How can I see if my oscillator works without using an oscilloscope?

Question

I am an electronics engineering student and for my own experimental reasons I wanted to design a sine wave oscillator.

I actually want to create a home-made wave generator later on bringing all my little oscillators together and switching between them in time of need. I began with a Wien Bridge Oscillator for my sine wave generator.

I have really minimal practical and zero circuit design experience. After some simulations and tests I put together the circuit but I don't know if it really works or not because I don't have an oscilloscope. That's why I need an answer which doesn't involve one.

I tried bringing the frequency down to 1.5 Hz and attached an LED at the output of my circuit to see if there is any flicker. The LED is on constantly. On the other hand, the simulation works. Not a clean sine wave, not even close but at least it oscillates in the sim.

I put together a voltage divider to get three different outputs since I use a UA741CP op-amp. Later, I just attached a voltage regulator. I used 100k resistors to decrease the frequency. I will add both simulation schematic and my real circuit below.

Answer 1

Some options (not in any particular order):

1. Use some kind of audio output like headphones or speakers. Keep in mind that those equipments are usually rated for 1 Vpp, so you should use a clamp if you don't really know what the output is like. Also, specially for headphones and speakers without amplifiers, you may need to use some sort of buffer (usually an OpAmp in voltage follower configuration) to avoid the low impedance of the output to trash your circuits' behavior. Caveat: only works up to your hearing+equipment range, and you lose DC content. It might work very fast to know if the oscillator works, but most people can't distinguish a frequency by just hearing it.

2. You can use a mid-cost multimeter. Some (maybe most) intermediaries (and even some cheap ones) have a frequency meter. Sometimes they are off by a few Hz, but most are usable nonetheless. You may find some that get to 20 kHz and a few that get up to 50 kHz and higher. Beware that if your oscillator is near 60 Hz, or not really oscillating, some meters "cheat" that the output is 60 Hz, or mostly just pick up your electrical grid's emissions. Caveat: if your output isn't a well-behaved waveform, it might give you wrong results, and you might not be able to tell it.

3. You can use a USB sound card. A USB one would avoid damaging your computer's (built-in or expensive discrete) sound card, and it's cheap. You can search Ebay or chinese shops for very cheap ones. Plenty of software out there to use a sound input as a scope, but you can also use a sound processing software like Audacity to see your waves or compute the FFT. If it is cheap enough, and you have the courage, you might be able to remove the card's high-pass filter and get a scope that gets to DC. You may also want to clamp and buffer the input, though if the card allows switching to line-in (instead of mic) input, the impedance will be higher and the buffer will be less necessary. If the card has stereo input, you may also get two channels! Caveat: most cards will only sample at up to 48 kHz, which may or may not be enough for you. High-end cards will get you higher, but some will cost you more than an oscilloscope and still get only up to 192 kHz. Beware that some cheaper cards have a fixed sample rate, and resample (very badly) to other rates. Try to find the card's native sampling rate, which is usually 48000 Hz for some cheaper ones.

4. Do a hybrid of 1 and 3: use a speaker and then use your computer's mic to display the waveform graphically. Advantage: there's another layer of protection for the sound card. Caveats: you have the disadvantages of both methods and usually introduce distortion and noise.

5. Use an Arduino, if you have one at hand. The biggest problem, though its sample rate is very low and you have to build the antialiasing filter, if you have it at hand and your frequencies are low, it may be enough. Boards based on the Atmega328PB, like the Uno, may get up to 76.9 kSPS (up to 15 kSPS at maximum resolution, according to the datasheet). The ones based on the Atmega32U4 like the Leonardo appear to only support 15 kSPS (at least the datasheet does not mention the 76.9 kSPS configuration). Boards like the Due may support 1 MSPS (datasheet). I don't know if there's a full solution around the net, but you may have to make a firmware+software to sample, transfer and display the waveform in a computer. The advantage is that you will have DC content. Caveat: you may have to build an AC pass + bias circuit (or add a constant offset voltage) if your oscillator's output gets lower than zero volts. You may also need a clamp "just to be sure" not to fry your ADC. Beware that your antialiasing filter must respect Nyquist frequency (if you build the filter) and since it will not be perfect, you will get to a little less than half the sample rate. If you don't make the filter, you will get more noise, but it might still be enough for your needs.

Answer 2

Often if you can't see an oscillator output you can hear it. Years ago an headphone was a detector of choice for many interesting behaviours. Of course it depends on the level and the frequency but using the audio route is extremely cheap.

Answer 3

See limitations stated below but...

You can connect a small speaker or earphones to your oscillator output.

