# Help on a Project Design Question

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#### Kib_

##### Newbie level 2
I'm a mechanical engineering student adventuring on the mechatronics field and i'm still new and struggling a bit into all that electronic field, so I'd like some help/feedback on a problem design on a module i'm currently studying:

a proximity sensor, Using a LED flashing at 1.8KHz should reflect on the object near and be reflected to a Photodiode, the photodiode then sends a current signal max amplitude of 5microamps, Also take into consideration the ambient light that the photodiode is measuring and how you could minimize its effect.

So far i've broke the problem in 3 steps: 1- current to voltage conversion
2- Amplification
3-Filtering

for the the current to voltage conversion, i'm using a transimpedance circuit to convert that current into voltage and here my question is: is it ok to convert microamps to Volts? at ideal level i could do it, but in practical what sort of amplification i would be aiming here, microamps to milivolts right? searched a lot and could not find an answer, always end up in discussions/topics on higher level of knowledge than mine, i guess that's a basic question but couldn't find proper explanation

for the amplification part, if i'm doing straight microamps to Volts on the transimpedance i guess i don't need it, but is that ok? i've read about AC/DC coupling amplification and was thinking to use it here, is it appropriate for this particular case, I guess the photodiode is generating a DC current than AC coupling is not suitable right? need some light here.

On the filtering part I'm inclined to use a passband filter, using a passive Lowpass followed by a passive highpass filter properly separeted by voltage followers, but i've read it's not an elegant solution and maybe i could use an active filter, but is it really necessary? since my frequency of interest is 1.8Khz(LED) should i make it my center frequency of the passband right? doing so would also get rid of the ambient light interference or a least minimize it?

I'd like to hear your feedback, maybe i forgot to consider something, i might try to build this later on and see how it works but for now i'd like to know if i'm on the right track.

thanks for your attention and such a big post, but i'm feeling a bit confused with all those options, will upload a schematic of circuit later on when i get home.

#### FvM

##### Super Moderator
Staff member
Many questions at once. I suggest to start with a simple but basic thing. The ambient light level and respective photodiode DC current commands the feasible I/V converter gain. You should clarify this point first.

tpetar

### tpetar

Points: 2

#### Kib_

##### Newbie level 2
Many questions at once. I suggest to start with a simple but basic thing. The ambient light level and respective photodiode DC current commands the feasible I/V converter gain. You should clarify this point first.

Well sorry for being brief, but the problem is also brief on the details, it only states that the photodiode will detect ambient light(no explanation on high/low frenquency interference or any hint whatsoever) and that the Photodiode will generate a current of maximum amplitude of 5 microamps, my question was if it was ok to use a transimpedance amp. (ideal one) with a feedback resistor of 1Megaohm, and if in a practical circuit that conversion would be possible/usual or it would take more steps, like convert the microamperes current to milivolts and them amplify that voltage, can't find an answer to that.

And i don't really mind someone answering everything, if you can just give me a little help on one the 3 points that will already help me a lot thanks

#### FvM

##### Super Moderator
Staff member
Microcampere to volts in a single amplifier stage isn't a problem for kHz frequencies, presumed the ambient light doesn't cause DC currents larger than microamperes. That's somehow unlikely. I would definitely expect a maximum Idc specification.

Another point is that you stated maximum signal amplitude of 5 µA. What's minimal detectable amplitude?

tpetar

Points: 2