I was looking to create my own interface with perfboard and liked the idea of a built in timeout timer as shown in and included below. However, I would also like it if there was also a PTT LED and was wondering if anyone had any suggestions where to add it. Thanks!

I assume you want it on the actual control circuit, and not on the timer request input.
Adding an LED and limiting (say 1k) resistor between pin 3 and 2k7 resistor would work.
This put the LED circuit in parallel with 2N2222 base bias.

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Rob KB8RCO

开云体育

Hello Matthew,

If I understand that circuit correctly, connect a 1k resistor to pin 3 of the 555 IC.? On the other side of the resistor, connect it to the longer lead of the LED (anode).? Connect the shorter lead of the LED (cathode) to ground.? Btw, here is a designed PCB that includes a PTT and DCD LED and makes the delay adjustable too:

??

--David
KI6ZHD

>Adding an LED and limiting (say 1k) resistor between pin 3 and 2k7 resistor would work.

And it might not.

There may not be enough voltage (3v3) to forward bias the transistor and an LED if they are in series.

David's idea of adding the LED from pin 3 to ground with a resistor would work (parallel to the transistor).

Danny
K5CG

On Tue, Aug 8, 2023 at 11:12 AM, Rob Giuliano wrote:

Adding an LED and limiting (say 1k) resistor between pin 3 and 2k7 resistor would work.
This put the LED circuit in parallel with 2N2222 base bias.

Poor description.
I didn't state 'to ground', but indicating 'This puts the LED in parallel with the wN2222 base bias' was to pass the information.
David's description was much more descriptive.
?
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Rob KB8RCO

Hi,

The NE555 timer chip supports strong output currents (TI specifies +/- 225mA) and therefore adding a parallel LED circuit is of no concern and in fact the preferred solution. It will work equally well with small signal LEDs as well as super-bright LEDs (which require higher currents but will be clearly seen even in sunshine). The only concern in that case is the load on the on-board 5V to 3.3V regulator in the Raspberry Pi to avoid overloading it (early Pi's had a linear regulator that only supported 50mA. now they have a switching regulator that will support 500mA).

However care must be taken when using outputs from other chips (such as low-power micro-controllers) that aren't designed to drive large loads. I designed a simple circuit that uses the drive current for the PTT transistor to illuminate a small signal LED. The original resistor is replaced with a LED and resistor in series. The value of the new resistor is lower to compensate for the voltage drop of the LED. The current through the LED is large enough to illuminate a small signal LED but also small enough to be within the design limits of the GPIO output and the transistor input. This is also an efficient design for battery operation since there is no increase in power consumption.

Thanks
Thomas
KK6FPP