Compact digital display frequency meter, tachometer

This is a very small electronic product, but it is very powerful. It can be easily embedded and installed on the instrument panel. It is suitable for hobbyists to make their own or to modify old instruments.

The circuit only uses two integrated circuits . CD4069 is used for amplification and shaping of small signals, and AT90S2313 is a high-speed single -chip microcomputer with simplified instructions . Its performance at 8M clock exceeds that of the 51 series at 100M clock. It is used for measurement, calculation and display in the circuit . Most digital frequency meters use a "second gate" to count, so the measurement error increases at low frequencies and decimals cannot be displayed. This circuit uses pulse width and density dual measurement technology, supplemented by high-speed floating-point operations, so the measurement accuracy is high, and the range can be automatically converted to display decimals. For example, the measurement result may be 0.543, 261.4, etc. When using it to measure the frequency of AC power, you can see that the result generally changes slowly between 49.82-50.17. Therefore, this circuit is very suitable for installation in generators and distribution rooms for AC frequency display.

The circuit also has a speed measurement function, which can be switched with a toggle switch . With the addition of Hall, LED , photoelectric, electromagnetic and other sensors , the speed can be measured, and the measurement results are also accurate and fast. If an appropriate signal shaping circuit is added, you can even take the signal directly next to the ignition high-voltage wire of the gasoline engine to measure the engine speed of motorcycles and cars.

The signal is added from C3. If it is a low-frequency or low-speed signal, it will be more effective to skip C3 input. The adjustable resistor P1 is used to adjust the measurement sensitivity. S1 is used for frequency-speed measurement conversion. The display uses a 0.56-inch 4-digit LED common cathode digital tube.

Technical indicators: Power supply Voltage 8-15V DC

Supply current 100mA

Input sensitivity 100mV-5V

Input impedance 500KΩ

Frequency measurement range 0.4-1000000Hz

Speed ​​measurement range: 20 rpm - 1000000 rpm

Note:

1. Because the input impedance is high, you must be careful to prevent interference and reduce the input impedance if necessary.

2. Capacitor C3 is used to block DC. When measuring low frequency or low speed, it is more effective to skip the C3 input.

3. When measuring strong electricity, isolation should be added and attention should be paid to anti-interference shielding.

4. When taking the induction signal directly from the gasoline engine ignition high-voltage line to measure the engine speed, a suitable signal shaping circuit should be added, and attention should also be paid to preventing interference.

5. Since a 4-digit digital tube is used, only 4 significant digits can be displayed. For results exceeding 9999, only the highest 4 significant digits are retained, and the number of decimal points after the number is used to indicate the multiple. For example, "6666" means 6666, "6666." has one decimal point, indicating 6666X101, "666.6." has two decimal points, indicating 6666X102, and so on. However, the decimal point in the number indicates the real number, so this method is not applicable, such as "50.12".

6. When the circuit is started, it displays "0.0.0.0.". Therefore, if "0.0.0.0." appears frequently during use, it means that the environmental interference is large and the circuit should be shielded.

The following are the component layout diagram and PCB diagram. It should be noted that the PCB diagram is a view from the TOP side. If viewed from the BOTTOM side, it should be reversed. Be sure to understand this clearly when copying.