Then you can install a free frequency measurement app on your phone/tablet.

That app will use the phone/tablet microphone to listen to the speaker tone and measure its frequency.

There's inaccuracies in all that but it's not a bad lash-up for almost free, if you've got no way at the moment. There's a good few free Android apps for measurement of the mic frequency.

This only works (a) if your currently-unstated oscillator frequency is below the max. speaker response frequency, (b) the phone mic max. frequency and app capability are up to it, (c) your oscillator can drive the speaker/earphones load suitably.

Answer 4

You can't easily see if it works correctly without an oscilloscope.

Otherwise, some hobbyist solution might be to use one or several binary counter ICs and connect the oscillator to both clock & input. Check analog characteristics to ensure that the IC can keep up. Then if you clock it for lets say 10 seconds, then stop, you can read the binary value to see how many pulses you got.

Answer 5

Just for fun, an approach completely without electronics is to built a stroboscope. I'd borrow my daughter's Lego Technic back and use that to spin a disc of black card with a white line, illuminated by the LED in a dark room. When you spin the disc at the same rate as the LED flickers (or an integer multiple of it), the mark will appear to stand still. Geared down from that I'd have a pointer; counting the number of revolutions the pointer makes in 10 seconds, multiplying by the gear ratio and dividing by 10 will give you the frequency.

Hand-crank and use a mechanical stopwatch for your 10s if you want to be completely off-topic. Actually I'd use a Lego motor and variable power supply for more even spinning and easier reading of pointers.

Answer 6

You got no measurement equipment. Therefore you have to rely on your senses. It means, that you could transform the electrical signal into a visual or acoustic one. A human ear is capable to detect sounds from a few tens Hz up to more than 10kHz. Lower frequencies can be "measured" visually, just by turning on/off a LED after a comparator (on/off is easier to detect than a sinusoidal wave). If the frequency is higher, you might use frequency dividers. The simplest one is an inversely fed back D-flipflop. Of course these methods might require some signal conditioning circuitry, but probably that's your the best choice.

If you are hobbyist and you really love electronics, you really should invested into an oscilloscope, even into an entry level one, as soon as you can afford it.

Answer 7

• Buy a cheap multimeter (it must have an amperemeter) and connect it in such a way as to measure the current drawn by your oscillator from the source (the battery).

• Replace one of the resistors in the reaction loop of your oscillator with a potentiometer and adjust the value of the potentiometer in such a way that your oscillator should not satisfy the Barkhausen oscillation condition.

• Then looking at the amperemeter, adjust the potentiometer for a value that will allow oscillations.

• If the current drawn from the source increases it means that your oscillator works.

Answer 8

Consider using your computer's audio input and Audacity as a cheap oscilloscope.

This is assuming the voltage is not too high (you might need a voltage divider - most cards expect about 1 V p-p) and in the audio range.

Answer 9

I would just put two LEDs with high enought resistors to the output - one to ground, other to power.

The resistors should be high enought, that none of the LED is shinig (at least not much visibly), when the output is not connected, but if the output is grounded, or is high, the respective LED would shine visibly.

If you connect the oscilator now, if it is oscilating, BOTH LEDs would shine (well they would switch visibly with slow oscilator but with fast one your eye would not be able to follow and would see shining BOTH).

If you want better detection, use transistors to switch those LEDs.

I wrote something about it in Czech ( at http://robodoupe.cz/2018/drobnicky-003-dvojity-emitorovy-sledovac/ ) using following scheme - nearly any common transistor works, the top scheme works that on unconnected, or ~2V both LEDs shine just a little, (on 0V or 5V only one LED shines), on PWM/oscilator they BOTH shine bright. And it nearly does not affect the oscilator output (point in the middle) Input resistance around 100 KOhm.

The bottom scheme shines both LEDS bright for oscilator, or unconnected, but if you exchange the second parts (block with Q5 and block with Q9) BOTH LEDs would shine bright only on oscilator, while they went black for constant power of around 1.6V-2.8V

It is supposed to work to approx 50 MHz+ with nearly any normal cheap small transistors. Input resistance over 1 MOhm.

Answer 10

Another answer suggested using the AC-volts setting of a multimeter, taking advantage of the DC-blocking filter it will use in AC-volts mode.

You could also build your own DC-blocking filter in front of an LED, e.g. a non-electrolytic capacitor and resistor in series with a pair of LEDs in opposite polarity. This is basically a probe that detects if any AC voltage is present.

