Lee “I Void Warranties” N9LO ran across this circuit when looking for a transmit indicator for his Small Wonder Lab PSK31.
[Note the circuit was originally drawn incorrectly, but has now been updated]
Lee threw it together (there are not many parts) and it lights up bright and solid for CW and you an watch your modulation on SSB. I can really see the difference when I switch in my SSM2167 module.
Jerry KE7ER recommends placing the circuit before the transmit LPF, not at the antenna port. The diodes potentially create a bit of harmonic content that the LPF’s would remove.