Useful Mobile Apps

Strobe light tachometer

When the flicker frequency of light coincides with the speed of any object, such as a fan, the rotating fan may appear to be stationary. Changing the frequency of the flicker of light may help detect the rotation speed.


You may easily use the flicker frequency control buttons to quickly assign the flicker frequency with an accuracy of 0.01Hz (1 RPM):

• Up to 1000 Hz (60 000 RPM) for a connected flashlight (see the video on YouTube).
• Up to 100 Hz (6000 RPM) for the iPhone flashlight (iOS < 9).
• Up to 40 Hz (2500 RPM) for the iPhone flashlight (iOS 9+).


To change the frequency, you should simply press and hold the button. The flicker frequency can be displayed either in Hz or RPM.

The image you see will be sharper in lower external light.

After a long period of usage, in case of overheating, the iPhone flashlight turns off automatically.

Connecting a flashlight

Follow these guidelines:
• Choose an LED flashlight with a low supply voltage (4.5 to 6 V).
• If you are not good at soldering, watch the following video.
• Do not use aluminum foil as it is difficult to solder wire to.
• If anything is unclear, be sure to drop me an email.


Connecting a high-powered light requires more complex circuitry. However, the circuit design is still simple enough for a beginner. The key point is to pick a light with a focusing lens (a zoom lens).

In the video, the first light produces 2000 lumens (according to the seller), and the second one produces 1000 lumens. Both lights support several modes of operation (high and low power, as well as flashing at various frequencies). The switching between modes occurs when the light is quickly turned off and back on. For this reason, to avoid changes of mode during the operation of the stroboscope, a resistor is used to close the light's circuit. Before using the stroboscope, one needs to set the light into the full power mode by opening its circuit as shown in the video. In order to be able to pick the correct mode it is important to remember the order in which modes are activated.

Laser tachometer

• Measurement range from 0.3 Hz (18 RPM) to 1000 Hz (60 000 RPM).
• Step 0.01-0.0001 Hz (1-0.01 RPM) .
• Update time 1 sec.
• 5 mW laser power required.

Follow these guidelines:
• IT IS ABSOLUTELY NECESSARY TO SOLDER THE PARTS TOGETHER; otherwise, there will be a lot of noise.
• If you are not good at soldering, watch the following video.
• If anything is unclear, be sure to drop me an email.

Do not buy a laser pointer with small batteries (button cells), because you will "go broke" buying batteries.

Without soldering iron and multimeter

Magnetic tachometer

This tachometer operates by measuring the magnetic field from a spinning magnet. The magnetic field is measured by the iPhone's magnetometer. The magnetic tachometer is best suited for measuring low rotation speeds (from 6 revolutions per minute). It can also be used for measuring medium rotation speeds (up to 2700 RPM), but at such speeds there is a risk of spontaneous tearing away of the magnet and strong vibration due to center-of-gravity shift.

• Measurement range from 0.1 Hz (6 RPM) to 45 Hz (2700 RPM).
• Step 0.2-2 %.
• Update time 2-5 sec.

Intended use instructions:
1. The magnet has to be secured in such a way that one of its poles is directed toward the iPhone when the magnet is closest to the iPhone.
2. The iPhone needs to be fixed.
3. At high rotation speeds, protective eyewear must be worn. It is also necessary to protect the iPhone in case the magnet tears away.
4. Strong magnetic field from the attached magnet might affect rotation speeds of electric motors.
5. Electric motors produce fluctuating magnetic fields; therefore, the attached magnet must be powerful enough to create a magnetic field that is stronger than that of the electric motor.
6. For best results, iPhone should be placed at right angle to the rotating magnet.
7. Neodymium magnets are the most suitable due to their ability to produce strong magnetic fields despite a magnet's small size. However, it is important to keep in mind that electric motors are affected by strong magnetic fields; a rotating magnetic field may produce electric current in conducting materials. The maximum permissible size of a neodymium magnet is about 1x0.5x0.5 cm.
8. Magnetometer sensitivity varies substantially between iPhone models. This results in some iPhone models attempting to measure the background magnetic field.
9. If anything is unclear, be sure to drop me an email.

Measurement of rotational speed by stroboscopic tachometer

To determine the rotational speed (e.g., to determine the fan rotation speed), a plane perpendicular to the axis of rotation should be visible, and only one mark should be on this plane. For example:

You should begin measurements from the largest frequency that is possible for the tested fan. To reduce the flicker frequency, simply click and hold the button.

The observed pattern will change. For example, if the actual speed of the fan is 1100 RPM, and you began measuring the flicker frequency at 4400 RPM, then as you reduce the flicker frequency, the following still images will appear:

Four or three marks can be seen at other flicker light frequency; for example, three fixed marks can be seen at a flicker frequency of 1650 rpm (real RPMs multiplied by 1.5), four marks at a flicker frequency of 1466 rpm (real RPMs multiplied by 1.333.) and five marks at a flicker frequency of 1375 rpm (real RPMs multiplied by 1.25).

The wrong conclusion can be made when defining the RPMs by increasing the occurrence of the flicker frequency of the flash. For example, at a flicker frequency of 367 rpm (real RPMs divided by 3), one mark is visible:

This frequency can be misleading, since after multiplication or division of this reading by 2, one may obtain the same pattern as what is observed at the correct frequency (in this case, at 1100 RPM). Therefore, you should not start with a small flicker frequency and then increase it.

After determining the speed of rotation, you can save your readings by taking a photo with the result of the measurement written on it.

Calculating a speed of rotation that is higher than upper limit

When the speed of rotation is more than upper limit, it may be calculated by using a simple formula. Let us take an example where the actual rotational speed is 11000 RPM. When you decrease the flicker frequency of light from 6000 RPM, you should determine the frequency at which it is possible to observe a single fixed mark for the first time:

This occurs at a frequency of 5500 RPM (F1). With a further reduction of the frequency, the reading at which one fixed mark can be observed will be at 3667 RPM (F2). And the next time it happens, it will be at 2750 RPM (F3). A calculation is then performed by using two adjacent measurements:

RPM = (F1 • F2)/(F1 – F2)
5500•3667/(5500–3667)=11003 RPM


RPM = (F2 • F3)/(F2 – F3)
3667•2750/(3667–2750)=10997 RPM

This method can also be used when the speed of revolution less than upper limit. When the standard way was used, some difficulties occurred (e.g. when the approximate rpm is not available).



AliExpress: "2000 Lumen Flashlight XML T6 LED zoomable"


AliExpress: "Mini Flashlight XM-L T6 LED 18650"


Building supplies store:
Twisted pair


AliExpress: "breadboard 170"


AliExpress: "breadboard jumper wires 65 pcs"


AliExpress: "TIP31"


AliExpress: "TIP32"


AliExpress: "IRLB3034"


eBay: "resistor 2K 1W"


eBay: "resistor 1W kit"


eBay: "resistor 10K 1W"


eBay: "resistor 51 1W"


eBay: "cable 3.5mm 4 pole male to male"


AliExpress: "laser pen red"


AliExpress: "battery box switch aaa 4"


AliExpress: "laser module red"


AliExpress: "phototransistor 3DU5C"


AliExpress: "resistor 5.1K 1/4W 100pcs"


AliExpress: "100pcs 2.54mm jumper male pin"


AliExpress: "100pcs 1p 2.54mm plastic head"


eBay: "neodymium magnet"