0
\$\begingroup\$

I am trying to make a receiver circuit that is able to receive an LED input (further down the line receiver input from a laser).

Firstly, I am trying to test the photodiode that will be used. I am using the FDS02 (silicon high speed photodiode with FC/PC bulkhead) photodiode component from Thorlabs.

Experiment:

  • Connect the detector bare to an oscilloscope and make an amplifier circuit that will filter out DC signal before amplifying (this will filter out the ambient DC light from windows etc.)
  • As a light source use a strobe light from your phone and aim into the photodiode.
  • Connect the oscilloscope and determine the frequency.

How do I make a filter to be able to filter out the ambient light? What should the output look on the oscilloscope?

I have tested this circuit here (it is where the photodiode is and I am using the LM741 op-amp component):

enter image description here

When testing the photodiode with a simple transimpedance amplifier circuit with a high pass filter (Rf = 1 kΩ, Cf = 104 pF, Cs = 0.1 μF, Rs = 10 kΩ, V+ = 12 V and V- = -12 V) and shining a strobe light into the photodiode the oscilloscope shows that the photodiode stays at a frequency of around 60 Hz but the output of the amplifier shows just nothing/low signal. It just looks as if nothing is being put through the amplifier. Is this correct or am I going to this all wrong? Should I use a band-pass filter and if so how should I do that?

\$\endgroup\$
5
  • 1
    \$\begingroup\$ Unless you're doing it for the academic exercise, this problem is easily solved with a cheap IC. Such as: pololu.com/product/837 The "filter to get out ambient light" is done electronically, by putting your signal on a fixed frequency carrier (such as 56kHz). \$\endgroup\$
    – Kyle B
    Commented Apr 17, 2022 at 0:42
  • \$\begingroup\$ The main source of 60 Hz is from the room lighting. You need to modulate your transmitter at a frequency well separated from 60 Hz. \$\endgroup\$
    – user69795
    Commented Apr 17, 2022 at 3:56
  • \$\begingroup\$ "High speed" and "LM741" don't belong in the same chapter, paragraph nor sentence. LM741 will perform extremely poorly in this application, even with fairly modest requirements placed on the receiver. If you need to use the cheapest possible op-amps, then a TL081 will do the job as it has a FET input suitable for use with photodiodes, and is an order of magnitude faster than a 741 - and costs about the same or less. What will be the frequency or bit rate of the laser signal you want to receive? Who transmits the signal? What is the content of the signal (data? physical measurement? ...) \$\endgroup\$ Commented Apr 17, 2022 at 8:06
  • \$\begingroup\$ @Kubahasn'tforgottenMonica we have tested the built receiver circuit with a transmitter circuit that is connected to a function generator with a sine wave input with frequency set to 10kHz and 250mVpp. A blue LED is then shinned into the FDS02 photodiode. The oscilloscope shows that the input freq from the transmitter is 10.187kHz and the input freq from the photodiode is 13.228kHZ but then the output of the op-amp is "low signal". Could this be due to using the LM741 op-amp? Let me know if you would like me to post the pictures of the data I have gotten. \$\endgroup\$
    – Martial
    Commented Apr 17, 2022 at 17:56
  • \$\begingroup\$ @Martial Step 1: Eliminate out of band ambient light that may saturate the detector with baffling, optics, thin film filters, etc. Step 2: Use narrow band electronic filtering to tighten the acceptance bandwidth. Step 3: If the transmitter drifts in frequency, include a PLL to first lock and then track it. \$\endgroup\$
    – jonk
    Commented Apr 18, 2022 at 8:04

1 Answer 1

0
\$\begingroup\$

There are many ways to block 60 Hz if you can isolated the source of the noise;

  1. conducted common-mode noise (lack of isolation, shielding or grounding)
  2. FL light emitted noise ( usually 120Hz) using a Daylight Blocking Filter
  3. Radiated Noise ( same fixes as 1) but also with a ground guard ring around the differential input of the Op Amp.

You may also wish to consider a comparator that can use the BW of your detector, or use a Sharp/Vishay PD with a daylight blocking filter that has the same sensitivity of 0.5A/W and only costs pennies but isn't as fast.

There are also IRDA IC's ~ $6 115 kbaud also IRDA2 > 1 MBd which seem to be scarce or obsolete.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.