The SKC is designed to provide basic keyer functions for simple qrp rigs. It is particularly useful in Direct Conversion rigs, as it provides a means of generating side tone. Separate outputs for audio mute and transmitter keying provide sequenced audio muting and oscillator shift, allowing these to happen before and after the transmitter is keyed. This eliminates the need for R/C time constants in the auido mute switch(es) in the rig.

When not active, the keyer is in a low power sleep mode. While "asleep" the chip draws just a few microamps. This not only reduces current comsumption when the keyer is not being used, it keeps the internal oscillator from being heard in the receiver section of the rig its installed into. Sleep mode was added to chips shipped after 6/1/06.

The SKC could also be handy as a keyer to operate separate transmitter and receiver set ups, solid or hollow state. The audio mute output can be used to both mute the receiver, shift the Tx oscillator frequency (if needed) and operate the antenna change over relay, before the transmitter is keyed. It also provides the side tone, which separate Rx/Tx setups generally lack.

New external keyer mode - included in chips shipped after 3/7/07

Two message memories added to SKC using Tiny13 chip


There are five possible functions which are selected using the “Function” Switch. These are:

1. Send message (not available in original, Tiny11 chip version of the SKC)
2. Set code speed
3. Enter and exit Tune Mode
4. Enter store message mode (not available in original, Tiny11 chip version of SKC)
5. Change from Iambic B to A mode.

Sending messages:

After a short, quick click of the function switch, tap the Dot paddle to send message 1 or tap the Dash paddle to send message 2. Once a message has started to be sent, it can be paused, stopped or set to beacon mode. Note: only message 1 can be used for beacons.

Pause: Close and hold the Dash paddle.
Stop: Close and hold the Dot paddle.

NOTE: these actions will occur after a character currently being sent has finished sending.

Beacon mode:

Click and hold the function button as Message 1 is being sent. The letter “B” will sound from the side tone when the mode is activated. There is a fixed 3 second delay between repeating the message. Closing either paddle during the delay will terminate beacon mode. In addition, message Pause and Stop may be used during the sending of the message.

Change code speed:

A short, quick click of the switch enters change code speed mode. There is a short pause to see if you want to send a message, then letter “S” will be sounded by the side tone.

• Closing the Dash paddle will increase the code speed, a dot will sound each time the
speed in incremented by 1 wpm.
• Closing the Dot paddle will decrease the code speed. Again, a dot will sound each time
the speed in decremented by 1 wpm.

• At the upper and lower speed limits, a double dot (I) will sound.
• The change code speed mode will automatically exit after the paddles have been
released for about 1 second.

Tune mode:

Tune mode is used when you want to key the transmitter continuously to say adjust an antenna tuner or make a power output measurement.

To enter Tune mode:
• Click and hold closed the function switch until the letter “T” is sounded by the side tone (about 1 second)
• To key the transmitter on, tap the Dash paddle closed.
• To turn the transmitter off, tap the Dot paddle closed.
• Repeat as needed.
• To exit Tune mode and return to normal operation, click the Function switch.

Entering Messages:

• Click and hold the function switch closed until the letter “M” is sounded by the side tone, about 1 second after the tune mode annunciation of “T”.

• A message of up to 29 characters (including word spaces) may now be entered via the paddle. If you exceed the maximum character limit, “EM” will sound and you will have to start again, making the message shorter. If you have a 2X3 call, you won't be able to quite fit a 3x3 CQ into the memory.

• “Ideal” timing of 7 dot times for character space and 21 dot times for word spacing is used to determine the completion of a character or word space. To insure a word space in inserted, it is best to pause slightly longer they you would otherwise between words.

• Click the Function switch when you are finished entering the message. The message will repeat so you can check for timing errors. If you need to re-enter the message, click the Function button and “EM” will sound. Since many CW ops then to run thier letters together by leaving insuficent space between the end of one letter element group and the start of the next letter, the message may come out quite garbled. It may take several trys and some practice to get the message to record properly.

• If the message was entered correctly, tap the Dot paddle to store the message in location 1 or the Dash paddle to store in location 2. “MS” (message stored) will sound and normal operation will resume.

Selecting Iambic A/B mode:

Click and hold closed the Function switch until the letter “A” or “B” is sent by the side tone, about 3 seconds. If the keyer was in “A” mode, “B” will sound, indicating it has switched to B mode. With the new Tiny13 chip, the mode selection is stored in EEPROM memory and the selected mode used on power up. With the old Tiny11 SKC, iambic mode has to be reset each time the chip is powered up if you want to use "A" mode.

Disabling Side tone output:

If using the SKC with a rig which generates its own side tone when transmitting, the SKC side tone output can be disabled by grounding the audio mute pin (pin5). This pin is checked when power is applied to the chip. Side tone is only disabled when transmitting, so that annuciation characters needed when using the Function switch or entering in messages can still be heard. Disabling the side tone would generally only be needed to be done if using the SKC as an external keyer.

Straight key mode:

Grounding the DASH input on power up will put the SKC into straight key mode. This is most easialy done by insterting a mono plug into the paddle jack. In this mode, the Tx keying output and side tone will follow the input to the DOT pin. The Function switch is disabled in this mode, as the paddle is needed to control the functions it enables. Straight key mode also alows using another, more advanced keyer if the SKC is built into a rig.

Connecting up the SKC.

The pin out and a typical wiring schematic is show below;


Tx keying and Audio mute output from chip are active high. 2N7000 mosfets are supplied to invert the signal as most rigs key and mute with active low signal. They also allow for switching voltages greater than the 5 volts the SKC operates at.

R1 is a required pull up resistor, as this port pin has no internal pull up. The value is not at all critical, anything in the range of 10 K to 100 K will work.

C2, 3 and 4 are required for static protection and help debounce the switch and paddle inputs.

R2, C5 and C6 are used to interface the side tone output with the audio stages of the rig. R2 and C6 form a low pass filter which helps make the square wave output of the side tone less raspy. Typical values for R2 and C6 would be 1 meg ohm and 0.1 uF. C5 is mostly used as a DC blocking cap, but can also be used to help set the side tone level. Depending of the gain of the audio stages following side tone injection, this cap maybe as small as 100 pF. If your rig uses a LM386, typically the pin 3 input is not used and is a good place to inject side tone into.

A small pizeo speaker could also be connected directly to the side tone output pin 7. You can not direclty drive a low impedance dynamic speaker from this pin.

The +5 volts needed to power the SKC can come from a 5 volt regulator such as a 78L05 or a 5.1 volt zener and dropping resistor. Timing for the side tone and keying speed is derived from an internal R/C clock. Both voltage and temperature will affect the clock speed. For proper side tone pitch and keyer speeds, the supply voltage must be near 5 volts and be stable, so the use of a 5V regulator is recommended.

Printed circuit board layout:

A X1 circuit board layout is available for down load by clicking here. The board is shown in "through board" view, so it can be directly printed onto toner transfer film. Transfering the image to the board copper reverses the image. The layout includes a spot for a 78L05 voltage regulator and reverse polarity protection diode, not shown on the schematic above. A PCB mount stereo jack (Mouser part # 161-3507-E) and a right angle TACT pushbutton (Mouser part # 612-TL1105S) are also included on the board. The board can be mounted to a front panel using the stereo jack. The two pads on the layout diagram labeled "NTS" (no side tone) are include so you can add a jumper there to disable the side tone if required.


Send $3.00 for each chip wanted, plus $2.00 for postage. DIP package only. Instructions do not come with the chip, print this page, as its all you need to know.

Steven Weber KD1JV

580 Durand Rd

Randolph, NH 03593 USA