OscilloPhone: Use your Smartphone as an Oscilloscope / Signal Generator
by Loann BOUDIN | 2015
Oscilloscopes and Signal Generators are two essential electronics devices to create and test electronic circuits. Unfortunately, these devices are very expensives for students starting electronics, or makers who want use electronic circuits in their creations just once...
The idea of my project is to rethink your smartphone as a portable, powerful and secured platform, able to simulate an oscilloscope and a signal generator for your electronic circuits. Here, your phone isn't just an accessorize or an ordinairy remote control : it becomes the center of the project, and a tool to help you create better things in the future.
The OscilloPhone project includes:
- an oscilloscope input for visualize electronic signals ranging from 150 Hz to 15kHz. Beyond this bandwidth, the displayed signals have a lower quality. Signals up to ±50V up can be injected on the input of the circuit. A warning LED alerts the user when a too high signal is measured.
- a signal generator output, able to generate sinusoidal, square and triangular signals up to 15kHz. A potentiometer is used to adjust the signal amplitude.
- a signal generator power output, able to generate sinusoidal, square and triangular signals up to 15kHz and 2A. A potentiometer is used to adjust the signal amplitude.
When writing this tutorial, I wanted to provide excellent quality pictures to facilitate the making of my project. I retouched some photos to get a pure white background (see step 2) using the recent Antzy Carmasaic's Instructable: "Pure White Background Photography Using Smartphone"
Step 1: Why Those Electronic Devices Are So Important?
Those devices are very important for an engineer on the go or any maker who wants to build electronic circuits. They are the only way to test that your electronic circuit operates as well as you want with electronic signals.
The Signal Generator :
"Signal generators, also known variously as function generators or waveform generators, are electronic devices that generate repeating or non-repeating electronic signals. They are generally used in designing, testing, troubleshooting and repairing electronic or electroacoustic devices." - definition of signal generator by Wikipedia
As the Wikipedia definition says, a signal generator is used to generate various electronic signals into electronic devices.
On the Signal Generator, you can choose the kind of signal you want (sine wave is the most common waveform but sawtooth, square and triangular waves are commonly available), his amplitude in volts or percent, and his frequency.
The Oscilloscope :
"An oscilloscope, previously called an oscillograph, is a type of electronic test instrument that allows observation of constantly varying signal voltages. Oscilloscopes are used to observe the change of an electrical signalled time." - definition of oscilloscope by Wikipedia
More famous than the signal generator, the oscilloscope can in effect allows observation of electrical signals over time. It's a kind of voltmeter for varying and non-varying signals with a screen for the visualization. On the oscilloscope screen, you can observe the form, the amplitude and the frequency of the signal studied.
How to use these devices :
Now that we know what these devices are and what they can do, how to use them?
Instead to try to explain this with a long and boring paragraph, I'll use an example of a circuit that need to be tested.
Let's test the circuit called "non-inverting amplifier" : here, this circuit must amplify twice the input voltage. To check if it operates correctly, a sine wave is generated on the circuit input with the signal generator. Then, the output signal of the circuit is connected to the oscilloscope, measured and displayed on the oscilloscope screen.
If the circuit operates correctly, the output signal shall be sinusoidal and has the same frequency as the input signal generated by the waveform generator. The output signal shall be amplified twice compared to the input.
Conclusion : for testing an electronic circuit, an electronic signal is generated on the circuit input by the signal generator. The output signal of the circuit is measured by the oscilloscope and if it meets the requirements, then the circuit operates correctly.
By this manipulation, we visualize the impact that the electronic circuit has on the input signals.
Step 2: Parts, Tools, & Skills
Here are the materials, tools and skills that you will need for this project.
• (1x) Phone with an internet connection
• (1x) Old CD Driver (for the box)
• (1x) 30 x 30 cm Wood Plank
• (1x) 10 x 4.5 cm Stripboard
• (1x) Heat Sink
• (1x) Switch
• (1x) SPDT Switch
• (30 cm long) 4 Wires Cable
• (1.50 m long) Electrical Wires
• (1x) 3.5 mm Bipolar LED
• (3x) 4 mm Black Banana Sockets
• (3x) 4 mm Red Banana Sockets
• (4x) Male Jack Connector (TRRS)
• (2x) Female Jack Connector (TRRS)
• (3x) Potentiometer Knobs
• (1x) Quadruple OP Amplifier (LM324) or (4x) Single OP Amplifiers
• (1x) +8V Voltage Regulator (7808)
• (1x) -8V Voltage Regulator (7908)
• (1x) Power NPN Transistor (D44H11)
• (1x) Power PNP Transistor (D45H11)
• (1x) 1MΩ Potentiometer
• (2x) 100kΩ Potentiometers
• (1x) 330kΩ Resistor
• (x1) 100kΩ Resistor
• (x1) 33kΩ Resistor
• (4x) 10kΩ Resistors
• (x1) 3.3kΩ Resistor
• (1x) 1.5kΩ Resistor
• (1x) 100Ω Resistor
• (1x) 22µF Capacitor
• (2x) 10µF Capacitors
• (1x) 4.7µF Capacitor
• (1x) 1µF Capacitor
• soldering iron and solder wire
• flat pliers
• cutting pliers
• duct tape
• glue gun and glue sticks
• basic soldering skills
• basic woodworking skills
This project can be built for under $25 by purchasing all the parts, but you can make it for less by using leftover parts. The potentiometers, the capacitors, the bipolar LED, the wires, and some resistors used in this project are from old electronics.
