NT7S Code Practice Oscillator

NT7S Code Practice Oscillator - In Altoids Tin

Here it is, a completed version of the simple discrete component code practice oscillator that I promised. I tweaked the circuit just a little bit and made a Manhattan layout that will enable the CPO, a 9 volt battery, and all of the required controls to fit into a standard sized Altoids tin. This CPO produces a nice sine wave at about 600 Hz, unlike many of the other CPOs that output a buzzy square wave tone. There are no exotic parts used in this project, only a couple of generic NPN transistors, a handful of common resistors and capacitors, and a trim pot. The output level is sufficent for headphone use, although it will not blow your eardrums out, even at full volume. If you need to use a speaker with this oscillator, just plug it into a set of amplified speakers, like those used for a computer. This project would also make a good oscillator for CW practice on a VHF/UHF FM repeater. The volume control should allow you to adjust the output level to one that is appropriate for the microphone jack of a FM rig.

NT7S Code Practice Oscillator - Tin Open
Inside view of the CPO

I’ve attached a PDF schematic and layout diagram below. I haven’t created any build instructions, but it should be an easy build for anyone who has any experience with Manhattan construction. Let me know if you plan on using this design for a group build to help people learn CW, I might be able to work with you to create such a document. Print out the layout diagram at 100% scale, and you should be able to use it to size your copper clad board and mark the locations of your pads. I hope this is helpful to you and can help you to introduce new operators to CW.



Preliminary CPO Schematic

Code Practice Oscillator Schematic
Code Practice Oscillator Schematic

Here’s quick update to post the schematic of the code practice oscillator that I mentioned previously. As you can see, I just paired a twin-T sine wave oscillator with a buffer amp that feeds directly into headphones. The twin-T provides plenty of voltage, so the buffer is all we need to provide enough current to drive headphones. You can also download a PDF file of the schematic here.

Thanks to a suggestion from David KB0ZKE, I’ve decided to rework the layout to fit in an Altoids tin (an excellent idea!). I’m also going to try to come up with an easily reproduced straight key made from a paper clip, knob, wooden base, and wood screws. This idea was inspired by KE6GS, who has a great example of such a key right here next to his Willamette. I’ll try to get the layout changes finished real soon. The detailed documentation will have to wait until after I finish the Willamette documentation (but I have started on it again, so it will come in the near future).


Back to Basics

I know that the blog updates have been a bit light over the last week or so. Although we have been in our new house for three weeks now, it seems like the chores just keep piling up. However, I have done a little bit of work in the homebrewing department. Inspired by messages from WB9VTB and KB9BVN, I decided it might be nice to create a simple code practice oscillator based on the twin-T sidetone oscillator from the Willamette. The discrete component CPO published by the ARRL is really neat because it is very simple and has a unique build method. However, the circuit is your traditional astable multivibrator, which produces a near-square wave. I guess I’m spoiled, but I like listening to a clean sine wave. It certainly doesn’t take any more components to build a twin-T compared to an astable multivibrator.

I experimented with a few different ideas for simple, discrete component audio amplifiers to pair with the oscillator, but settled on perhaps the simplest of all: an emitter follower. The twin-T oscillator puts out a waveform with a fairly large voltage, so all I really needed to do was tack on an emitter follower to provide some high impedance buffering for the oscillator. The entire circuit is extremely simple and produces a pleasant tone at 600 Hz, which you can sample here. The circuit can easily drive low impedance headphones, but if you wanted to listen on a speaker, you would need more amplification. An easy solution would be to plug in a set of amplified speakers, but it wouldn’t be hard to add another stage of amplification.

I’ve created a Manhattan layout for the circuit, and I think that I would like to develop a complete kit with full build instructions at some time in the future. Something that would make it easy for the complete homebrewing novice to successfully build. I know that CPOs are a dime-a-dozen, but I think that the simplicity of this design (2 NPN transistors, handful of resistors and caps, a few 1/8″ phone jacks) is a bit unique.