Meteo in APRS with Arduino - project schematic question

Thread Starter


Joined Mar 28, 2019
Hi all
I've found a project that should do what I've written in the subject of the thread.
Arduino is connected to a VHF Intek transceiver.
What do you think the circuit in the red circle do?
I've asked the guy who designed this project and he has told me that it's used since 90s with TNC packet radio.
I don't know... Is that diode necessary? Has that cap the right polarity?



Joined Jan 29, 2010
Hi Lucky,
The Atmega program, while running correctly, sends High going pulses out on pin#15 continuously at a mSecs rate, the circuit detects the pulses and keeps the Watch Dog circuit disabled, so Vptt output is held Low.

In the event of a program/system latch up, the Atmega will not send the V(vp) pulses, so within approx 10mSec the Watch Dog circuit will set Vptt High.

The Diode discharges a very small current when Q1C switches OFF, when the Watch Dog circuit detects a fault.

Edit: The Cap polarity is reversed, polarity added.
EG 1322.png
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Joined Jan 23, 2018
That circuit gives the impression that it pulls low the PTT (Push To Talk) line and thus keys the transmitter. That is what APRS is about Amateur Position Reporting System.