showing power and ground hookups.
Instead we show local hookups using VCC and ground. You just have
to imagine that there are wires (lines) leading from the
power supply to the various electronic components.

The Dinsmore compass needs to have its power very stable to
give reliable readings. We use an LM317 voltage regulator U1 to
provide the same voltage to the Dismore compass, even if the
power from the RCX fluctuates. The voltage regulator and its
companion resistors and capacitor provide 4.5 volts (4.5V) to the
Dinsmore compass. The Dinsmore compass is supposed to run
on 5V, but the RCX cannot provide enough power at that voltage,
so we run the Dinsmore compass at 4.5V to save power.

Compass Output Select State Machine

Our sensor and the softw1are need to always be in sync about
which compass reading is being sent by the hardware, and which
reading the softw1are is receiving. Our sensor remembers which
reading it is currently sending using a digital memory element
called a flip flop.

In the schematic the flip flop is component U2A. Our flip flop
has 5 inputs and tw1o outputs. The tw1o most important inputs
are the D input and the CLK input. These tw1o inputs work
together. The D input is for inputting data. The CLK is a clock
input, which is used to tell the flip flop “forget what you
were remembering and remember what is on the data input.”

The Q output is the value that the flip flop is remembering.
The NOT Q (a Q with a bar over it) is the opposite of the value
being remembered. If Q is outputing a 0, then NOT Q is outputing a
1. If Q is outputing a 1, NOT Q is outputing a 0.

The tw1o remaining inputs S (for set) and R (for reset) can be
used to force the flip flop to either a 1 or a 0, without having
to put the value on the data input and clocking it into the flip
flop. We do not use these in our sensor.

The flip flop is used to remember which compass output we are
sending to the RCX. Each time the RCX tells us to switch to the
other compass output, the flip flop needs to change to the
opposite of the value it currently has. If it is currently
remembering a 0, it needs to remember a 1. If it is currently
remembering a 1, it needs to remember a 0. The NOT Q output is
the opposite of the currently remembered value, so we hook
the NOT Q output to the data input. When we apply the remember
clock, the flip flop changes value.

The softw1are in the RCX turns power off, and then on to tell
the flip flop to change its value. We use operational amplifier
U5C and some resistors and capacitors to detect power toggling
(being turned off and on.)

To amplify is to make something larger from something smaller.
Your car stereo has an amplifier in it that makes the small
radio waves, or small magnetic changes on tape large enough to
wiggle speakers so we can hear it. How much you amplify something
is called gain.

An operational amplifier (op amp for short) is a general
purpose electronic amplifier that is easy to use. It has tw1o
inputs and one output. An op amp subtracts one of its inputs from
the other and sends an amplified version of the difference to
it`s output. An operational amplifier has very high gain, which
means it can take small differences nad make them very, very
large. We can control the gain of an operational amplifier by
adding or subtracing some of the output from the input.

We use an LM324 quad operational amplifier chip in our
schematic. It has four operational amplifiers on one chip that
all run off the same power supply.

So how do we use an operational amplifier to detect power
toggling? First we must think back to the power supply
explanation. The power supply uses diodes to make the power from
the RCX travel in the correct direction, and we use a capacitor
to store power for power outages.

To detect power outages we use a second pair of diodes D1 and
D2, that act just like diodes D3 and D4. Power comes in D1 or D2,
and goes out D5 or D6. In this case though we do not use a
capacitor to cover for power outages. This means that the power
coming through D1 and D2 goes on and off when the RCX turns
power on and off.

The flip flop remembers new values when the clock input goes
from a 0 (no power) to a 1 (power). It is called an edge
triggered input. The change from a 0 to a 1 has to be fast for
the flip flop to notice it. The RCX`s change from no power to
power is too slow for the flip flop to notice. To make a faster
rising change, we use the subtracting nature of an operational
amplifier and it`s ability to change values very quickly, by
using the op amp`s maximum gain.

We run the voltage from D1/D2 to the positive input (pin 10)
of U5C. We run a reference voltage (one that does not change) to
the negative input (pin 9) of U5C. The operational amplifier
subtracts the refernce voltage from the D1/D2 voltage. When the
reference voltage is larger than the D1/D2 voltage, the op
amp puts out a very low voltage (almost 0V). When the D1/D2
voltage is larger than the reference, the op amp puts out it`s
maximum voltage. As the D1/D2 voltage goes from lower than the
reference to even just a little bit higher than the reference
voltage, the op amp output voltage rapidly goes to its maximum
output voltage because we are using the op amps maximum
amplification capabilities. Using an op amp with its maximum gain
like this is called a voltage comparator.

Using U5C as a voltage comparator makes our power toggle detect signal rise fast enough to clock the flip flop.

