Circuit :  Dan Christensen, Ontario, Canada
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Description:
This circuit, from Dan Christensen is a rebuild of my earlier Mark 5 design. He has designed and tested each stage and his circuit responds to IR signals and re-transmits them accurately.

Download the original circuit large view.

Notes:
In the Mark 5 circuit, the output of IC1 looked great, but the output of C1 was holding a DC charge after each pulse was over, since it had no low resistance path to ground. A 100k pulldown resistor was added after C1, which brings the output voltage back to ground, and improves the filtering of ambient light.

The second stage amplifier based on IC2 is now a non-inverting amplifier with a positive gain of x11 and the output pulse shape is good.

The final output stage has also been modified. The original circuit included a diode and two output transistors. As the second stage is no longer inverted, a single NPN amplifying stage is fed by two 1k resistors. There is no capacitor and no phase shift change and Dan found the output very nice and clean, in phase with the original input signal and works beautifully in his living room!

Some other minor changes were to power the circuit from a single 12V wall wart adapter, and reduce the 5.6M resistors to 4.7M, and reduce the filter capacitor from 22n to 2n. Dan used his XMEGA oscilloscope to test each stage of the design.

Update:
During later testing Dan has found that by changing R17 from 100k to 10k or even 4.7k has improved the filtering of ambient light and helped noise immunity.