Design Success?

The rethink of the battery warning design took only a few minutes in the workshop to prove out. First, I took measurements of the actual current draw of each section of the design. The pulse generator, with no load on the output, requires 1.8mA. The low voltage detector requires about 1.6mA. Hardly going to break the bank with a 4Ah battery! So, its not a big issue if the pulse generator is running all the time.

This made life much easier! A quick rearrange of the breadboard and the output of the pulse generator is now only capable of turning on the optocoupler when the detectors output turns on the control transistor!

This did mean a very simple yet fundamental change to the detector - I had to swap the inverting and non-inverting inputs over. Now, the output of the detector is high when the battery is below threshold. This turns on the transistor and grounds the cathode of the optocoupler.

Of course, at present on the breadboard with 741s in use, the transistor is never actually switched off! I measure 0.7v on the base due to the poor swing limits of the 741. With any luck, the LM358s will not only cure that, but the dual device means I can design the PCB much, much smaller! A surface mount NPN transistor takes up much less space than a SOIC-8 package! With the potential divider changed back to individual fixed resistors, to be selected on test (SoT), my current PCB design is down to slightly less than 24mm x 13mm!

I eventually managed to get my turn on the EasyEDA autorouter, and now have a fully designed PCB! I'll check it all over to ensure ive not made any silly design mistakes, and then see about getting some boards etched.