View Single Post
Old 23rd Mar 2019, 11:29 am   #11
Ambientnoise
Heptode
 
Join Date: Feb 2006
Location: Marlborough, Wiltshire, UK.
Posts: 915
Default 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
Ambientnoise is online now