The circuit is designed as a colour sensor, using a 3-channel output photodiode to detect the RGB light intensity.
In order to save component costs and in an attempt to produce a more compact solution, I included a 4-1 multiplexer to switch between the 3-channels of the photodiode sensor
Please do not be overly concerned about the PIC, I couldn't find the particular PIC I wanted so I just selected a random one. The pin numbers are wrong, but the pin names are correct.
I'm not sure if this multiplexing would work, can someone please point me in the right direction?
p.s. the power supply consists of 6V AAA batteries with PC78L05 as a low dropout regulartor
I think you need to amplify the output from each photodiode separately, and feed the amplified signals into the MUX.
The way you've got it, there's a lot of low-pass filtering between the MUX output and the PIC input, so you would have to switch very slowly to avoid just getting an average of the three signals.
I think you need to amplify the output from each photodiode separately, and feed the amplified signals into the MUX.
The way you've got it, there's a lot of low-pass filtering between the MUX output and the PIC input, so you would have to switch very slowly to avoid just getting an average of the three signals.
Yes indeed, didn't notice that the input current has to be in uA range...
I found a low bias current op-amp but forgot about the mux..
This stupid error cost me 20% for my design report :'(
Anyway, since I only need my system bandwidth to be 20Hz, the switching time I made it to be 6ms for each channel.
This part should be okay though is it?
high gain bandwidth Transimpedance ( current to voltage ) amplifier is essential .
THen you need to decide on bandwidth of the signal and response time of your detectors.
Where is your Design Spec? Looking forward to a well thought out Functional Spec with measurable parameters.
high gain bandwidth Transimpedance ( current to voltage ) amplifier is essential .
THen you need to decide on bandwidth of the signal and response time of your detectors.
Where is your Design Spec? Looking forward to a well thought out Functional Spec with measurable parameters.
I actually didn't develop a spec for the system, the aim is to design a sensor that can detect the colour of any physical object and replicate the colour using RGB LEDs as accurately as possible (yeah think about it now it is indeed very vague...)
I initially designed with three transimpedance amplifier using the low bias current OPA2338, and the MCP601 is designed to have a -3dB bandwidth at 140Hz for 1% aliasing error. Then I decided that it's probably a good idea just to switch between the three since the signal bandwidth is <20Hz. And my system bandwidth is pretty much dominated by the low pass at 140 Hz. But I completely forgot about the input current to the mux is way too small...
Anyway, if I scrape the whole idea of multiplexing, or put three transimpedance amplifier first then the mux, the system should work though? Well at least the analogue circuit part?
Are you sure you need such a high transimpedance? You have a potential divider in the feedback implying a current of maybe 10nA.
In principle you can multiplex photodiodes but leakage will kill the idea if your currents really are that low, in which case you need 3 transimpedance amplifiers as has already been suggested.