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FEZ experiences and connecting to a transmitter


I am working on a morse code keyboard which will connect to a transmitter.

I did a similar project on an Arduino and it took me weeks of spare moments to get it done. I had to write a program on a PC to act as the keyboard.

While I had more functionality on the Arduino version (so far) I estimate it took me about one tenth of the time to get it done with a FEZ. Hooking a keyboard to the FEZ took minutes. An interesting part was the development of a methodology to do user command inputs with morse code responses. No lcd for this project.

Of course knowing .NET, C# and having threads available did have something to do with it. Oh, I have about 20 years experience with c/c++ so language was not a big issue.

I am now thinking about how to do the actual connection to the transmitter.

It appears that the input to the transmitter is TTL like. On open key there is +5V present. To key the transmitter, and send, the input has to be grounded. When grounded, about 90 micro-amps of current are flowing. The input must be taken to less than .35V.

If I was using TTL logic I would use an open collector output to ground the input.

I could use a 2n2222 transistor and a resistor to ground the input driven by a digital output.

I thought about the relay component, which would the easiest/cleanest, but I don’t know if it can be toggled fast enough and whether it is rugged enough to take abuse of a keying relay. The fastest I would use it would be at 40ms and 120ms pulses, which is 30 words per minute.

I noticed that some of the Domino pins are open drain. I think these might be equivalent to open collector? Can I use one to ground the Tx input?

Anyone have a suggestion on the best/easy way to proceed?


We are always happy to hear the good stories :smiley:

With the description you gave, there isn’t enough information on what the transmitter expects…or I didn’t understand your post.

Even if the pin is TTL why not connect it directly to any pin? Do you have the datasheet of the transmitter?


The keying input to the transmitter appears to be a TTL input with a pull up resistor. When not transmitting there is 5V on the keying input. To transmit, the keying input needs to be pulled to grounded. In the un-keyed state the keying input does not require an external voltage to be applied.

When grounded, the unit which is actually a transceiver, goes from receive mode to transmit mode, and transmits a signal as long as the keying input is held to ground. When ungrounded the unit stops transmitting and goes back to receive mode.

I used a digital meter to measure the current that goes between the keying input and ground and found out that 90 microamps are being drawn when transmitting.

The instruction manual for the transmitter says that if using an external electronic keying circuit the drop across the external keying circuit must be less than 35 millivolts.

The transceiver is a commercial piece of amateur radio equipment by ICOM.

Bottom… In the unkeyed mode the FEZ driving circuit should at high impedance. When keyed the FEZ driving circuit should go to ground or within 35 millivolts of ground.

I did look at the schematic and saw that there was not a TTL circuit in the input. It was a transistor with a resistor to +5V.

Could I use a TriState port without a pull-up resistor and start in output mode high and then go to output low to transmit?


The I2C pins can be used on this case or you can use any pin through the Tristate Port.