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An old thread show this schematic by JimB here attached. It uses a 74121 but I have a hard time finding it and Mouser shows it is obsolete. I did some searching and wonder if it can be replaced in this circuit by a 74HC4538? Or anything more similar to the 74121?
Looking at the datasheets, I see no reason why the newer 74HC4538 should not be used.
The reasons that I used a 74121 are:
1 I had several in the" junk box"
2 Many years ago (about 1976) I built a capacitance measurement jig which used two or maybe three 74121s.
I wish I could remember the circuit, it was something out an electronics trade magazine.
The boss came along one day and asked me to build it. It worked very well.
His application for it was to measure the capacitance between a strain gauge and the metal that it was bonded to, and look for variations if the strain gauge became detached.
The problems were:
There would only have been a few pico-Farads between the gauge and the metal
He wanted to do it on the far end of many metres of multicore cable (several 100s of pF).
Hi JimB, thank you! This helps a lot because I think this is a genial little circuit.
1976?? .. we must be the same age? Although you look more like 35 on that picture..
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