Diode logic and audio amplifier


Most constructors have used stripboard to construct a diode logic matrix for BPF and LPF switching, the PCB presented here should provide a far neater solution and is the same footprint as the timer so can be stacked if required to save footprint. The board is connected to the Pic N Mix via a 17 way lead, note that there is a "straight" connection from the Pic N Mix keeping wiring neat.


In addition to the diode matrix is an audio amplifier using the LM4940TS device which is available at low cost from Farnell, series resistors for attenuator and best IP3 front panel LEDs are also provided. Note that the attenuator is switched by keying "72" and best IP3 mode by "48". A darlington output is provided to directly switch the attenuator relay.

D11 to D13 provide a user output from DDS key entries "23", "43" and "79", any or all diodes can be fitted to drive the spare darlington output marked as "user 1".

Construction notes

  1. Pin spacing on audio amp is quite close so care needs to be exercise whilst soldering.

  2. Complete audio amplifier section has been left ungrounded by the mounting holes, this is so the constructor can utilise single point grounding to minimise ground loops if required.

  3. Ensure that components are soldered top and bottom where indicated on the overlay as their leads may be used to interconnect the PCB.

  4. The uncommitted darlington output (user 1) may be operated by key inputs 23, 79 or 43 by fitting diodes D11, D12 and D13 respectively.

  5. A darlington output is reserved for the attenuator relay (if fitted), operated by keying 72.

  6. An output is provided to switch the DDS filter board if used.

  7. Series resistors (3K3) are incorporated to provide power for attenuator and best IP3 indicator LEDs if required, take LED cathodes to ground.

  8. Capacitors C11 - C26 may be fitted if RF feedback becomes a problem, their fitting hasn't so far been required.

  9. If the spare BPF section is unused ensure it's control input is grounded so it doesn't "float" and intermittently turn the section on.


PCB files for board manufacture

Component overlay

Component list

Circuit schematic

Design files

PCB plots

PCB Overlay

Component list

Circuit schematic

Not currently available

PCB dimensions - 108 x 50.9mm to the outer edge of PCB outline

Revision history


Item affected


06-01-07 PCB Overlay Labels "user 1 and Attenuator" swapped
    Added construction notes
13-02-07 PCB Overlay Labels "Best IP3 (to magic roundabout) and best IPs LED anode swapped