Electronic Melodist

How to build MonotrOndes Part 3

 

Sorry, It's been a while since my last post! It was a warm summer here in Finland and I didn't have a summer holiday. So I enjoyed sunny evenings and weekends. But I have also improved the design of MonotrOndes.

Now the expression lever also controls the filter cutoff!

The Pulley System

Mitsushi Abe's Ondes Martenot MIDI Controller was the first design I came across that uses horizontal pulley system. This site gave me inspiration to build MonotrOndes that way too.
http://gam.boo.jp/blog/archives/2012/08/midi_1.html
I don't understand japanese but the pictures were very informative. :)

Horizontal version simplifies construction a lot.
Check out Mark Steiner's awesome Artemis that also uses horizontal system.
http://www.youtube.com/watch?v=fRdTDTK3QTQ

The Pulley System. Top left pulley is connected to the multiturn pot. The other pulleys have ball bearings to ensure fast action. The tension spring is very important part. 

I'm not going to tell every possible detail about the mechanical construction. There are many ways to build this kind of system. The basic idea is quite simple. It's more fun when people add their own personal touches. I bought my pulleys from finnish online hobby shop. I had to enlarge holes. I don't own a drill press so holes are not perfectly centered but it doesn't affect performance. In the near future I will resin cast my custom pulleys - topic for future blog post. :)
The string is fake leather cord - same type that jewellery hobbyist are using. The ring is keychain ring that fits to my finger. I knitted a little "padding" for the ring. I covered ugly knots with aluminum tape. :)
Dana Countryman used piece of pipe and velcro for the ring. I think it's a much better idea.
http://www.danacountryman.com/martenot_project/martenot3.html

The Ring.

You need a tension spring. I have an assorted spring set which I found from hardware store. I'm sure you'll find similar sets in your country.
When the Ring is in the middle of the keyboard the Tension Spring should also be in the middle.
Attach the Tension Spring first. It's a little challenging to tie the Ring - there should be a little tension in the spring when the Ring is in place.

The Ring and the Tension Spring in the middle of the keyboard.

The Keyboard

Technically it's not a keyboard. It's only a reference for finding a correct pitch. But let's call it keyboard anyways. :)
Many people have wondered why the keys gets narrower in the higher octaves. Monotron has linear to exponential conversion. But it’s not perfect 1 volt/octave. It's not designed to have pitch range of many octaves. If you want to make keyboard more even you will need extra circuitry.  I haven't tried this but I think you can use an anti-log trick.
http://www.geofex.com/article_folders/potsecrets/potscret.htm
 
I'm also a guitarist and I like the fact that the keyboard gets thinner in the higher octaves like frets in stringed instrument. I get more violin like vibrato and larger note range that way.  

When adjusting the pitch range and intonation use Monotron's own intonation trimmer and that trimmer pot I introduced in part 2. 

I have four and half octaves in my keyboard.
Lowest octave in my keyboard is 20 centimeters wide from C to C. fourth octave is less than 9 cm wide.
Multiturn pot is adjusted to be in the end of it's travel when the Ring is in the highest point of the keyboard. 

Here is how I did the keyboard overlay:
I adjusted pitch range first. Then I attached paper strip under the string and listened reference pitches from other instruments. I marked painstakingly every note. The paper strip as a reference I drawed stylised keyboard with Inkscape and printed it with a laser printer. I laminated both sides with clear contact paper. For those tactile pitch points I used scrapbooking stickers.

I wanted to keep keyboard overlay changeable. For example I might want to experiment with micro tonal scales. I used a piece of a picture frame and Velcro stickers as a way to keep keyboard in place.