Clansman PRC344 Low Battery Alarm

Ive decided to slightly rename this project as a low battery alarm rather than a warning, as in use, once the battery is too low, the 2kHz CALL tone pretty much makes any further listening impossible!

Battery alarm circuit. NB - 'Battery' has to be a switched line!

The circuit is very simple. I make up a potential divider chain rather than leave one of my few stock presets in circuit, but of course if you have a 100k - 200k preset spare then using that will mean you can adjust the threshold to taste. In this case the alarm threshold is about 20v. This is ideal as it means the battery is not discharged lower than 1v/cell. Note in particular that 'battery' on the diagram refers to a switched 24v line inside the radio - not the raw battery connection!

I knocked the prototype up on matrix board. This is a bit too big really even with the edges trimmed off!.  Ideally to fit comfortably in the radio SMT parts would be used. But, wrapped in several layers of insulating tape (ive no heatshrink wide enough!), it is now installed in my '344. The red wire is the 24v, which ive connected to a handy pin6 on one of the regulators. Yellow is 3v from pin3 of the 3v regulator. Black is ground. The blue wire connects to module 17 pin 3. You can see how it fits and how it works in the little video below -

A later revision of this would be to use a double op-amp to pulse the mute open, similar to how the '351 low battery warning works. But that would have more components and would demand SMT parts to fit the radio.