Schematics for the Mic Amp and the 555 Modulator

I had promised last year that I would publish schematics for the microphone amplifier and the 555 PWM modulator.

I recently spent some time learning how to use ExpressPCB.  Their schematic tool makes a much better picture than my hand drawn schematic I posted last year.

I'll start with the Microphone Amplifier.  I used an LM833 OpAmp in my transmitter, mostly for two reasons; I had a bunch on hand, and I liked the sound it produced.
You can substitute any dual OpAmp (TL082, LF353, TL072, etc.) for the LM833 in this schematic and get good results.

This is a new version of the microphone amplifier for my transmitter from the previous version I posted.  I realized that I didn't need to add the low pass filter to the microphone amplifier.  My optical receiver has a low pass filter in it's amplifier chain, so adding it in the microphone chain seemed redundant.

This is the 555 timer PWM modulator

The LEDs I use are the SFH4550.  These have a 6 degree beamwidth so you can cover a lot of distance without a collimating lens.  My new 3D printed transmitter uses 2 parallel banks of 6 LEDs in series.  I've denoted this on the schematic as D1-Dx for the first 6 and D2-Dy for the second 6 LEDs.  R4 and R5 are valued to limit the current to the LEDs based on the voltage drop of the 6 LEDs and the duty cycle of the timer.

My modulator is set-up to run the LEDs at a duty cycle of 20% so I can run 400mA to the LEDs.  This is roughly 4 times the data sheet value for luminous intensity and light output.  I chose this value to increase the output well past the datasheet value while still providing some life to my portable batteries without having to carry large gel cells with me.

To get to the 20% duty cycle for the LEDs you need to set up the 555 timer to provide an 80% duty cycle. The MOSFET will inverse the duty cycle when switching the LEDs.
R1 and R2 will set the duty cycle of the timer.  I use 12k for R1 and 2.2k for R2.
Keep C3 connections as short and direct as possible.  If you want to add more bypass for the circuit, you could also add a bypass capacitor after the 1N4001 (mislabeled as D1) directly to ground.

I haven't worked up a PCB for these yet.  Currently I am using vero board and I can create a small foot print with those.