700Hz CW Audio filter board

Nik VK4PLN has yet another board design – this time for an audio CW filter similar to the QCX and HiPerMite.  Nik says “he is  loving the cheap and easy EasyEDA/JLPCB boards and simple web designer”.

He “takes no credit for the design, just copied it and added a switching relay.   He has ordered 10 boards at $10 posted”.

Details of the filter

Center Frequency: 700 Hz
Bandwidth: 200 Hz
DC Power: 5VDC
GAIN: Upto 20dB via R11 and R12 + trimpot.

Resistors 1206 SMD:
R1 33k
R2 33k
R3 1M
R4 47k
R5 47k
R6 36k
R7 36k
R8 10k
R9 750k
R10 10
R11 100K
R12 100K

Capacitors 1206 SMD:
C1 0.1u 104
C2 47n 473
C3 36n 363 (33n + 3n3 mount on side together)
C4 1n 102
C5 39n 393 (33n + 5n8 mount on side together)
C6 1n 102
C7 2n2 222
C8 2n2 222
C11 0.1u 104
C14 220u Electrolytic

Relay OMRON G6S-2-DC5
200K/100k trimpot

R11 and R12 are optional to parallel the 200k trimpot to make it a logarithmic 100k.   Or just use a linear 100k pot.

Use a switch connected to 5V to turn on the filter.

This powers the opamp and switches the relay feeding the audio into the circuit.  When off the relay just passes the audio straight through unfiltered.


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