LFO Prime--Reverse Engineering my Own Design--Works!

Want to Build The Module in this blog post? To download a Eurorack version of the LFO Prime's gerber, BOM, schematic, etc., please go to my Sponsor PCBWay's Project Site, here.  To build a Frac or Large Format version, go to my github page, here. 

OK here's today's post:

Hello again!! 

  

This is a continuation of the post here. To summarize: I built a basic LFO in 2003 or so I like a lot but forgot to document any of it.  

Using a continuity tester and common sense could I recreate my own work?  After a false start (post here), answer is yes. My First LFO--"LFO PRIME" has been successfully reverse engineered, and the new version of this old design works.

My builds always start with PCBs from PCBway. They have fast turnaround and offer cool services like metal PCB's. Please help support this blog by checking them out.  

In my previous attempts to bring the 2003 LFO design back to life what was wrong?  We need 10V peak to peak unipolar or bipolar output signals, but that wasn't being seen with a scope.

Once I thought about how a total newbie would correct this sort of bias issue I reduced the voltage of the square wave by about 80% then boosted both signals back up 400%--after that, everything worked.  

 

The front panel was left over and unused from last time

Ready to test....there is an LED the blinks for each LFO pulse, but I have too many LEDs in my rig so I left it off this build.

Front Panel still fits....

The features: Tri or Square, frequency control with course frequency switch, and a bias offset (bipolar or unipolar).  Three identical outputs. Super Simple.

Calibration: Build this LFO then attach any of the LFO's 3 outputs to a scope. Adjust the "shape" trimmer until the triangle wave looks the way you want.  

Next, adjust the "offset" trimmer so when you throw the 0V/5V switch the peak to output's peak to peak voltage is offset by 5V. Otherwise, the output waveform should straddle ground.  

Finally, adjust "Speed" so the frequency switch (fast/up), with pot fully CW, is whatever you want.  The speed trim impacts the P/P voltage of the circuit so you may have to re-adjust the other trimmers once you settle on the speed you want.

Mods: Download the schematic then take a look at C3 and C4. Adjust these caps' values: larger value capacitors mean slower frequencies; smaller values will be faster. I used 1000uF for the slow speed and 33uF for fast, but any reasonable values work.  

However, you will need to stay with 25K for the frequency pot--going to a higher value here will slow down the frequency of the LFO, but I found it got harder to get a good triangle shape if you use a large value for the "Frequency" pot.

For perfectionists, tweak R3 and R2 to make sure the triangle and square are exactly the same amplitude (I didn't care that much).  This circuit could be easily customized as well: for bias, the BIAS 0V/5V switch could be eliminated and the offset wiper tied to R6 for fully adjustable bias control. 

Overall, this is a simple design; experiment and have fun. 

Again thanks to PCBWAY for hanging in there while I tried to figure out my own work. From now on I will document, document, document!