# Have I got the schematics right?

I’m playing around with driver designs in Eagle and just want to make sure if I have got the schematics right?
This should be schematics for zener mod and off time capacitor:

Hm, no reverse polarity protection, and the voltage divider will always measure the zener voltage, not battery voltage. You'd probably want to connect the top side of R1 to the left of R3, and use values for R1/R2 appropriate for the battery voltage.

Ahh, the reverse polarity protection. I forgot that with Eagle it’s not modding drivers, it’s designing them.

But about the voltage divider, what exactly is it needed for? Is it for low voltage protection only? Zener modding an existing nangj illuminates low voltage protection. Is the divider still needed in that case?

Yes, low voltage protection for the batteries. When designing your own driver you can do it right and have working voltage monitoring, which might be especially helpful in a 2s setup.

You need the voltage divider to connect before anything else so it sees the raw voltage then adjust the two R’s so that it works using the same set point as before.

On the original 105C battery monitoring occurs after D1 so the voltage read is already down by~ .5 V so you do the math to see what pin 7 sees when monitoring kicks in and then recompute the R values for the # of cells you plan on using. This has been done before by some of us and there’s the obligatory thread around someplace but it’s a good exercise to go through to help cement your understanding of the circuit. I have to run but will check back this evening and hunt it down if it hasn’t been pointed out for you.

In this Post (Post 917), BLF member wight tell us how to calculate LVP resisters for multiple cells and for with/without protection diode.

You can adjust the FW to fine tune from there..

This is excellent stuff! Thanks!

I read the discussion between you and wight, but one thing still eludes me. What happens if cells are inserted incorrectly into a circuit with the divider before the polarity diode? I know there will be very little current flowing through the divider resistors, but what about the capacitor connected to pin 2 (for off time) and other connections to the MCU? Nothing funky will happen?

No, the GPIO pin used for the vdivide input can accept a [small] + or - voltage, not like the + pin which can ONLY accept a + voltage in.