LEDs emit light. But did you know you can actually use an LED to sense light?
We are going to use this trick and check how to build a laser tachometer circuit. You can use this technique to measure the RPM of any rotating mechanical device.
Here’s an ordinary red LED, and an ordinary red laser pointer module. The LED is connected to my multimeter which is set to measure current, and as you’d expect, no current is flowing through the LED. But if we shine the laser onto the LED, it induces a current of about 4 microamps. That’s a very small current, but it’s enough to allow us to use an LED as a light sensor!
You can find circuits on the internet to do this, but the easiest way to use an LED as a light sensor is to just hook it straight to your oscilloscope. Make sure you are using a 10x attenuation oscilloscope probe. Now if we shine the laser onto the LED. We get a DC voltage appearing on our oscilloscope! It’s 1.5 volts, which is roughly the same as the LED voltage. Now if an object interrupts the laser beam, we can see it on the oscilloscope. The response time of the LED is way faster than a photoresistor, so you can use this trick to build a laser tachometer to measure the RPM of a rapidly rotating device.
Here’s an example with a PC case fan. You can see that the laser is being interrupted at around 133Hz. You may have to adjust your trigger holdoff setting in the oscilloscope to stabilize the waveform. Now this fan has 7 blades. This means that for every full rotation, there are going to be 7 laser interruptions. So to calculate the RPM of the fan, we do 133Hz divided by 7, which gives us 19Hz. Since there are 60 seconds in a minute,
if you multiply the frequency in Hz by 60, you get the rotations per minute. So 19Hz times 60 tells us that this fan is rotating at 1140RPM. In conclusion, LEDs make surprisingly good light sensors, and lasers are fun! You can use this technique to measure the RPM of wheels in your robots, or propellors in radio control planes.