Introduction

This stage was arbitrarily chosen as the place to go ahead and install the remaining SMT capacitors (now that the SMT ICs have all been installed).

This and the next three stages wrap up some miscellaneous enabling functionality to handle switching between RX and TX and to provide audio listening) output for the board, should that be desired.

Schematic

PTT Circuit Schematic

Bill of Materials

DesignationValueOrientation
C54 0.1uF, smt 1206 smt
C55 0.1uF, smt 1206 smt
C56 0.1uF, smt 1206 smt
C57 0.1uF, smt 1206 smt
C58 0.1uF, smt 1206 smt
C59 0.1uF, smt 1206 smt
C60 0.1uF, smt 1206 smt
C61 0.1uF, smt 1206 smt
C62 0.1uF, smt 1206 smt
C68 0.1uF, smt 1206 smt
C36 0.033uF, ceramic, 10% n/a
D3 1N4003 West-East  (cathode=hairpin)
Q09 2N3904 NPN TO 92
Q10 2N3906 PNP TO 92
R33 2.21 K, 1/4W, 1% East-West
R34 4.99 K, 1/4W, 1% South-North
R35 10.0 K, 1/4 W, 1% West-East
R36 10.0 K, 1/4 W, 1% West-East

Build Notes

PTT Circuit Topside

  • When mounting D3, mount it so the lead going to the square pad (cathode) is a hairpin style lead
  • Be sure to distinguish between the 2N3904 and the 2N3906 (the markings look very similar).

PTT Circuit Bottomside

DesignationValueOrientation
C54 0.1uF, smt 1206 smt
C55 0.1uF, smt 1206 smt
C56 0.1uF, smt 1206 smt
C57 0.1uF, smt 1206 smt
C58 0.1uF, smt 1206 smt
C59 0.1uF, smt 1206 smt
C60 0.1uF, smt 1206 smt
C61 0.1uF, smt 1206 smt
C62 0.1uF, smt 1206 smt
C68 0.1uF, smt 1206 smt

Testing

Current Draw

  • With current limiting resistor:
  • Without current limiting resistor you should see ~38 mA

Switching

  • Before powering up, attach a clip lead between the hairpin of R33 and the hairpin of R35 (this sets PTT_IN = 12 Vdc, ON)
  • Apply power
  • Measure the voltage at the hairpin of R34.  You should get 9-10 Vdc
  • Measure the voltage at the S12V pad. You should get a high signal ~12 Vdc (can go as low as 10 Vdc, depending upon battery)
  • Unclip the lead at the R33 hairpin (this sets PTT OFF)
  • Measure the voltage at R34 again.  You should get 9-10 Vdc
  • Measure the voltage at the S12V pad again.  You should get a low signal, ~0 Vdc