The bias and metering circuits are based on work done by G3SEK and others.
(Why re-invent the wheel ?)
Additional logic (wired OR) has been added to control the PTT actuation and power supply trip
circuits required by the power supply controller. All of the critical monitoring and trip
logic is based on the ULN2003 (MC1413) IC's and does not use the microcontroller.
Leds are provided on the PCB to indicate PTT applied, PTT ok, Ig over-current and Ia
over-current. The PTT ok led will normally be activated (with PTT) but will extinguish if any
of the fault conditions exist along with the deactivation of the PTT output line.
A PIC 16F84 microcontroller provides a sequencer output with 4 possible sequence times
available. All of the sequence timers are adjustable in software. The last of the outputs
also controls the PTT activation, this can be bypassed with an on-board jumper (visible
between the relays) Release of PTT deactivates the timer in the reverse of the activation.
A PCB header is provided to remote the PTT sequence indications.
Surface mount components have been used where appropriate.