LFO

This is a relatively simple project. It's a low frequency oscillator (LFO) that gives square and triangle out. There are three controls; one potentiometer for LFO rate (or frequency if you prefer), one potentiometer for adjusting pulse width (or changing triangle to sawtooth), and one switch for selecting two different frequency ranges. Many thanks to Eric G and Magnus Danielson for ideas, suggestions, and thoughts.

Schematic
Component placement for stripboard
  1. First measure the stripboard.

    The printout of the component placement in the background of the photo is incorrect. The above file is corrected.

  2. Use a small hacksaw to saw it to the right size, and trim the edges with a small file to avoid leftovers from the sawing to shorten any copper strips.

  3. Snap off the small metallic bits from the potentiometers using a flat plier.

  4. Mark up the stripboard. This will make it much easier to insert the components in the right place.

  5. Then make links between strips. This can be done with or without the isolation, I usually do it without isolation.

  6. Use the flat plier to bend the links.

  7. Insert it (in the right place) and expand (or collapse) the pins of the link to hold it in place. Solder.

  8. When all but two (!) links are inserted it should look something like this.

  9. Then it's time for the resistors. Resistors are not polar and can be placed in any direction, however, it's easier to for example troubleshoot if all resistors are placed in the same direction. Note that some resistors are more standing than others.

  10. When the resistors are in place the final two links should be placed. The reason for doing this now and not earlier is because the links are a bit too thick to be placed to the right under the IC. Therefore I use one pin from one of the resistors there.

  11. Now the IC-holder should be soldered in place.

  12. When these parts have been soldered it's time to break/interrupt the strips. Turn the stripboard upside down and use a 3mm (or something about that size) drill. Do this by spinning the drill between your fingers. Then use a sharp knife to cut off any burring. As you can see I made a mistake when soldering of the links...

  13. Continue to solder the rest of the components. The trimpotentiometer and the power connector. Set the trim potentiometer to the middle. Mark + and - for the power connection!

  14. Bend the pins of the electrolytic capacitors. Electrolytes are polar and must be mounted in the correct direction. In the schematic the positive side are marked while the negative side are marked on the electrolytic capacitor. Usually the negative pin is slightly shorter.

  15. Solder the electrolytic capacitors in place. -12V (volt) is negative to ground (GND), while GND are negative to +12V. The ceramic disc capacitors can be placed in any direction, however it's good practice to place both in the same direction. Also solder the diodes (1N4148). The comment about the direction referred to an old printout of the component placement. If the above link is followed, everything is correct. The diodes should be placed as shown in the photo. The black line on the diodes mark the cathode.

  16. Solder the rest of the components. You'll need to give the LED some support to be able to solder it in the same hight as the potentiometers. These potentiometers take some space between the potentiometer "house" and the board. For some of the LFOs I've build, I've bent the pins on the potentiometer and soldered it from underneath the stripboard. I've done this to save panel space, being able to mount the LFOs closer together.

  17. All components on the stripboard are soldered. It should look something like this, with the double opamp (the TL072) inserted, and the NPN transistor (the BC550C) in place. Note, if you use 2N3904 instead this should be mounted turned 180 degrees compared to the BC550C. Please refer to the data sheets of the transistors if you're unsure.

  18. The two capacitors for the Range switch are soldered at the switch. Both capacitors (22uF and 1uF) are rather high giving a very (extremely) slow LFO rates. Decrease the value for higher LFO rates, maybe 1uF and 100nF or even 100nF and 10nF, would be more suitable. At it's highest rate the 1uF capacitor makes the LFO swing in 0.5Hz... First cut and bend the pins.

  19. Solder the capacitors to the switch. The switch is closing in diagonal to the switch arm. So if you want to have a faster rate when the switch arm is upward you should solder the 1uF (the smaller) capacitor on the bottommost pin. Solder the other pin of both capacitors together.

  20. Then solder cables from the switch and the joined capacitors to the stripboard. I used some heat shrink to isolate the pins of the switch.

  21. Then it's time for the connectors, the phone jacks. Usually mono jacks are used, and then the tip is for the signal and the sleeve for the ground. These are marked on the photo. I used switched jacks, but I don't use the switching function so I just leave it.

  22. I solder a link between both ground connection of the phone jacks, that's why the are mounted mirroring each other.

  23. Finally it's time to solder three cables between the phono jacks and the stripboard. One cable for the square wave, one for the triangle wave, and one for ground. It is good practice to have a resistor in series with the output to prevent the opamp to be destroyed if the output is shorted, which might happen if you patch the modules while the power is on (which you of course will do if you use the LFO in a modular system). Add a 1k resistor between the opamps and the output. This resistor is not (!) in the schematic or in the component placement illustrations. The resistor can be soldered standing up from the stripboard, solder one side of the resistor to the board and the other side to the cable and add some heat shrink to isolate the resistor. Or, you can solder the resistor directly to the jack and then to the cable that runs to the board, add heat shrink around the resistor. Or, you could solder the resistor bent, but a bit standing, just like the 47k resistors. Break/interrupt the strips under the resistors, and solder the cables to the jacks.

    As you can see in that photo I've replaced the 22uF with a 100nF capacitor.

  24. When all is soldered in place it's a good time to check all soldering. Use a multimeter to seek for any short circuits between copper strips and over interruptions. Refer to the schematic and the component placement, and don't forget that in some places you've added a link between strips.

  25. Finally it's testing time! If needed adjust the triangle wave output with the trim potentiometer. This sets the amplitude (and it also affects the frequency) of the triangle wave. Adjust the trim potentiometer until you're pleased with the output.

  26. You'll need some kind of cables for the power connectors. This is how I usually make them.

  27. If you're doing more than one LFO you can save some space on the stripboard by skipping the power connection and soldering cables between LFO 1 and LFO 2, and also skip the capacitors next to the power connection (on the other hand, it's always good to have a stable power supply so you might want to keep them). A good thing with this is that you can make the second stripboard a tad more narrow, allowing access to the first boards power connector. It could also be fun to use other capacitors to set the frequency, maybe 220nF and 22nF could be good values. I built four LFOs and the capacitors I used is: LFO1 - 1uF and 100nF, LFO2 - 220nF and 22nF, LFO3 - 100nF and 10nF, and LFO4 - 10nF and 4.7nF.

  28. The four finished LFOs.

That's it! Have fun and happy modulating!

/Niklas

Updated - 2013-08-31 21:35
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