Simple digital frequency meter circuit diagram

**Simple Digital Frequency Meter Circuit Diagram (1)** This article introduces a digital frequency meter that is modified using a six-function electronic watch. The design is low-cost, easy to assemble, and ideal for beginners. It offers simple debugging and convenient reading, making it a great project for electronics enthusiasts. The working principle is based on the operation of the electronic watch. When the watch is in the "running timekeeping" mode, pressing the "ADVANCE" button twice allows the watch to measure the interval between two key presses. If this interval is T, and the displayed value is M, then M = T × F / N, where F is the input signal frequency and N is the division factor of the watch. When T equals N or T equals N × 10⁻ˣ, then F = M or F = M × 10ˣ, which means the displayed value directly represents the frequency of the input signal. The circuit diagram is shown below. A 9V power supply provides stable 5V through IC1, which is used to power IC2 and IC3. The 5V is further stepped down by R7 and LED2 to provide 1.5V for the watch. IC2 and IC3 are responsible for resetting the watch and generating standard timing pulses of T = N × 10⁻ˣ (e.g., 32.80 seconds and 3.28 seconds). S2 is a switch for selecting measurement ranges (×10 or ×100), SB is the measurement button, and S1'–S3' are the "MODE," "SET," and "ADVANCE" buttons of the watch. When powered on, the oscillator made from IC2 (NE555) with resistors R1–R4 and capacitor C3 starts oscillating. IC3 (CD4017) begins counting after being cleared by C5. After a certain time, Q9 goes high, stopping the count. Pressing S1' switches the watch to "running time." During measurement, the signal is connected to terminals A and B. After being limited by R11, D1, and D2, the signal is coupled to the watch via C7. Pressing SB clears IC3 and restarts the counting process. As IC2 continues to output pulses, IC3's Q1–Q9 sequentially go high, simulating button presses to control the watch. When Q8 is high, the timer stops, and the displayed value indicates the number of pulses counted within five IC2 cycles. Once Q9 is high, the measurement completes, and LED1 lights up to indicate the end of the measurement. Component selection includes S1 as the power switch, S2 and S3 as single-pole double-throw switches, and SB as a push-button switch. To ensure stability and accuracy, C3 should be a tantalum capacitor with low leakage and 16V rating. Resistors R1–R2 should be 1/4W metal film types, while R3 and R4 are 1/2W 51K linear resistors. The watch should be a BP-type model with a "running time" function. Other components are selected accordingly. For production and adjustment, open the watch and remove the circuit board. Identify the power (+) and (-) terminals, along with the contacts for the "SET" and "ADVANCE" buttons. Use thin wires to connect them to the main circuit. Disconnect one end of the internal crystal oscillator. Connect S3 according to the diagram and fix it on the side of the case. After assembly, adjust R3 when S2 is in ×10 mode to get 32.80 seconds, and R4 when in ×100 mode to get 3.28 seconds. Usage tips: After measurement, multiply the display value by the gear position (×10 or ×100) to get the frequency in Hz. When the display exceeds 59.99 seconds, use ×6000 and add the second digit. For low-frequency signals, the display may not work properly; wait for LED1 to light before reading. Keep connections short, especially between C7 and S3. The input signal should be less than 1 MHz. Function expansion: To measure higher frequencies, add a frequency divider before the limit. For example, use CD4017 to extend the range to 10 MHz. Replace components in the IC2 oscillator with the component under test, and calculate its value based on the pulse period. **Simple Digital Frequency Meter Circuit Diagram (2)** This paper presents a novel, fully automatic RF frequency measurement system using the prescaler SAB6456A and high-speed digital divider 74HC390, combined with the MSP430F449 microcontroller. The design offers accurate, real-time frequency measurement with an intuitive digital display. The main device is the MSP430F449, a 16-bit RISC microcontroller with ultra-low power consumption, strong computing power, and rich peripherals. It operates at 1.8–3.6V, supports five low-power modes, and has a 125ns instruction cycle at 8MHz. It also features a built-in hardware multiplier, 64KB Flash, and 2KB RAM, making it ideal for complex tasks. The SAB6456A is a UHF/VHF prescaler that divides frequencies between 70MHz and 1GHz by 64 or 256, depending on the MC pin state. It offers high sensitivity and good harmonic suppression. The system works by first clipping and dividing the input signal with the SAB6456A. The divided signal is further processed by two 74HC390 dividers, reducing the frequency to below 10kHz. This low-frequency signal is then fed into the MSP430F449, which uses its 16-bit timer A for timing and counting. By measuring the time between N pulses, the frequency is calculated and displayed. The hardware includes a reset circuit, power supply, and crystal oscillator (8MHz and 32.768kHz). The 8MHz clock drives the timer, while the 32.768kHz is used for the display. The 74LS373 latch drives the digital tube directly. In software, the divided signal is connected to the MCU’s input. After a delay for stabilization, the timer and capture interrupts are enabled. The program counts N pulses, calculates the frequency, multiplies by the division coefficient, and displays the result. For lower frequencies, the LED may flicker, so fewer pulses or shorter delays can be used. Dynamic scanning display ensures each digit is lit in turn, creating a smooth visual effect. The main measurement routine handles the timing, counting, and display processes. Testing confirmed high accuracy, low cost, and simplicity, making this a practical and economical frequency meter.

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