Performing some 9V battery characterisation I was in the need of a switchable load. Not having $$$ lying around to buy a proper load which can be controlled externally, I built a small constant-resistance load switched using logic level signals.

The device has two logic-level FETs that enable two possible resistive loads resulting in four configurations: high-impedance, load 1, load 2, or both loads in parallel.

The main problem is the parallel impedance of the FETs is only around 5 MΩ which isn't as high as I'd like to approximate an open circuit. Only stuffing one FET brings this up but reduces functionality.

Another issue is if the load needs to switched quickly, there is overshoot from the FETs that can distort measurements if a small time resolution is required. Luckily for battery measurements the resolution is sufficiently large to prevent significant error.