| Electronics > Projects, Designs, and Technical Stuff |
| Photodiodes saturating in ambient light |
| << < (5/10) > >> |
| twospoons:
It was around the 800nm wavelength. This was a while ago so some details are a little hazy . The version of BPW34 we were using had a built-in longpass daylight filter. Just a dye incorporated in the package. Running reverse bias does usually yield more signal - we were using photovoltaic mode to save power in a battery operated system. We would have needed to supply over 30mA otherwise (lots of photodiodes). Attached is one trick to getting good signal in photovoltaic mode. The transformer takes care of the DC current, and the transistor acts as a trans-impedance amplifier. |
| aussie_laser_dude:
--- Quote ---The version of BPW34 we were using had a built-in longpass daylight filter. --- End quote --- ahhh haha, numbers make a lot more sense to me now. Thanks guys, figured out most of the details, using a 1 kohm resistor in reverse bias mode does actually work quite well. The signal is very low (~5mV100mV), definitely needs amplification, but now the AC signal remains a constant amplitude when the ambient sunlight changes. I've attached a pic of it operating in sunlight nicely and also a pic of my photodiode math/theory. Thanks tggzzz, ejeffrey, TimFox, T3sl4co1l & StillTrying for help on understanding this stuff :) Sweet, that's half the problem solved! Now onto the amplification side of things, pretty hyped to try out some different amplification methods, I do also plan to save battery using the photovoltaic approach (but happy to try out reverse bias circuits in early prototypes, whatever gets the prototype working), thanks twospoons, will definitely look into the circuits you've given. Thanks once again guys, one step closer turning an idea into reality :-+ |
| aussie_laser_dude:
Could I get some help with understanding twospoons small pulse transformer photovoltaic circuit in "reply 16". Transformers scare me, I don't understand transformer details very well. My understanding is the transformer will have a limited bandwidth, I have no idea how to calculate transformer properties and load resistor R1 that will give a bandwidth of say >3 MHz for the photovoltaic photodiode. Could R1 be changed from 1 Kohm to 10 Kohm without ruining the circuit bandwidth? My reasoning is that the large DC current from the photodiode won't be passing through the R1 resistor, so we can use a higher value resistor to get a higher potential difference. I'm worried this will sacrifice the bandwidth of the transformer or photovoltaic photodiode, but I'm too dumb to understand how to figure this out. I have no idea of the math. If anyone has a good link that explains these properties of transformers I'd love to have a look. Also, could a 1:4 transformer be used, ie. 1mV AC in and 4mV ac out on other side? Would a transformer like this ruin the bandwidth somehow? Is it better to stick to 1:1 transformers? Thanks for any help |
| SparkyFX:
You could try a polarizing filter in front of the receiver, those filter out ambient stray light very efficient. I can't tell if all lasers are polarized in general or you need to arrange the laser in a specific orientation, could you work with that? |
| TimFox:
Be careful with Polaroid-type (absorptive) polarizing filters and lasers. As a young graduate student, I burned a hole in a Polaroid by sending a low-power laser beam through it: the polarizer works by absorbing the power with the wrong polarization. |
| Navigation |
| Message Index |
| Next page |
| Previous page |