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  1. Q900
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[Q900] PTT jack on the back

 

The design is wrong, and China manufacturer knows it. It need a resistor value changed from 4.7k to 560-1000 ohms.?


On Tuesday, June 13, 2023, Edouard Griffiths <f4exb06@...> wrote:
You will not damage anything but it simply won't work. The XPA125B needs to sense less than 0.1V to switch to Tx and it seems the darlington mount on the Q900 side cannot get that low when turned on. One would need an intermediate relay which is a pain.

 

You mean R81 then that connects the LK1801S input to the PTT signal. This means that if you don't change this resistor it will not turn on the Darlingtons no matter what...
Brgds, Edouard F4EXB.?
 

Yes! I simulated that chip in ltspice, and the highest value of the resistor to make a relay work is 1k. I got about 100mA thru ptt with 1k.? Plenty for a relay.? Now, the problem is with lowering the value. The bus that R81 is getting the voltage from might be affected, and other block circuits might be affected if too much current is drawn. So I thin think 910 to 1000 ohms should do.?


On Thursday, June 15, 2023, Edouard Griffiths <f4exb06@...> wrote:
You mean R81 then that connects the LK1801S input to the PTT signal. This means that if you don't change this resistor it will not turn on the Darlingtons no matter what...
Brgds, Edouard F4EXB.?

 

Thanks for the details. There are some 5V relay modules that take a trigger signal and can be set to trigger if low or high. Then you'd need much less current through a pull up resistor. I have ordered such a module and intend to take the 5V source from the USB host socket. We'll see how it goes...

Brgds, Edouard F4EXB

On Thu, Jun 15, 2023, 19:55 N1COZ COZ <Callsign.n1coz@...> wrote:
Yes! I simulated that chip in ltspice, and the highest value of the resistor to make a relay work is 1k. I got about 100mA thru ptt with 1k.? Plenty for a relay.? Now, the problem is with lowering the value. The bus that R81 is getting the voltage from might be affected, and other block circuits might be affected if too much current is drawn. So I thin think 910 to 1000 ohms should do.?

On Thursday, June 15, 2023, Edouard Griffiths <f4exb06@...> wrote:
You mean R81 then that connects the LK1801S input to the PTT signal. This means that if you don't change this resistor it will not turn on the Darlingtons no matter what...
Brgds, Edouard F4EXB.?

 

I tried this but the current is so small when PTT is on that it cannot even lower voltage by more that a few mV even with a 470k pull up resistor. This PTT output is just useless.
Brgds, Edouard F4EXB.
 

470kohms is way too big. 910 ohms, under 1k.



On Monday, June 19, 2023, Edouard Griffiths <f4exb06@...> wrote:
I tried this but the current is so small when PTT is on that it cannot even lower voltage by more that a few mV even with a 470k pull up resistor. This PTT output is just useless.
Brgds, Edouard F4EXB.

 

This is not to replace the internal R81 on input of the LK1801S but a pull up resistor between +5V and the PTT tip.

Brgds, Edouard F4EXB.

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