Minimal transceiver with Hellschreiber interface

Some years ago Stephen, VK2BLQ, and I experimented with using low cost CW transceivers and the Hellschreiber mode. This mode works with simple transceivers as it is simply keyed on and off.

Having moved house several times since then I had to re-build the interface. I think this version is neater. FlDigi keys a hellschreiber transmitter by keying a tone on and off and to key the transmitter an interface needs to rectify this audio and activate a transistor. 

I've just had a contact with Richard, VK3LRJ:

He's using an IC-7300 at his end but my side is simpler:

The transceiver, lower left, is a micro mountaineer kit, from an original design by W7ZOI. It's a step up from the Pixie kits and the receiver works much better than on them. There's no keyer to get around so it's well suited to use with Hellschreiber. Here's the interface board:

At first I used silicon diodes but found the audio level was too low. Substituting germanium diodes works better but is even a bit too sensitive so I have to reduce the audio level a bit. Here is the circuit I ended up with:

The 8R:1K audio transformer was purchased at Jaycar.