QRP iambic keyer (initial discussion)

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QRP iambic keyer (initial discussion)

Postby Eamon » Sun Apr 29, 2012 1:11 pm

Since we have one more session left over and most people have finished the Ramsey 40m receiver, I thought we would try cramming another mini-project into our busy schedule: a QRP keyer using the KD1JV "Simple Keyer Chip".

The SKC is an Atmel ATTiny13 (see http://www.atmel.com/devices/attiny13.aspx), programmed with KD1JV's firmware. It has a lot of interesting features including variable speed and a message bugger. The instructions and features can be seen here http://kd1jv.qrpradio.com/skc/SKC.HTM.

A keyer can be used with a QRP radio (many of which don't have a built-in keyer), or with an older big radio.

It can also be used stand-alone to learn how to send CW with a keyer.

Of course, you still need an iambic key (paddle). Maybe we will build those as well, a bit later.

Charles managed to build it and make it work yesterday from nothing more than the connection diagram projected onto the screen. With a bit more preparation, we should be able to help anyone build one who wants to, at our next session. This is a good introduction to how to just put things together from a schematic. It is ultra-simple, but we are not being given a kit with instructions. We have to figure out where everything goes, ourselves.

The exact cost for the parts is TBD but will be between $5 and $10, a major part of which will be $3 per chip we'll send to KD1JV. The rest is stuff from Addison or Abra which we'll pick up, or Digikey which we'll order.

Please email me at ve2egn at gmail dot com to let me know you are interested - I have two weeks to get the chips, but I need to order them right away to be sure they get here on time.

- Eamon
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Feeding a CMOS chip power or GND through its I/O pins

Postby Eamon » Sun Apr 29, 2012 7:22 pm

This is not at all recommended, but since Charles did it and it worked, I thought I would talk about why.

CMOS I/O pins typically have diode clamps to the internal power or ground pins for ESD. This way, if a positive ESD pulse comes along, the positive ESD diode will conduct it to the positive rail. If a negative one comes along, the negative diode will conduct it to the GND rail.

If, as Charles did, you ground pin 5 (an I/O pin) instead of pin 4 (the proper GND pin), the positive current wants to go somewhere. It goes through the chip, and onto the chip ground. The ground pin isn't connected, so it can't go out on the ground pin. So it finds the ESD diode connected to pin 5, and flows through the diode and out on pin 5.

The circuit will work OK with the following limitations:

- all the GND current is flowing out through the ESD diode on pin 5, instead of on pin 4 (the proper GND pin). There might be a limit to how much current this diode is happy conducting.

- there will be an approx. 0.6V diode drop, so the chip GND will be about 0.6V above the GND connected on pin 5. So the chip will run on 0.6V less than the full supply rail.

Note this won't work unconditionally. If the firmware were to enable a HIGH output on pin 5, it would start sucking a LOT more current and might not end up very well.

- Eamon
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Re: QRP iambic keyer

Postby ve2rfi » Sun Apr 29, 2012 8:06 pm

I actually fed 5 volt supply to pin 5 (wrong end of the chip)


Charles
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Re: Feeding a CMOS chip power or GND through its I/O pins

Postby vernerle » Sun Apr 29, 2012 9:38 pm

Eamon wrote:This is not at all recommended, but since Charles did it and it worked, I thought I would talk about why.

Just a question on why we are using 5 volts vs say 8 to 15 volts. I always thought CMOS could run on anything from 3 to 15 volts. When I took digital tech at (mumble mumble) in TO we usaed variable voltage supplies to power projects.

VEI

- Eamon
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Re: QRP iambic keyer

Postby VE2EVN » Sun Apr 29, 2012 9:48 pm

As per Eamon's request, moving discussion to the forum.
ve2rfi wrote:Marc-Andre,

A 5.1 volt zener is recommended in the instructions by KD1JV for voltage regulation. Is a zener not good to keep a stable voltage?

A regulator is better with varying currents, and generally does a better job especially if the input voltage varies.
The 78L05 needs a minimum input of 8V to work correctly (7v is pushing it) but can go as high at 18V. The higher the input voltage and current drawn, the hotter it will become.
You can also use a regular 7805 (TO-220), but the 78L (TO92) might be better for a small QRP board.

ve2rfi wrote:Eamon,

Maybe we can build a add-on Wein Bridge sine wave oscillator using the LM386 for a practice keyer that would have a good tone.
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Re: QRP iambic keyer

Postby Eamon » Sun Apr 29, 2012 10:36 pm

ve2rfi wrote:I actually fed 5 volt supply to pin 5 (wrong end of the chip)


Charles


Same principle applies. Except in this case, it's the ESD diode from pin 5 to the + supply that conducts.

Maybe pin 5 was getting grounded by the firmware at some point, and that's where we saw it taking 60mA.

60mA is actually a LOT for this processor to take. Now that I think of it, it seems to me that the only way it would take that much would be in a case like we're discussing. But, very likely not heavy enough to destroy the chip.

- Eamon
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Re: Feeding a CMOS chip power or GND through its I/O pins

Postby Eamon » Sun Apr 29, 2012 10:42 pm

vernerle wrote:
Just a question on why we are using 5 volts vs say 8 to 15 volts. I always thought CMOS could run on anything from 3 to 15 volts. When I took digital tech at (mumble mumble) in TO we usaed variable voltage supplies to power projects.

VEI



You are correct Verne regarding 4000 series CMOS logic, but this is not true for CMOS in general. In particular, the ATTiny13A runs from 1.8 to 5.5V.

- Eamon
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Re: QRP iambic keyer

Postby ve2rfi » Tue May 08, 2012 5:15 pm

Hello, I have finished my keyer and seems to work well with a 12 volt power supply, I added a 5 volt regulator and a small filter cap. I am using a KBS27DA5C56 button piezo glued inside the case for side tone, I here it well enought. I started building a practice oscillator using a 1458 op amp, I have it oscillating around 700hz with a decent sinewave. But it looks like I may need to built a small amp for it.

Charles
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Re: QRP iambic keyer

Postby Eamon » Tue May 08, 2012 6:55 pm

I'm not sure why you would want to build a separate oscillator, Charles, when pin 7 gives you a side tone.

I tested out the keyer chip last night with a couple of piezo speakers I had lying around.

I used the pin 7 side-tone output and it seemed to me as if it was probably loud enough.


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Re: QRP iambic keyer

Postby ve2rfi » Tue May 08, 2012 8:06 pm

The oscillator is for practice, WIARC needs extra oscillators for science day and the tone from the oscillator sounds more pleasant than the piezo.

Charles
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