'344/'320 low battery circuit

Well it didnt take me long to become dissatisfied with my low battery alarm circuit!

I realised almost straight away that the circuit as built is pretty much game over if the battery reaches the threshold. The tone being constant means that no further listening is possible!

The PRC351 does its low battery warning by pulsing the mute circuit open, causing a brief burst of noise in the audio gear, every few seconds or so. Since my circuit uses an optocoupler output, in theory this could be connected to the '344s mute circuit to achieve the same, if the ouput could be pulsed.

So, last night I threw this little beastie onto the breadboard -

 With the values shown I found this lit the LED for about 1sec every 3-4sec. Not sure if that duty cycle is adjustable, it would be nice to be able to choose to lengthen the 'off' time. This simple circuit therefore is ideal for giving a pulsed warning that wont disturb actual radio listening too badly, but will still indicate that the user should change the battery.

Next step then, is to couple it to the low battery detector circuit previously built.

As the detector output goes low when the threshold is  reached, my thought is that I can put it in the ground connection of the pulse generator, in order to switch that on. Im not sure this will work, but i'll give it a go. If not then i'll need to add a switching transistor, unless I can juggle the inputs of the detector such that the output goes high when the threshold is reached, and I could then just drive the pulse generator supply line. But my thought is that with the detector output high, I will be putting roughly equal positive voltage on both the supply and ground pins, with a net result of no effective supply.

This design does of course add a minimum of four extra parts into an already grossly cramped space. So to fit in the radio this will have to be made surface mount. All the parts can easily be done this way, although some are not very cheap (the opto and the preset resistor if I make the threshold adjustable again) The 741s will be replaced by a LM358 dual op-amp in a SOIC package.

If im going to do this in SMT, then I may as well also design a PCB  and take advantage of the rediculously low cost PCB offers available, make a few up and sell them on!

Im pretty sure the same circuit could be used to put a low battery warning on the PRC320 as well, but i'll look into that later.