Ramsey Electronics COMPUTEMP CT255 - Manual (Page 8)

CT255
• 8
How the counter oscillator works
U1:D is set up as a simple oscillator to generate our clock pulses that drive the
ripple counter. When the comparator U1:B output is low, this circuit will run
normally, generating pulses for our counter. When the U1:B comparator
output is high, the oscillator stops. This is how the count is “held” for display.
This oscillator works through the same principle as the comparator, by
comparing the charge voltage across C1 charged through R1 to the voltage at
pin 12. When the output of U1:D is our max of 5V and the output of U1:B is 0V
(running mode), the voltage at pin 12 should be about 3V (R4, R2 and R3 act
as a voltage divider). At this point C1 is charging through R1 over a period of
time. Eventually its charge will surpass our 3V at pin 12, where the output of
the opamp comparator will switch to low. This makes our voltage at pin 12
about 1.6V. Now the capacitor C1 discharges until it surpasses 1.6V in the
opposite direction, and the output switches back to high. This repeats over
and over and is the basis of our oscillation. The reason why the oscillator
stops when the output of U1:B goes high is the voltage at pin 12 becomes a
level that the charge on C1 can never reach. It is greater than 5.0V due to R15
limiting the voltage on C1 to 3.4V, so the comparator function is disabled.
Restarting the conversion cycle
Now our count stops at a certain voltage, but how do we make the count reset
every so often so we can get a new reading? Simple. We have another opamp
stage that is set to be yet another comparator. In this case we are comparing
the voltage between R30 and R34, in this case 2.5V, to the charge on C2. C2
charges through R24 in about 5-6 seconds. This capacitor charges while the
output of U1:B is in the disable count mode with the output being high. Once
the charge on the capacitor surpasses the 2.5V threshold, the output goes
high, clearing the count on the ripple DAC.
This in turn re-enables the oscillator of U1:D since U1:B compare values are
reset, and C2 then discharges quickly through D9 to get ready for the next
round starting with the first clock pulse in U1:D. C2 does not charge until the
count is complete, so the reset cycle will hold the finished count long enough
to be seen.
For the complicated Fahrenheit design
Skip this section if you don’t want a bad headache.
Now how do we switch to Fahrenheit? Not a simple as it sounds. For one our
Celsius to Fahrenheit conversion is listed as:
F = 9/5 * C + 32
We see by this formula that not only do we have to scale by 9/5 we also have