Hi guys! I've started designing an automatic led backlight. Simple as a transistor with high hfe switching between states. I planned to use some logical state (perhaps power signal on the lcd screen). But I stopped liking my idea when I calculated power consumption, because most of the time the calculator is used with natural light and this will be a huge power waste.

So, my question is: Is any way to control any part of the z80 (ASIC) to switch between logical states and control it with a program? Programing isn't a problem. In fact I'm designing all related to hardware and one friend who knows programming in assembler is ready to do software. I thought I could use the link port, but I've read something about silent checking for transfers so I assume that the port isn't all time low (about 5V) when unused. Main problem is I can't find any hardware info related to the ASIC as it's TI propietary. Another thing I can't know is if it will be possible to keep this logical state running low or high in the background without hanging the cpu and letting me use the calculator.

What do you think about? This is all because I really dislike the idea of cutting case to put a microswitch.

PD: I'm sorry if this is the wrong section to post this. Feel free to move it wherever you consider better

My first idea would be using a photoresistor connected to a transistor... But it would be hard to nicely hide it somewhere.

Well, you want something that will definitely leave the backlight off when the calculator is off, and can be on or off when the calculator is on. A photoresistor combined with a tap on the LCD's power line sounds like a good solution to me, because then you'll only have your backlight on when both the LCD is on and the LCD is in darkness.

Not exactly. I want something that could polarice a transistor when I like. In other words: go to prgm, choose an app, set low or high any physical logic state and remain it in that state until I choose to change. And it can't be switched if I decide to exit program and go to home screen to work. That's the main approach.
The second choice would be using a LDR as you both said. But it can't be done with just one transistor because of the gradualness of LDRs. Transistor will remain most of time in conduction or led will be dimming. I can do this with a few more components, that's not a problem.
The third choice will be beg to get the most beautiful microswitch in the world and drilling the front of the case to put in there. Some logic to switch and there I go. But despite beeing the most easy solution, I really dislike the idea of permanent drilling my case.

What I would really like to try is implementing the first option, if it's possible at all. My main problem is I can't find any usefull information about the ASIC to see if there is any line (AKA pin) unused that could be programmed to switch between states. I know it's a long shot, but if this can be done, it would be the most professional and confortable backlighting solution.

EDIT: I opened up my 83+ to take a look at flex cables and I've found a discrete Z80. Maybe it could be programmed directly. I'm afraid that my regular 83 (1st edition) has the embedded Z80+ASIC.