[minima] Re: PTT Schematic
- From: Ashhar Farhan <farhanbox@xxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Mon, 20 Jan 2014 22:13:34 +0530
mark,
we were running out of pins. we should have had a separate PTT input and
another T/R line as an output. But I had to make do with a single line.
This is quite confusing and i had to keep reminding myself of the plan.
the plan is like this:
1. In SSB mode, the PTT line is in input mode. it has an internal 10K
resistor pulling it up. The ptt switch provides the high/low switching.
2. In CW mode, the PTT line is put into output mode and pulled down,
putting the transceiver into transmit mode. Then, after a delay to let the
relays settle, the sidetone is keyed.
- f
On Mon, Jan 20, 2014 at 9:47 PM, Mark G0MGX <mark.g0mgx@xxxxxxxxx> wrote:
> Hi Gang
>
> At the risk of getting flamed (again) I have been muddling over the PTT
> part of the schematic and the main T/R relay control and provide the
> following to enable other to understand the workings of this bit....
>
> The top of D3 must be connected to +12 and Q24 pulls the other side of the
> relay coil to ground when the base is high. This is pulled high by the 2.2K
> resistor. So far so good. Now, Q23 then pulls the base of Q24 to ground
> when it's base is high - following so far?
>
> So, it seems that in RX mode the base (via the 4K7) of Q23 must be high
> and in TX the base must be grounded - we can see this is the case from the
> PTT switch which does the grounding in TX. So how is this line held to a
> HIGH state when in RX mode?
>
> Looking at the code we see:
>
> #define TX_RX (3)
>
> this just defines the constant TX_RX to have value 3 - which says we are
> using pin 3 on the Arduino Uno board which is pin 5 of the processor.
>
> then we see:
>
> pinMode(TX_RX, INPUT);
>
> so this pin is being used as an input and will therefore read HIGH (equal
> to 1) or LOW (equal to 0) when using the digitalRead(TX_RX) command in
> Arduino.
>
> a few lines later in the code we see:
>
> digitalWrite(TX_RX, 1); //old way to enable the built-in pull-ups
>
> This instruction enables the internal pull-up on the Arduino pin 3; this
> is the software equivelant to an external pull up resistor on that line
> i.e. it is HIGH by default when no connection.
>
> Hence the base of Q23 is pulled HIGH in software and will be taken LOW by
> pressing the PTT switch......
>
> Simples!
>
> Mark
> G0MGX
>
>
>
>
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