Circuit Description - Ramsey Electronics COMPUTEMP CT255 Manual

CIRCUIT DESCRIPTION

There's a lot of circuitry in the CT255 that is in use to this day in a variety of
circuits. We will begin with the temperature sensor itself, detailing how we did
this 20 years ago and how we do it now.
The actual temperature sensor is the LM35DZ. It is a specialized part that is
pre-calibrated at the factory to output 10mV per degree Celsius. For example if
we connected the part to power and measured the output at 0 degrees Celsius,
the output would be 0mV. If the temperature were then raised to 50 degrees
Celsius, the output would become 500mV, which is 10mV * 50C. Really simple
isn't it?
In the past we used a diode junction to do the sensing, because a diode
junction has a very predictable change of voltage drop versus temperature. In
the case of silicon, this drop works out to be 2.1mV for every degree Celsius.
Unfortunately using diodes caused us to use calibration to compensate for
offset and gain, which was a real pain.
Now it comes down to needing to display this output in a form our eyes can
understand, and this means driving a display. In our case we will be displaying
the value in a way that both humans and computers can understand: binary.
How do we do this easily? Hang on, here we go...
The binary counter
U2:A and U2:B form a cascading ripple counter which is simply designed to
count pulses on the clock pin (pin 1), and output the count as binary. The high
order bit of U2:A (pin 6) is then tied to the clock input of U2:B to make an 8 bit
counter in total.
A binary counter is a very simple set of flip-flops that are fed from one to
another in sequence. The lowest order (bit 0) is tied to the input of the next (bit
1) and the output of bit 1 is tied to the input of bit 2 and so on. For every two
pulses on the input to the flip flop, the output switches once. When the flip flops
are cascaded, they make a "ripple" counter, meaning all outputs are effected by
the single input, but all in succession, not simultaneously. When we put the kit
together, this count sequence will make much more sense since you can see it
occur!
The binary count is then tied to a resistor "ladder". Believe it or not, this makes
a very simple digital to analog converter! As the count increases on the outputs
of U2, the voltage increases in a linear step across R38 and C5. This is set up
here to be an eight bit digital to analog converter. Many digital to analog
converters use a ladder style resistor network just like this. To increase number
of bits, they just use more precision on the resistors, and increase the count.
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