                             +--->|------+
 [oscillator]---||----^v^----|   D1, D2  |---|
   |            C1     R1    +---|<------+   |
   |                                         |
   +-----------------------------------------|

A one-diode rectifier would just charge the cap and get stuck with no current flowing; that's why you use 2 diodes in opposite polarities, so current can flow in either direction, through one or the other. (Only one of the diodes needs to be an LED.)

Choose a plenty-large capacitor (but it has to pass current both ways so it can't be electrolytic, and a resistor appropriate to not burn your LED if the full DC voltage was applied, and to not load your oscillator so much that it stops oscillating. (Each diode will actually only conduct about half the time.)

In case of very high frequency the capacitor will be basically a short circuit, but at low frequency it will have significant impedance. Still, with a large enough cap, you should still get enough current to make the LED visible. If your frequency is too high for the LEDs to really rectify, that's fine I think: they'll just both light.

You could build a bridge rectifier out of fast signal diodes as a power supply for a single LED (still with a cap in series before the rectifier as a DC-blocking filter, and standard filter cap + series resistor after the rectifier).

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With the circuit diagram in the updated question, the LED connectsion seem strange to me.

You have the LED connected between the outputs of two op-amps with no series resistor. It will probably be lit at max brightness any time the voltage difference is above its forward voltage. (Surprised you didn't burn it out.)

You say you measured some DC volts and more AC volts with a voltmeter. That means you have oscillation, but also a DC bias. That DC bias might be enough to keep the LED permanently lit, so you don't see much variation from the AC voltage changes.

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Other comments and answers have pointed out that you need to be careful that your measurement attempt doesn't break the oscillator itself: that usually means putting a decent-sized resistor in series with the LED if you want to connect it between points in your circuit that aren't like ground or an op-amp output.

Or power it from a high input impedance amplifier, e.g. using another op-amp as a voltage follower, if you want a high RC time constant in your oscillator but still need enough current to your LED.

But probably you can just connect it from an op-amp output to ground, with a series resistor, so you're not affecting the voltage across any of the higher-resistance resistors.

Answer 11

Completing what someone else said above: You should see something using a DC voltmeter (and the same reading after reversing the leads) at the output of your circuit, and nothing when using an AC voltmeter.

Edit

I'm sorry, I made a confusing mistake. I juggled in my head analog voltmeters, digital voltmeters and the ADC microprocessor project I'm working on; each can report differently. Please disregard my errant response--learning electronics is confusing enough as is.

An aside: Borrowing a 'scope or using one in a lab (like at a Hacker Space) is a good idea, too.

Answer 12

Firstly, I would simplify the whole setup by removing the 741 and its associated components and run the oscillator straight from the power source.

Remove the LED - it loads up the oscillator.

Change the 210k resistor to 100K with a series rheostat-wired potentiometer (any value from 500K to 1M) to give you some control over the gain of the oscillator amp so you can ensure oscillation. It is common for Wein-Bridge oscillators to have some form of automatic gain-control at this point to accommodate operating variations.

Set your frequency-determining components to a frequency about 1KHz.

Get hold of some form of audio amplifier. An old transistor radio with a potentiometer volume control will do. Open it up and connect your oscillator to the earth and "hot" end of the radio's potentiometer via a capacitor (100nF will do). Advance the radio volume to the middle. Adjust the oscillator gain potentiometer until you hear the tone.

If you are looking to make a career in electronics, it would be a good idea to collect a set of instruments. They do not have to be "top spec"; you can upgrade as your earnings and skillset advance. My oscilloscope is 10MHz and a single trace. It does all I need. If I am working with frequencies above that there are other instruments and techniques. Cheap oscilloscopes are available from online shops from $27Australian (about $24US). Cheap multimeters from about $6. Many of my test instruments are DIY (or kits) and building instruments is a great way to learn and save money at the same time.

Best of luck with your electronics career. I have been an electronics nerd for 66 years and it is the one thing that I keep coming back to.

Answer 13

Is checking with a low-frequent pulse sufficient? Put a 1 Hz clock timer on it, see what the imagery looks like and whether that's as expected.

If you have a trigger setting on your scope, the flanks of any digital pulse will tell you something about resolution and timing. If the scope is broken far enough, this simple test would tell you. If not, at least you got a baseline that works and work your way up from there. The key is in working with stable sources which you know to be working correctly.

Answer 14

In the distant past when test gear was unobtanium like mid seventies I used a simple half wave diode pump and measured the DC output voltage .If there is no AC there will be no reading because the input coupling cap blocks DC.I used Ge diodes because they were plentiful and low drop and good frequency response.For your job you could get away with 1N4148 or similar which is cheap.