An easy-to-make counting frequency meter

This frequency meter measures frequency by counting input pulses. The measuring range is 1Hz~10MHz in three gears. Since quartz crystal is used as the frequency source, the counting gate time is accurate and the display accuracy is high. It is low in cost, easy to make and does not require debugging. It can also be used as an addition counter alone.

The circuit is shown in the figure. Ten integrated circuits are used in the circuit. IC1-IC5 are CD40110 decimal addition and subtraction counting/storage/decoding/driving four-in-one integrated circuits, and the rest are commonly used digital circuits. The digital tube uses 5 common cathode digital tubes BSR510C.

The power supply uses a 220V/12V transformer to output a 12V voltage which is full-wave rectified and filtered, and then stabilized by a 7809 to obtain a 9V voltage (not shown in the figure).

IC9 forms the clock circuit. C2, 32768Hz crystal, R4, R5 and IC9 internal circuit together form an oscillation circuit, generating 32768Hz pulses, which are divided by 14 levels inside IC9 and then output 2Hz pulses from pin ③ to pin 14 of IC8. C3 and R6 are the power-on reset circuit of IC8. Because the input pulse is 2Hz, IC8 output terminals Y0-Y9 generate high levels for 0.5s in sequence.

The first pulse makes IC8⑧ pin high, and resets ICl~IC5 and IC6, IC7 at the same time. At this time, the digital tube displays zero. The first and second pulses are combined and widened by VD3 and VD4 to a standard pulse with a width of }s. After being inverted by IC10-1, it becomes a low level to control the counting gate, that is, to control the TE end of IC5's ④ pin. The gate opens and the counter starts counting. When the third pulse arrives, IC5's ④ pin becomes high, the counting stops, and the measured value is displayed, which lasts until the tenth pulse, so the reading cycle is 3.5s. When the eleventh pulse arrives, the counter is cleared again and the next measurement starts, and the cycle repeats over and over again.

Since the gate time is 1s, the number of pulses counted within 1s is the measured frequency. R1, R2, C1, VD1, and VD2 form the input circuit. VD1 and VD2 are used to limit the input pulses. IC10-2 and R3 form an amplifier circuit. IC10-3 and IC10-4 shape the input signal to make it a square wave signal, and then send it to the "1" end of K1 and to IC6. After being divided by IC610, it is sent to the "2" end of K1 and to IC7. After being divided by IC710, it is sent to the "3" end of K1. K1 is a three-speed multiplier selection switch. The measured signal is sent to the CPU end of IC5's 9th pin through Kl for counting, and then the count value (frequency) is displayed by the digital tube.

Obviously, when K1 is set to 1, 2, and 3, the measurement range is 1Hz-100kHz, 10Hz~1MHz, and 100Hz~10MHz respectively. Pin 10 of IC1 is the overflow terminal. When overflow occurs, the LED lights up. At this time, the position of K1 should be changed to expand the range. The reading multiplied by the multiplication factor of K1 is the actual frequency.

When used as a counter alone, as long as the system is automatically reset, close the switch K2 to start adding counts, the maximum count value is 99999.