Digital thermometer circuit. A simple digital thermometer circuit with out a micro controller and having a seven segment LED read out is shown here. The IC has an internal timing circuitry and hold function. When the hold function is enables, the output IC latches itself to the present state. It can directly drive a seven segment display and there is no need for current limiting resistors. LM35 is a three terminal precision temperature sensor IC from National semiconductors.
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It had the same chip too. Some more digging, and I found a digital panel meter. This had a 7 segment LED display, but the chip was again from the same family. In short, everything you needed to take a DC analog signal and display it. The ICL71xx was the go-to device for a reason. Lawsuit First, a little bit of history. This chip was developed by Fluke in collaboration with the original Intersil, Inc. This made the unsuitable as a direct, drop in replacement for the But Fluke found out that the silicon inside the still sported a Fluke logo, and a lawsuit ensued.
But the was now available to all for use and would end up inside a large number of products built in those days.
You can read [drtaylor] who worked at Fluke during the time reminisce about this over at the EEVblog forum. But the and others in its range are still available today, almost 40 years after they were first introduced. It seems there may still be a lot of use cases where selecting this device is justified.
The differential inputs and adjustable reference voltage allow sensors such as strain gauges, load cells or similar ratiometric bridge type inputs to be easily interfaced.
A lot of these chips got used for weighing scales, for example. This Wiki stub explains the process succinctly. In order to achieve 50Hz noise rejection, the signal integrate phase needs to be a multiple of 50Hz. For example, to get good rejection of 50Hz, 60Hz, Hz and Hz noise, the oscillator needs to be at 40kHz and will result in 5 readings every 2 seconds. The oscillator parts R3, C4 , thus, need special selection. In many cases, a simple voltage divider is connected to the regulated supply rail to derive the reference voltage.
But for improved performance, a more stable reference, using a zener diode or a dedicated reference voltage band gap device was highly recommended. Walking down memory lane is quite fun, especially for parts like this which were hiding in so many kits and products alike. Do you folks have any interesting ICL71xx stories to share in the comments?
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Get To Know 3½ Digit ADCs With The ICL71xx
Digital thermometer circuit