Greetings all,
I am working on a project which will trigger the PTT on 50W PAs using pins on an Adafruit Trinket M0. I can use a BSS138-based converter to bring the 3.3V logic levels to 5V, however, for my proof of concept using an Arduino UNO (5V logic), while it works -most- of the time, occasionally, judging by the red LED on the front of the PA, it struggles - the LED dims and/or flickers. Since the Trinket M0 can only manage ~7 mA per output port, whilst the UNO can manage 20 mA, after looking at schematics and sifting through spec sheets, the following questions have emerged:
1) Assuming a forward voltage of 2.2V and 5V on the PTT line, with the 470-ohm current limiting resistor, the transmit LED on the PA draws ~6 mA. Try as I might, I can’t seem to find the necessary amount of current to turn on the (2) BS-170s that the PTT signal is also connected to. How much current is necessary for the PTT line to function? I don’t recall having any trouble when going straight from a QDX to the amplifier, but I can’t sort out how much current the QDX is outputting on the PTT line.
2) Since the UNO, which can output 20 mA per pin, seemed to struggle a bit from time to time, I’m inclined to think that 20 mA will not be enough. While I don’t know how much additional current will be necessary, I don’t think that I’ll need a LOT (I’m not running a motor here!), so suggestions on how to boost 5-7 mA to, say, 40-50 mA would be welcome. Since the logic level will be fixed at 5V, then using a MOSFET with a low current on the source (correct pin?) and then 5V + resistor wired to the gate -sounds- like a good solution, but which MOSFET should I use and/or is there an easier (and/or correct) way? **
I appreciate the help!
Charles Johnson
KF4AYT
** After writing this, I realized that if the MOSFET had a 5V source, then a 3.3V + (probable) resistor connected to the gate might be sufficient, as this is just an on/off scenario, thus allowing me to skip the 3.3/5V conversion. Would that be realistic/correct/workable?
