Thomas Henry 555 VCO

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.

Thomas Henry 555 VCO circuit working outs

Thomas Henry 555 VCO circuit working outs

Using highlighters I transferred the circuit on to strip-board using the Lochmaster strip-board design software.

Thomas Henry 555 VCO design on stripboard from lochmaster

Thomas Henry 555 VCO design on strip-board from lochmaster

After a few hours the finished board was ready to test out.

The Thomas Henry 555 VCO not working

The Thomas Henry 555 VCO not working

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.

Thomas Henry 555 VCO basic core on breadboard

Thomas Henry 555 VCO basic core on breadboard

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.

The Exponential generator on stripboard

The Exponential generator on stripboard

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 Thomas Henry 555VCO on stripboard producing a triangle wave

The Thomas Henry 555 VCO on strip-board producing a triangle wave

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.

ARM DSO201Pocket Digital Oscilloscope

ARM DSO201Pocket Digital Oscilloscope

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.

Thomas Henry 555 VCO with triangle, sawtooth and square wave circuits

Thomas Henry 555 VCO with triangle, sawtooth and square wave circuits

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.

 

The working VCO and Exponential generator stripboards

The working VCO and Exponential generator stripboards