ABSOLUTE_TEMPERATURE_LOG_WITH_RS_232

Published:2009/6/24 4:08:00 Author:Jessie | From:SeekIC

In the setup, T1 (an LM334 temperature sensor) generates a constant current that's propor-tional to absolute temperature, and equal to 25μA at 323 Kelvin (50℃). RI sets this constant of pro-portionality. The current discharges the parallel combination of C1 and C2 connected to the trigger and the threshold pins of UI, which is a CMOS implementation of the venerable 555 analog timer.The negative-going ramp is compared by UI to an internal2.S-V trigger level controlled by 21. When the ramp gets there, U1 triggers. The output pin (3) will go high, presenting a start bit to the con-nected communications port, which causes Q1 to source 100 μA to the timing node. At the same time, the discharge pin (7) will open, allowing R2 and the bottom end of C1 and T1 to be supplied by C1, and isolates it from C2. Consequently, the current supplied by transistor Q1 will go solely to C2 so that when the result-ing positive-going ramp reaches 5V, exactly 25nC (2.5 V x C2) will have been deposited in the tim-ing node by the recharge cycle because its threshold level (6) will have been reached. This causes both the output pin to return to the negative rail, restoring the marking condition of the R2-232 in-terface, and Q1 to stop recharging C2. U1's discharge pin (7) now connects R2 to the negative rail, causing the charge that was deposited by T1 on C1 during the recharge interval to rapidly redistrib-ute between C1 and C2. This arrangement creates a very linear (0.01%) relationship between T1 cur-rent and pulse output frequency. While this is happening, the PC's communications port hardware assembles a valid (although meaningless) character because the positive pulse output by UI looks like the start bit of a charac-ter. A simple program running in the PC can then count the frequency of these characters and con-vert the resulting rate into a direct readout of temperature. Because of the 3.1-Hz/degree slope of frequency versus temperature, a 30-second average suffices for 0.01°resolution.



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