Hey guys! First time posting here and I would like some advice for my final year project. I'm really bad at designing circuits and English are not my main language but feel free to criticise any mistakes or stupidity. So here goes nothing.

I am trying to use light as a medium to transmit digital data. I chose the ASK modulating system as it seems to be the simplest to construct.

1) I finished designing the transmitting side: Used the 555 chip to generate the carrier and a 7408 chip (And gate) as a multiplier to generate the ASK signal. 2 white leds were used as the transmitter. I would like a suggestion for what to make to be used as the digital data source, besides just connecting a on/off switch to the circuit.

2)I am so far having a nightmare designing on the reciever side though. A photodiode is used as the receiver but I'm not sure if I can connect it directly to the detector circuit. For the detector circuit I'm following the block diagram of my school's textbook but they do not have circuits diagram for it. The detector consist of, in order, a Bandpass filter---->Full wave rectifier---->Low pass filter----->Threshold detector

My teacher told me to use a 2nd order bandpass filter for the first part but although I have found circuits to construct one I'm still not sure how it to calculate the components value for it. This link http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm for example says:

[ First order low or high pass cutoff frequency (-3dB point) = 1/(2pi*R*C)
2nd order low or high pass cutoff frequency (-3dB point) = 1/2pi(R1*R2*C1*C2)^.5

Example for 200 Hz cutoff frequency - R1=R2=7.95K, C1=C2=0.1uF ]


I don't want to sound silly but does his example apply to both the 1st and 2nd Order circuits?

Continuing on, I think I should have no problems regarding the full wave rectifier and the low pass filter but can I just use a normal low pass filter instead of a 2nd order version?

Lastly, I have no idea how to design a threshold detector at all. Googling reveals to me a Threshold detector with hysteresis and a Zero crossing threshold detector. Can I use them?

//en.wikipedia.org/wiki/Threshold_detector_with_hysteresis
//en.wikipedia.org/wiki/Zero_crossing_threshold_detector

I would use 2 white leds to show the recovered data at the end.

Thank you all who took the time and effort to read this. I feel much appreciated.

 

The output of a photo-diode is extremely low unless the LED transmitter is glued to it.
So it needs to have a pretty high gain amplifier if you want any range. IR receiver ICs use automatic-gain-control to reduce interference and to avoid overload when the transmitter is close.

With the lowpass and highpass sections of the 2nd order filters tuned to the same frequency then their 3dB losses add and produce a loss of 6dB (half the output).

 

I will see if I can pass along some things to consider.

Without knowing parameters like distance and the nature of the data, like work length and frequency, it hard to say much about your modulation. ASK has at least two variations, and you don't say if you are going to use on/off or multilevel.

As for the receiver, look at TAOS light to voltage converters. They are three pin devices with sensitivity from IR to UV. The output is a voltage. look up the spec sheet on a TSL12S-LF and see if that isn't interesting.

Anything but a PB switch gets elaborate, unless you go with a microcontroller. As the data will be transmitted asynchronously, you might think of framing bits to delineate the beginning and end of words/bytes.

Voltage comparators make nice level detectors.

Remotes use IR with a 38 KHz carrier. You could look into how they work to get more ideas.

 

You need a 'transimpedance' amplifier circuit to measure the photodiode. My definition, it causes the variable current induced by light to be transformed into a varying voltage. Then, you'd have to clean it up via level detection. I'd experiment with different filter types, pretty easy to do on a breadboard.

Steve