Re: Low frequency indicator?
TTL is not easy to drive direct from a transducer. The input drive needs to sink around 2mA to pull low so you usually need some buffering, usually a comparator. The 121 does have a Schmitt trigger input so that can help in some cases. A cmos version of the 555 timer may work with a simple RC low pass filter and protection diodes on the input. This needs negligible trigger current. Your signal level is a bit low at low freq though, it might need a boost as the trigger is related to supply voltage.
Ken
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