Electrical – How to check for quick oscillations without an oscilloscope

555oscillator

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.

Best Answer

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.

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