After fiddling with the ATtiny based optical transmitter with mixed results for almost 2 months, I put that design on the shelf and pulled out a 555 timer.
I had a solid PWM duty cycle with the ATtiny45. I could modify it to produce any duty cycle up to 100%. I had a nice 600Hz tone playing through it with a 20% duty cycle, firing an SFH4550 LED with 400mA of current. What I couldn't get to work was intelligible audio when adding a microphone and Op-amp. I got close, but not close enough for me, so I took a break and created this.
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| 555 timer based Optical PWM transmitter |
It is a PWM transmitter based on the 555 timer. The output is fed through a .1uf capacitor to an IRF510 MOSFET, which switches an SFH4550 IR LED. Power is a 9v battery, so I have a 5watt 18 ohm current limiting resistor which brings the current to just under 400mA. To be able to feed that much current to the LED I need to have the duty cycle at 20%. The 555 timer can be used for less than a 50% duty cycle if you put a diode from pin 7 (discharge pin) to pin 2 (trigger pin). Also R1 needs to be smaller than R2, which is reversed from the traditional timer circuit. Audio is connected to pin 5 (control pin) through a .1uf capacitor.
The PWM Frequency is about 85 kHz.
These are the values I am using to set the frequency and duty cycle;
R1 = 2200 ohms
R2 = 12000 ohms
C1 = .001uf
For testing I've been using my iPhone to provide audio. Yes I am using music, but only for the initial tests. Once I build out the audio amplifier, I will post the completed circuit. The plan now is to create an amplifier that will be used for an electret microphone as well as a level for a line input from a laptop for digital modes.
Until next time - 73, Warren.
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