Step 3: Downloading the Signal Generator App
Before putting on the soldering iron and starting assembly, download the applications to your phone. Check the compatibility of these apps on your phone is the first step of this project.
It just takes a few minutes, and can give you the motivation and conviction to make the project if you hesitate.
Note: I have an Android phone and I used for this project Android apps from the Google Play Store. There are similar applications for Apple users, but I haven't tested them yet.
The Signal Generator app:
To generate various waveforms with your phone, you'll need an application that generate a sound (a signal) to the audio output of your phone. Hopefully, many apps can do this job!
For Android devices:
For Apple devices:
The signals generated by these apps go directly to the audio output of the phone : to the phone speaker if there isn't headphones plugged, or to the jack output if the phone detects headphones.
I wanted an electronic signal (not a sound) in exit of the phone, so I decided to use the jack output. So, the electronic signal comes out the phone by the jack output and goes to the electronic circuit with a jack cable.
The signal generator application chosen for this project (Function Generator) takes advantage of the smartphone stereo output : the first channel of the signal generator is connected to the left audio side, and the second one is connected to the right audio side.
So, you dispose of 2 separated signal channels (left and right) using the single audio output of your phone!
Step 4: Downloading the Oscilloscope App
As for the Signal Generator applications, I haven't tested the Oscilloscope ones for Apple devices yet.
The Oscilloscope app:
To visualize an electronical signal on your phone, you'll need an application able to receive and display the waveform of the sound (the signal). Some apps have been created for this purpose, here is a list of them.
For Android devices:
For Apple devices:
These apps allow you to visualize the signal coming from the mic input.
Step 5: The Electronic Circuit
The Electronic Circuit of this project is composed of 4 parts:
1. The power supply circuit
2. The Oscilloscope input circuit
3. The first output circuit of the Signal Generator
4. The second output circuit of the Signal Generator
• The power supply circuit
To support and process sinusoidal signals, the OP Amplifiers in the system need to be supplied with asymmetric voltages. The entire project is supplied with +12V and -12V, which are converted to + 8V and -8V by the 7808 positive voltage regulator and the 7908 negative voltage regulator.
The capacitors C1, C2 and C3 are used to avoid voltage drops when a high current is requested from an output.
• The Oscilloscope input circuit
The actual Oscilloscope input circuit is a slightly modified version of the chipstein's circuit in his "Preamplifier for Smartphone Oscilloscopes", where I changed a few of the resistor and the capacitors values.
First, the 1µF capacitor blocks any DC input and then the potentiometer adjusts the signal attenuation. If the incoming signal is still too high for the phone's microphone input, a second signal attenuation by 10 is performed by swaping the switch between the low and high ranges.
An OP amplifier in unity-gain buffer mode receives the attenuated signal, and adjusts the high impedance circuit. It also helps provide some extra power to the rest of the electronic assembly.
The 100 Ω resistor and the bipolar LED limit the voltages reaching the phone to about ±1.8 V, and also warn you when signals higher than that are present.
A 4.7µF capacitor is used to isolate the mike input from DC on the preamp output and a 1.5 kΩ resistor is connected between the output and ground to allows the phone recognize an external source.
If the final signal is not too large for the phone and that the warning led is off, the last switch allows you to connect the phone to the circuit
I recommend to see his tutorial to find out more informations about the possibility of this circuit !
• The first output circuit of the Signal Generator
The first output circuit of the Signal Generator use a potentiometer to amplify the signal coming out the phone audio output, up to 11 times. The OP Amplifier isn't able to provide high current output of this assembly. It is therefore intended to provide a signal to a small circuit.
• The second output circuit of the Signal Generator
The second output of the signal generator circuit uses the same circuit as above for the signal amplification. The waveform generated by the phone can be amplified up to 11 times using the AOP, the potentiometer and resistors. This circuit also includes a power stage (called push-pull), using two power transistors and voltages coming from the voltage regulators. It therefore allows to feed important circuits up to 2A, like a loudspeaker, an audio amplifier, or even bigger...
Note : I included at this step the Eagle schematic file if you want to make modifications and improvements.
Step 6: Populating the Stripboard
Start by soldering an IC carrier in the stripboard center.
Solder the resistors, capacitors, transistors and voltage regulators, while respecting the electronic schematic.
The switches, the LED, the potentiometers, the jack female connectors and the power connectors will be connected to the project enclosure. To connect them to the stripboard, it is necessary to use electrical wires: cut as much electrical wires as required and use duct tape and pen to identify them.
