The < 100nA leakage "power off" circuit I meant is in the transistor tester circuit.
The pdf in the link below shows at least 20 different display variants, and it also has the 3 BJT power switch circuit.
https://github.com/Upcycle-Electronics/AVR-Transistor-Tester/blob/master/Final%20Draft%20LCD%20Master%20Transistor%20Tester.pdfMine has been laying in a drawer for 2 or more years and the 9V battery is still good.
(It shows battery voltage on startup
.
BJT's do need a continuous base current to turn them on, but in applications such as the transistor tester the low leakage while off is the predominant factor. In the few seconds the transistor tester is on, the base current of those transistors is still < 1 % of the current draw and therefore pretty insignificant.
Mine version is also one of the oldest / simplest versions with an HD44780 display.
Battery life is so long that I simply glued it with hot snot to the PCB.
I also glued a piece of corrugated cardboard to the bottom of the circuit:
1). First press the wires of the THT components in the cardboard.
2). Deform the cardboard a bit more where the wires are.
3). Glue the cardboard to the bottom of the PCB.
4). Cut cardboard to the size of the PCB.
5). Smear some more glue around the sides of the cardboard to make it more durable.
You can also make "rubber feet" from a dot of hot snot. Once it's cooled and especially some dust has accumulated on the outside it does not stick anymore, and they hold better to the underside of your equipment than those "self adhesevive" rubber feet, which always fall off after some time.
Last time I used my transistor tester was for some IR power MOSfets.
For some kind of weird reason they do not specify the pinout of the TO220 power MOSfet's in their datasheets
and with this tester it's easy to check which pin is where.
Some transistors also come in different pinouts, with the same part number, and some different suffix and then this tester also helps to get more confidence in the actual component you have in your hands. I always like to verify info from datasheets with relality.