If your project now works properly, then the following observations are only for future consideration. They are from an "if it were me" perspective.
1. As far as I can tell you only use 3v3 for the phototransistor part of the circuit. Forgive me if I am missing something but I don't get the use of the 3v3 zener. I dont know what the input signal to the phototransistor looks like, but under most circumstances I think you could simply get rid of the 3v3 regulator and the 3v3 zener, connect the collector of the phototransistor to GPIO26, the emitter to ground and program an internal pull-up in the pico to pull GPIO26 up when the phototransistor is off.
2. In general terms I am uncomfortable about the idea of running a Linear regulator off the output of another one. It just doesn't feel right, and in this case I don't think it will be reliable because of the 3V3 regulator's drop out voltage.
3. If you use a Linear regulator in a future circuit then you should consider the dropout voltage of the regulator. For example a MulticompPro L78L33 has a drop out voltage of 2V. As its a 3.3V regulator that means its absolute minimum input voltage is 5.3V for correct operation. Data sheet states regulation accuracy for Vin 5.8V to 20V but personally, because of component tolerances I would never go less than 4V margin with a 2V dropout voltage. Fair enough, you need to bear in mind power efficiency. A 3.3V linear regulator with a 7V input would lose more than half the used power as heat. In cases like this with a tiny current, its such a small amount of extra power I feel it can be ignored. For circuit with a 5V output regulator running at 3A its a different ball game.
For the tiny current you seem to be using, if the existing board suffers from problems with 3v3 regulation then you could just take the L78L33 input from 12v, which would probably be the simplest mod.
Hope you find this useful and best of luck with your projects.
1. As far as I can tell you only use 3v3 for the phototransistor part of the circuit. Forgive me if I am missing something but I don't get the use of the 3v3 zener. I dont know what the input signal to the phototransistor looks like, but under most circumstances I think you could simply get rid of the 3v3 regulator and the 3v3 zener, connect the collector of the phototransistor to GPIO26, the emitter to ground and program an internal pull-up in the pico to pull GPIO26 up when the phototransistor is off.
2. In general terms I am uncomfortable about the idea of running a Linear regulator off the output of another one. It just doesn't feel right, and in this case I don't think it will be reliable because of the 3V3 regulator's drop out voltage.
3. If you use a Linear regulator in a future circuit then you should consider the dropout voltage of the regulator. For example a MulticompPro L78L33 has a drop out voltage of 2V. As its a 3.3V regulator that means its absolute minimum input voltage is 5.3V for correct operation. Data sheet states regulation accuracy for Vin 5.8V to 20V but personally, because of component tolerances I would never go less than 4V margin with a 2V dropout voltage. Fair enough, you need to bear in mind power efficiency. A 3.3V linear regulator with a 7V input would lose more than half the used power as heat. In cases like this with a tiny current, its such a small amount of extra power I feel it can be ignored. For circuit with a 5V output regulator running at 3A its a different ball game.
For the tiny current you seem to be using, if the existing board suffers from problems with 3v3 regulation then you could just take the L78L33 input from 12v, which would probably be the simplest mod.
Hope you find this useful and best of luck with your projects.
Statistics: Posted by Pi5_User — Thu Mar 27, 2025 10:44 pm