Ramsey Electronics COMPUTEMP CT255 - Manual (Page 10)

CT255
• 10
At 100.0 degrees Fahrenheit the temperature sensor will have an output of a
value related to Celsius, so to make life easier, we will convert to Celsius first.
So:
C = 5/9(F-32) or C = 5/9(100—32) which is 37.78 degrees Celsius.
This means the output of the sensor should be 377.8mV at 100.0 F.
So to convert 377.8mV to 1.965 volts we need to multiply by:
1.965 / 0.3778 or 5.201. This will be the gain we need from the non-inverting
amplifier.
We go back to our old non-inverting amplifier formula of:
A = 1 + Rf / Ri, which is the gain of the non-inverting opamp.
We already have Ri = 1.00K and A of 5.201 so we have to find Rf.
Rearranging we get Rf = (A-1) * Ri so Rf = (5.201—1) * 1000
So Rf = 4.201K. Our closest 1% value is 4.22K.
Binary math
Oh no! Your brain hurts and doesn’t want any more theory; on to the kit
building! Ok, go ahead and build the kit, but come back here later to review
how to read the display properly, and why it works that way.
Normally we would only deal with integer binary math, but we at Ramsey like
to mix things up a bit, and throw in fractional binary math so you learn
something new! It is the basis of how all math works on computers, including
that pesky floating point stuff, especially on DSPs. We will also learn why
computers are NOT perfectly accurate.
A binary number is either 1 or 0, on or off, meaning it is only two states. Two
LEDs will have 4 potential states, 3 will have 8 states, and 4 will have 16. The
relationship is 2 to the power of the number of bits. This is seen as 2^n when
n is the number of bits. In the case of our display we have 8 bits, so we have
2^8 or 256 potential states.
As the value counts up, you will see the right most bit (LED) toggle the fastest.
This LED is considered the least significant bit, meaning it has the least value
associated with it. In our case it is worth a measly 0.5, but normally would be
worth 1 if we didn’t throw in a decimal point to confuse you. The LED on the
far left is called the most significant bit, because it is worth the most. In our
case it is worth 64 but normally 128. This is because the .5 LED had to flash