Once the heat sink screwed on the voltage regulators and the transistors, it is very difficult to identify wires: now you understand why naming them is very helpful! ;)
Double check your wiring before going to the next step !
Step 7: Making the Cables
In this project, the exchange of signals between the electronic circuit and the phone is constant.
First, the signal generator simulated on the phone should be able to send waveforms to the right and left outputs of the stereo jack.
Then, the oscilloscope simulated on the phone must be able to receive an electrical signal using the microphone input of the phone.
The only way to make all these connections between the phone and the circuit is to use a 3.5 mm jack cable with 4 poles. Thus, the use of the oscilloscope or the signal generator is done with a single cable.
To make this cable, start by cutting a 4 wires cable of 30 cm long. Then solder to each end a jack male connector, respecting the same wiring on each end.
This cable will be like an extension cable between two female jacks.
An extra cable can be made to use a second phone with this project.
Step 8: Preparing the Box
The project Oscillophone uses an old CD driver as box. Once the CD driver open and its contents removed, it is large enough to hold the electronic circuit and all the connectors.
In addition, a metal box like this is extremely useful for this type of project: the electronic circuit maked for this project process with small electronic signals sensitive to external electric and electromagnetic pollution. The metal box of the CD player surrounding the circuit acts as a Faraday cage, preventing the external disturbances to change the shape of the signals.
Step 9: Drilling the Holes
The inside cover of the CD driver is lined with duct tape to prevent shorting with the bottom of the stripboard.
To arrange the potentiometer closer to the user, three holes are drilled on the top cover of the CD driver. Then, the potentiometers are inserted into the holes and secured with rubber washers. The potentiometer to the left is the 1MΩ (for the oscilloscope) while the other two are the 100kΩ (for the signal generator).
Step 10: Cutting the Wooden Parts
The front and the rear of the project box are made by cutting two strips of wood.
6 holes are drilled on the front face, then the banana sockets are implemented there.
A seventh hole is drilled above the connectors dedicated to the oscilloscope, to implement the warning led.
The rear face is pierced with 3 holes, 2 for female jacks and one for the power supply wires.
Step 11: Finalize the Box
To finalize the box, all the wires of the electronic circuit are connected and soldered to their connectors (switches, potentiometers, banana sockets, female jack, led, power supply wires...), then stripboard and wood strips are glued to the box using the glue gun.
Finally, the top cover of the CD driver is screwed and potentiometers knobs are installed.
Step 12: Making the Phone Stand
The last fabrication step is creating a stand for the phone. It consists of 3 pieces of wood glued together.
The assembly is then glued on the upper side of the CD driver using a glue gun.
Step 13: See the Result Works!
To see the result works, connect the male jack cable to the phone and to the female jack of the circuit.
To test the proper operation of the signal generator, connect a speaker to the signal generator output power, and generate a low frequency sinusoidal signal (between 300Hz and 1kHz). You should hear a sound from the speaker, becoming more and more acute when the applied frequency becomes high.
A more rigorous test consists to visualize a signal from the signal generator output on a professional oscilloscope, and compare it to the output of a professional signal generator.
The result is quite impressive: the output signal of this project is of course poor compared to professional signal generator, but remains usable and accurate enough.
The oscilloscope input of this project works perfectly: the warning LED limits the signal up to ±1.8V and allows the user to know when the input signal is too high. If this is the case, it is necessary to change the rank of the signal attenuation at a higher level. The signal can then be visualized on your phone, using the oscilloscope app.
Step 14: Limitations and Improvements
The audio bandwidth of mobile phones is reduced to human audible signals. This means that the signal generator and the oscilloscope apps will be able to generate and process signals from about 150Hz to 15kHz, although the signal generator app can generate signals up to 22kHz. Beyond this bandwidth, the signals are more and more weaker, with a lower quality.
Obviously, this project can not compete with the quality of professionals signal generators and oscilloscopes. The Oscillophone is dedicated for students studying electronics and makers. If you must work regularly with electronic signals, invest in oscilloscopes and signal generators is not a waste of money!
An important limitation of this project is the simultaneous use of the signal generator and the oscilloscope. On my Android phone, when the Function Generator app is opened and then reduced in the background, the sound generated is turned off. This prohibits the use of both applications at the same time and seriously undermines the usefulness of this project on some circuit.
However, some phones (like Galaxy S5) and other tablet are able to open two applications at a time. A next version of Android will extend this functionality to all phones.
Another solution is to use two phones: one as a signal generator, the other as oscilloscope. This option has been made during the assembling of this project, which integrate two jack female connectors in parallel to the circuit and 2 jack cables.
Even if the phone is highly protected from high voltages by the electronic circuit, it is not completely disconnected from the parts. A way to protect the phone against a very high voltage (even more than 1000 volts) is to use galvanic isolation: optocouplers. These component uses light to transmit a signal and thereby completely isolating the phone.
Another possible improvement concerns the power supply circuit. Currently, the circuit is powered by an DIY ATX power supply. An integrated power supply in the housing could improve the portability of the project.