HP-42S battery drain

[Dave Shaffer]

(07-13-2020 02:55 AM)Marcovecchio Wrote:  Another possibility: the processor certainly has a switching transistor to drive the IR led. Is there an easy way to test if the IR circuitry is ok, if I don't have an IR printer? This transistor might be damaged, and leaking current. Does that make sense?

Do you have an oscilloscope?

If so, you can make a very simple IR detector. You need a photodiode or phototransistor (typically less than $1 here in the US) and a resistor and a few volts from some batteries.

Something like this: https://electronics.stackexchange.com/qu...photodiode