Interactive electronics resource
Simple Function Generator Kit Instructions
Build, understand and experiment with the Simple Function Generator Kit. This resource brings the original kit material into the current MitchElectronics format with the schematic, circuit explanation, component and PCB information, construction guidance, troubleshooting and practical ideas where available.
Introduction
In just about every electronic circuit there will be some signal that changes with time and in many circuits, this signal may be continuous. For example, the electricity from the mains supply has a sinusoidal shape (like an ocean wave). Another example would be the clock for a computer which has a square shape.
Imagine a scenario where a digital circuit has been created and requires a clock from an external source. One could be built especially for the task however the clock source would most likely have a fixed frequency. Even if one was designed to have a variable frequency (which takes time), the device would only output a square wave. This is where the humble function generator steps in!
Not only does the SFG (Simple Function Generator), have both a square and triangle output but the frequency is also adjustable. The square and triangle waves are generated by the same source and thus are always in the same phase which means that the two waveforms follow each other constantly (useful if both waves are being used). But how does this circuit work? What active devices are used to create the oscillations and how is the frequency adjusted? Let's find out...
Schematic
How does the simple function generator work?
The Main Components
The Simple Function Generator circuit uses special component arrangements such as Schmitt triggers and integrators. To properly understand how this circuit operates it is important that the following is pre-studied:
- Op-Amps
- Schmitt triggers
- Integrators
How The Simple Function Generator Oscillates
The first op-amp with R1 and R2 is configured as an inverting Schmitt trigger. The second op-amp with C1 and the potentiometer (POT), form an integrator. Assuming the Schmitt trigger output is 0V the integrator will begin to discharge C1. This forms the downward slope of the triangle wave.
The Schmitt triggers output will rise to VCC once the output voltage on the second op-amp falls below the Schmitt triggers lower threshold voltage. As a result of this, the output of the integrator will begin to rise because C1 will begin to charge. This forms the upward slope on the triangle wave. Eventually, the output of the integrator will become larger than the Schmitt triggers upper threshold voltage.
When this happens the Schmitt trigger will output a voltage of 0V which will result in the output of the integrator to fall. This process results in the constant production of both a square wave and a triangular wave, in other words, it oscillates.
The rate at which the oscillation occurs (i.e. the frequency), depends on the resistance of the POT and the capacitance of C1. Since a variable capacitor would not be ideal in this situation (as they are only available with small ranges), a potentiometer is used instead. By adjusting the resistance of the POT the frequency of the produced wave will change.
Project ideas
Bench Function Generator
Function generators are extremely expensive pieces of equipment that will likely contain many features you don't need. As such, the Simple Function Generator Kit provides an excellent alternative that is not only much cheaper but easier to use and interface with. If you require more complexity, you can add extra circuits to the output of each waveform, and enclosing the Simple Function Generator in a case can make a very neat and practical project!
Synthesiser
The Simple Function Generator produces both square and triangle waves which are both highly popular with old consoles. As such, the Simple Function Generator kit can be used as a synthesiser either on its own, or combined with external analogue voltages. Furthermore, both the square and triangular waves can be mixed together to create some really unusual sounds!
Sine Wave Source
Did you know that if you pass a triangular wave through several low-pass filters you can create a sine wave? As such, a sine wave source can be made from the Simple Function Generator, and this can be used for creating inverters, modulation systems, and so much more!
Try it yourself
Circuit simulation
Not available in mobile view
What you need
Component List
| Component | Quantity | PCB Reference |
|---|---|---|
| 8 DIP Socket | 2 | U1, U2 |
| LM358 | 2 | U1, U2 |
| 100nF Capacitors | 3 | C1, C2, C3 |
| 47K Resistor | 1 | R2 |
| 100K Resistors | 3 | R1, R3, R4 |
| 100K Potentiometer | 1 | RV1 |
| Red Wire | 1 | VCC |
| Blue Wire | 1 | SQU |
| Green Wire | 1 | TRI |
| Black Wire | 2 | GND |
| SFG PCB | 1 | — |
Inspect the board
Interactive BOM
Board reference
PCB & assembly
The PCB silkscreen and component references should be checked against the component list before soldering each part.
Before applying power
- Check every component against its PCB reference.
- Confirm the orientation of all polarised components and ICs.
- Inspect for solder bridges, unsoldered pads and clipped leads that could cause a short.
- Check that no loose wire or solder debris remains on the board.
Build with confidence
Construction tips
Recommended build order
A reliable way to assemble Simple Function Generator Kit is to work from the lowest-profile components to the tallest. This keeps the PCB easy to access while you solder.
- Fit resistors, links and other low-profile components first.
- Fit small capacitors and diodes, checking polarity where applicable.
- Fit IC sockets and small semiconductors, observing the orientation markings.
- Fit larger capacitors, potentiometers, switches and other controls.
- Fit LEDs, connectors and the remaining taller components.
- Insert socketed ICs only after soldering around the socket is complete.
- Inspect every joint and check for solder bridges before applying power.
Electronics construction guidance
If you are new to kit construction, use the Soldering Guide alongside these instructions. Identify each component before fitting it and compare its reference with the component list and PCB silkscreen.
When it does not work
Troubleshooting
Nothing happens when power is applied
- Confirm the supply is connected to the correct input and with the correct polarity.
- Check that ICs, diodes, LEDs, transistors and electrolytic capacitors are fitted in the correct orientation.
- Compare component values and positions against the component list and PCB reference.
- Inspect for missed joints, dry joints and accidental solder bridges.
The circuit powers up but does not behave as expected
This kit is intended for multi-waveform function generator. If the output is stuck, unstable or outside the expected behaviour, use the schematic to trace the circuit a stage at a time rather than replacing several parts at once.
Check the components around the part of the circuit responsible for the output or timing first. A misplaced resistor, reversed semiconductor or poor connection can allow a circuit to power up while preventing it from operating correctly.
The circuit works intermittently
Intermittent behaviour is often caused by a marginal solder joint, a loose connector or a component lead that has not been fully soldered. Gently inspect the board with power removed and reflow any joint that looks dull, cracked or incomplete.
If the fault remains, compare the assembled board with the schematic and PCB reference one connection at a time.
Ready to test and experiment?
If you combine the SFG and the PWM Generator together with some hacking, you can create some rather interesting results such as LED breathers and other advanced signal controllers
Once the board is working, compare its behaviour with the schematic and the explanation above. Try changing only one input, control or permitted component value at a time so you can clearly see what effect that change has on the circuit.