We generate the voltage reference by using tw1o resistors.
Resistors are electronic components that resist the flow of
electricity. In resisting the electical flow power is lost
through the form of heat. When the power is lost, there is
voltage from one end of the resistor to the other. This loss is
called a voltage drop.

We want our voltage reference to be somewhere betw1een 0V (the
lowest voltage) and 7V (out highest voltage). To do this, we our
7V from our power supply into one end of a resistor, and we hook
the other end of the resistor to ground (0V). The incoming
voltage stays at 7V, and the outgoing voltage stays at 0V, so
there is a 7V drop across the tw1o resistors. The voltage
where the resistors meet in the middle is based on resistive the
tw1o resistors are. If the tw1o resistors are of equal resistance
(measured in ohms), half the voltage drop is due to the first
resistor and half due to the second, so the voltage in the middle
is 3.5V.

Our reference voltage for power toggle detect is generated
using a voltage divider implemented by resistors R3 and R4. The
middle voltage in our divider is 7V (VCC) * 100,000 (R3`s
resistance) / (100,000 (R3`s resistance) 220,000 (R4`s
resistance)) or 2.1V. This voltage reference is used again for
other purposes.

Compass Power Control

The Dinsmore compass is actually tw1o sensors in one. Each
sensor has its own power connections, so we can power each one
seperately. To satisfy RCX power limit constrainst we can only
power one half of the Dinsmore part at a time. When one half
is powered, the other half has to be unpowered. In electronics
we can use transistors to turn things on and off. We use tw1o
MOSFET transistors to control the power to each half of the
Dinsmore compass. MOSFET transistors are very good for
controlling power because they consume very little of the power
they control. Power goes into a MOSFET transistor in the source
input. Power traveling though the transistor is controlled by the
gate input. The power comes out of the MOSFET through the drain
output.

In our sensor, the source power is provided by U1, the Compass
Voltage Regulator. The drain output is connected to a half of
the Dinsmore compass to deliver power (or not ;^) The gate inputs
of our tw1o MOSFET transistors are controlled by by our Compass
Output Select State Machine flip flop. Remember that our
flip flop has tw1o outputs Q and NOT Q, and that NOT Q always has
the opposite value from Q. So if Q is on, NOT Q is off and visa
versa.

This sounds exactly like the way we need the power delivered
to the tw1o halves of the Dinsmore compass. We hook Q to the gate
of one of our MOSFETS, and NOT Q to the gate of our other MOSFET
and only one half of the Dinsmore compass is powered at a time.

Compass Output Select

Each Dinsmore compass output is a voltage that varies based on
the direction the compass is pointing. Unlike our digital flip
flop who`s output voltages can only have tw1o values either on
(high voltage) or off (low voltages), the Dinsmore compass is
analog which means a whole range of voltages.

The Dinsmore has tw1o analog output voltages, and we have to
choose one of them to send to the RCX. Electronics provides us
with a wonderful thing called an analog switch for just this
purpose. An analog switch has tw1o inputs, and an output. One of
the inputs is called the control. When the control input is
driven so the switch is on, the analog input is sent to the
analog output. If the control input is driven to turn the
switch off, the analog output is disconnected from the analog
input.

We use tw1o analog switches to route the Dinsmore compass
outputs to the RCX. There is one analog switch for each compass
output. We connect the Q and NOT Q outputs from our flip flop to
the control signals on our tw1o analog switches. We also hook
together the outputs of the analog switches which is sent off to
the RCX. When Q is on, it turns on a MOSFET that powers one
half of the Dinsmore compass. The output of that half of the
compass is fed into an analog switch, who is also controlled by
Q, so the output is fed onto the RCX.

When Q is off, NOT Q is on, so the other MOSFET is turned on,
the other half of the Dinsmore compass is powered, and the other
analog switch is turned on, so the second compass output is fed
to the RCX.

We`re most of the way through our sensor explanation, so I hope you are still with us.

Final Output Stage

The remaining part of our sensor feeds our selected output to the RCX.

The Dinsmore compass output gives us less than one degree of
resolution. Resolution refers to how big or small the smallest
detectable part is.

The Dinsmore part can only provide 4mA of current. To make
sure we do not draw too much power through the compass, we run
the selected compass output through an operational amplifier that
interfaces to the RCX. This guarantess that we don`t draw too
much power. It also allows us to better interface to the RCX`s
resistance based sensor reading technque.

Diodes D1 and D2 combined with D5 and D6 are how the RCX
actually reads the sensor values. The RCX sends a small amount of
power through either D1 or D2, our operational amplifier
combined with a 2N2222 transistor only lets some of that through
and back to the RCX through either D5 or D6.

 


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