Battery Warning Made Simple

I was going to incorporate a battery monitor function into the PRC344 scan controller project, but in order that the device itself wouldnt drain the battery, this would have involved so many power saving modes, wake-on-interrupt functions and shockingly expensive ultra low quiescent current regulators, unless the scan controller was actually in operation, that I decided against it. No doubt at a much later stage of development I will revisit this function, perhaps when incorporating the external control module, but for now ive decided against putting this function into the MCU.

But one of the flaws of the PRC344, and also the popular PRC320, is that unlike the PRC350, PRC351/2, and the PRC349, they have no warning of a low battery! For amateur use this is a big problem, as it makes it very easy to accidentally leave the set on, and end up potentially destroying a good battery by massively over discharging it.

So, I thought up a little add-on circuit that should be compact enough to fit into either the '344 or the '320, and will give a warning of an excessively low battery. Its very simple, just a cheap 741 op-amp acting as a comparator, a zener diode reference, a fixed potential divider (originally a 200k preset but fixed resistors are smaller and cheaper), and an optocoupler. This is set such that the optocoupler is turned ON when the supply to the circuit falls below around 20v.

Within the '344, there is an easily accessible 2kHz tone oscillator, used to provide the CALL function. As luck would have it, this is part of module 17, which is also the battery input module where the bizarre Schottky diode is! To activate the tone, 3v is connected to pin 3 of this module. All I have to do is connect my little device's optocoupler emitter to  Pin 3 of module 17, the collector to 3v, its supply to the switched 24v, and a convenient 0v. When the battery is too low, the CALL tone will be heard in the audio gear.

Battery Warning circuit mock-up

The battery warning circuit draws just 2mA, rising to 3mA in a low battery condition. Add to this 4.7mA to drive the PC817 optocoupler, for a maximum 7.7mA. At first it seems a bit counter intuitive that the battery low warning system should draw more current when the battery is low than when fresh, but this is of little consequence as the warning signal should cause the operator to either change the battery or turn the set off!

The Scan Controller code is now coming along, with much help from Mikroelectronika forum members Hexreader and Janni. These two kind persons have helped me with many code examples, and, to a greater extent, complete sections of workable code! Its not quite ready to implement in the radio yet, but it is now very close to a working system on breadboard. A few minor snags with interrupt handling need to be smoothed out.

How it will work in the radio itself ive yet to discover! Until I receive the TSSOP break out boards im unable to make use of the HV multiplexer ICs for testing, but I might be able to connect up the Synth Lock and Mute lines to test against the breadboard, plus the CALL switch for the over-ride interrupt.