# How to check for quick oscillations without an oscilloscope

I'm working on a 555 timer oscillator circuit, and I'm trying to get an LED to blink. The LED turns on, but I'm not sure if it's blinking. I do not own an oscilloscope I've tried a few things, such as recording with a slow motion camera. Is there a good way to test it? I was thinking I might be able to hook up a transformer to it and check for a constant voltage.

• You should share your schematic. – Billy Kalfus Nov 27 '17 at 23:31
• Try shaking the LED, you will see, as your eyes can detect pulsing if moved. – Marko Buršič Nov 27 '17 at 23:41
• Did you know Audacity (free) has a scope view of audio so you can use the AUX in port or maybe the Mic port with a suitable voltage divider and series R to input the signal – Tony Stewart Sunnyskyguy EE75 Nov 28 '17 at 1:42
• speakers connected with a resistor make noise in the presence of unsteady current – dandavis Nov 28 '17 at 2:37
• Use a scope probe 10:1 to Aux in high impedance 1V or 100K:11k or something like that, then choose input and press record – Tony Stewart Sunnyskyguy EE75 Nov 28 '17 at 7:12

Use a RC low-pass filter and a multimeter.

Here's how:

• Connect a 10 kΩ resistor to the point you believe is oscillating.
• Connect a 10 µF capacitor between the resistor and ground
• Measure the voltage across the capacitor, let's call this voltage $V_c$

If $V_c$ is around 20-80% of the max value of the signal you believe is oscillating, then it's most likely oscillating.

If $V_c$ is around +99% of the max value of the signal you believe is oscillating, then it is most likely not oscillating.

Extra info:

Using a low pass filter in this configuration will give you the average value of the oscillating signal. So if it is 50% high and 50% low and it is a square wave signal that is between 0 V and 5 V. Then in an ideal world you will read 2.5 V across the capacitor.

If it is high 80% of the time then you will in an ideal world read $0.8×5=4$ V across the capacitor.

If it is high 100% of the time, then it's not oscillating, and you will therefor read 5 V.

• Thanks! Could I do this with a 1 or a 100uF capacitor? – HXGamer Nov 27 '17 at 23:45
• @HXGamer The higher the capacitance, the smoother your reading will be. And the longer time it will take for it to reach the average value. I'm not sure what you mean with a "1", but with a 100 µF capacitor, sure. With a 100 µF capacitor + a 10 kΩ resistor, I believe you will see the average value within a second. Or somewhere around there. If it would've been 1 F then you should expect around a minute if not more. – Harry Svensson Nov 27 '17 at 23:49
• I used an 11.73kΩ resistor, and an 100uF capacitor. The supply voltage is 12.30V, and the output using the low pass filter was 3.41. That's ~27.7% of the supply voltage, however, I used a different resistor. Also, is there a way to determine the frequency and duty cycle of this signal? – HXGamer Nov 28 '17 at 2:38
• @HXGamer Then the duty cycle appears to be 27.7%, if you want to get the frequency then you will have to make some other changes, but this is outside of the scope of this question. So this will be my last comment. I will still attempt to point you in the right direction for acquiring the frequency. A) Buy an oscilloscope (at least a DSO138), it will be some investment. B) Use a microcontroller (e.g. Arduino) and its internal timers. C) Make a charge pump and measure the voltage with 0.1–1 kΩ load. D) Make a RC high-pass filter followed by envelope detector and trim R for -3dB. – Harry Svensson Nov 28 '17 at 2:55
• Also, you can try to measure an AC component of signal: put a 0.1 uF capacitor in series to multimeter in AC mode. If there is no oscillations, measured value wil be close to zero. – Eugene K Nov 28 '17 at 5:28

If you can't afford a scope a decent logic probe will tell you if a logic signal is pulsing. They are reasonably cheap and a quick test tool.

• I used a logic probe a lot to debug digital circuits before I can afford my first oscilloscope :) – Eugene K Nov 28 '17 at 5:20

You could try to take a movie of your LED at a high rate. I think most smartphones can film at 60 Hz. Plot the amplitude of the pixel associated with the LED and see if there is a distinct frequency. You might be able to say if your LED is blinking but might not be sure about the frequency due to aliasing.

• If there's oscillating then it will be most likely in kHz range. 60Hz camera won't help you. – Chupacabras Nov 28 '17 at 5:55
• I tried that already, but @Chupacabras is correct. If it is infact oscillating, it's probably in the kHz range. The circuit is supposed to make an LED blink but I used to little resistor values – HXGamer Nov 28 '17 at 16:28