We needed a function generator in PCBheaven and instead of buying one we decided to build one from scratch. The whole idea is based in maxim's MAX038 chip. This chip is rather expensive, yet has many capabilities. It can provide 3 different types of waveforms, triangular, rectangular and sine wave, as well as it provides a TTL output with 50% duty cycle for synchronization reasons. The frequency can be adjusted from 0.1 Hz up to 20 MHz.The frequency range can be selected by placing a different capacitor to the COSC input. A 12 positions rotary switch with 12 different capacitors is perfect for this reason. The output frequency can be selected from within the frequency range by a potentiometer and another pots is responsible for the fine tuning
The DADJ input of the chip is the control for the duty cycle of the waveform. It has a switch to either disable it (50% fixed duty cycle), or enable the duty cycle adjustment. If enabled, the duty cycle can be adjusted by a potentiometer.
The output waveform type can be selected from a "FUNCTION" switch. This is a 3-position rotary switch. The common of the switch is driven HIGH, while the two inputs of the chip, A0 and A1 are pulled LOW with 10K resistors. When both inputs are '0', the output is square wave. When A0 is '1', the output is triangular waveform, and when A1 is '1' and A0 is '0', the output is sine wave.
The output is buffered with an OP37Z OP-AMP. The peak to peak amplitude is also controlled a potentiometer. By default, the output waveform is symmetrical to the ground. A DC offset can be added to the output if the switch "OFFSET ENABLED" is closed. The DC offset is also controlled by a potensiometer.
Step 1: Materials
Here is a rough list of materials used for this project. Since completion some changes have been made but you can read them in the worklog.
R1 10K R13 75 Ohm C1 0,1uF C16 22uF D1 1N4001
R2 10K R14 330 Ohm C2 0,1uF C17 100uF D2 1N4001
R3 47 Ohm R15 6.8k C3 0,1uF C18 IC1 MAX038
R4 10K R16 820 Ohm C4 0,1uF C19 0,1uF IC3 OP372
R5 1K pots R17 100K C5 0,1uF C20 10nF IC4 A TL072P
R6 1K pots R18 100K C6 22pF C21 0,1uF IC4 B TL072P
R7 1K pots R19 100K C7 82pF C22 0,1uF IC5 LM741P
R8 5K pots R20 100K C8 330pF C23 0,1uF IC6 7805T
R9 10K R21 10K C9 1nF C24 0,1uF IC7 7805T
R10 1K R22 10K pots C10 4,7nF C25 1000uF S2 12 pos. rotary switch
R11 1K R23 500 Ohm C11 32nF C26 0,1uF
R12 10K R24 500 Ohm C12 68nF C27 1000uF
C13 330nF C28 0,1uF
C14 1uF C29 1000uF
C15 4,7uF C30 1000uF
Apart from the electronics, 2 acrylic glass plates 16x10cm used as face plate for the controls along with some colored buttons to give a more pro look.
Step 2: PCB
PCB and finished result.
Keen eye can observe some slight modifications form original schematic. Since this is going to be used in our labs near the main power supply, the 7805 and 7905 chips have been removed. Instead, the +- 5 volts come directly from power supply. Also the PCB was created with the IC2 (TL72P) to control the frequency select and fine tuning. But during the tests, didn't behave as expected and therefore was replaced with two 10K resistors. Updated schematic can be found here.
The etching came out perfect with our home made etching bath but that is another instructable to come.
The soldering of the parts had to be done very carefully, as the grounding layer had just .1'' insulation.
Step 3: Range Selector
The range selector is a 12-position single pole rotary switch. Each switch position corresponds to a different capacitor. So, either you solder the capacitors to the PCB and draw 12 (+1 for common) wires to the selector, or do this:
First, make a ring with a thick single core wire (2.5 mm2). The diameter should be about 10mm larger than the diameter of the rotary switch. Cut and solder the two edges of this wire to make a closed ring. Then, solder one terminal of a capacitor to one position of the rotary switch. The other terminal of the capacitor is free, outside the switch's diameter. On this terminal, solder the ring. Then, on the symmetrical pin of the switch, solder likewise another capacitor. Do that for all 12 capacitors. When finished, all 12 capacitors connected directly on the selector switch, and only 2 wires would now be required from the PCB, the 0V wire which is connected on the ring, and the COSC input for the chip.
Step 4: Artwork and Face Plate
The need of a casing is not in the plans since the whole project will be put in a big closet that houses the mains supply and other staff. The only thing needed is a plate to mount the pots and switches and a nice face plate so you know what that knob is doing, the design is kept to the bare essentials.
As mounting plate we took a 10x16 cm 10mm thick white acrylic. When artwork was done we made a cut-out mask for the holes and the wood that will hold all. After finishing the holes and wood cuts the whole thing was in place and another 10x16 transparent acrylic plate was put in front of all to give that nice proffesional look.
Step 5: Finished and Mounted
All done and mounted.
Complete worklog and details can be found here.