After perusing the internet for quite a while I have decided to build the “Thomas Henry 555 VCO”. Thomas is quite respected in the world of electronic synthesiser design and he has published the circuit for people to build for non commercial use, so thanks to Thomas for that, also there are a lot of comments from people who have built it so it gives me great confidence that the published circuit diagram actually works. Something you cant always say for a lot of internet designs! Also just about all of the components are easily obtainable and I know I have a lot of them already, except the temperature variable resistor. I intend to just use a normal one while I’m playing around with it.
Unfortunately, as with a lot of electronic designs it seems that the PCB s are not currently available or they are waiting for another production run, so I’m going to try and build it on strip-board. After a lot of head scratching I came up with a design as you can see from the diagrams below.
Using highlighters I transferred the circuit on to strip-board using the Lochmaster strip-board design software.
After a few hours the finished board was ready to test out.
Unfortunately it didn’t work. I’m sure its my fault and I think I was maybe a little rusty with my electronic construction skills and a little over optimistic to put so much onto the strip-board, so back to the drawing board!
This time I’m going to try to understand the circuit and just build the bare minimum of the core of the oscillator and get that working first. I’m also going to build it on to my new toy. that’s the Breadboard I always wanted to get one and this was the perfect opportunity to try it out.
I think I managed to basically grasp how the circuit was oscillating and just build that.
A much simpler circuit to start and guess what IT WORKS! producing a nice triangle wave.
The next stage was to transfer this bit of the circuit on to strip-board and then verify it again. I also decided to build the exponential generator on to a separate strip-board so I could play around with it.
As you can see the transistors are mounted in sockets as they are not matched yet. I just wanted to get a working circuit so thats something I’ll improve later when I come to putting the tempco resistor in instead of the two 1K resistors ( I couldn’t find a 2K one!)
The picture above shows the strip-board with the additional op-amp and resistors for the final triangle wave output wired in. Also note my new toy the SainSmart Pocket-Sized Digital Oscilloscope ARM DSO Nano DSO201 I’m really impressed with this. It makes testing electronic circuits a lot easier and it impressed me with how clearly you could see the wave forms.
Having got the main core of the 555 VCO working along with the triangle wave shaper and the exponential generator it was just a case of wiring in the other op-amps and associated components and checking everything still works.
The above picture shows the finished stripboard layout with all the componets for triangle, ramp and square wave. Note that there is a preset to adjust to set the shape of the sawtooth wave. My SainSmart Pocket-Sized Digital Oscilloscope ARM DSO Nano DSO201 proved invaluable for this, having never owned an oscilloscope before I’m not sure how I would have been able to correctly set this.
I wasn’t able to get the sine wave to function properly. It was more than likely down to my mistakes as in hindsight I was trying to cram too much into too small a space and you don’t get a lot of room down the side of the particular board that I was using. You also need to fit another couple of preset resistors. This maybe something that I’ll look at again in the future but to be honest I’m quite OK without sine-waves, my Sequential Circuits Pro One oscillators didn’t produce them and I made do with that! Anyway below is a picture of the finished strip-boards. I’m going to build another one now and then try to tune them in. Then I’ll probably match the transistors and experiment with tempco resistors.