Simple electric tachometer

The circuit of professional speedometer is complicated and expensive, and ordinary enthusiasts cannot afford it due to economic constraints. The author designed a simple electric speedometer that can be used to test the speed of general objects.

The circuit of the tachometer is shown in Figure 1. The circuit is very simple. It only uses a single transistor amplifier circuit. When the tachometer motor M is driven, a weak current will be generated. After the current is amplified by the transistor Q, it drives the microampere meter A to indicate. The faster the speed of the object being measured, the greater the current generated by the tachometer motor, and the greater the reading of the meter. W is used to adjust the value indicated by the meter.
 
When making it, select the components as shown in Figure 1, where M is a miniature toy DC motor; A uses a 100uA meter, and it is best to have the pointer in the middle of the dial, so that when measuring speed, it is not limited by the rotation direction of the object being measured. Otherwise, when the test direction is wrong, the needle will be reversed and damage the meter; W can choose a linear potentiometer with a power switch, so that it is easy to draw a uniform test gear scale on the panel; the power supply can choose a 9V laminated battery. The circuit is simple and does not require the production of a printed circuit board. It can be welded. After welding, the entire device is placed in an appropriate small box. The appearance is shown in Figure 2. The speed measuring head uses a flexible rubber connector, such as the tape recorder's pressure wheel, which is tightly fitted on the motor shaft.

After production is completed, use a standard tachometer for calibration and mark the scales of each test range on the panel, see Figure 3.

When in use, press the speed measuring head onto the object to be measured, adjust the W button so that the pointer is at 2/3 of the scale of the meter head, and use the test gear value marked by the W button to estimate the speed of the object to be measured. For example, to test the speed of a motor, just press the speed measuring head onto the axis of the motor shaft.

This tachometer can also be used to test wind speed, liquid flow rate and other items when combined with an appropriate tachometer head. Enthusiasts can use their imagination to design their own designs, which will not be elaborated here.