The following simple analogue frequency meter circuits can be used for measuring frequencies which may be either sine wave or square wave. The input frequency which is to be measured must be at least 25 mV RMS, for optimal detection and measurement.
The design facilitates a relatively wide range of frequency measurement, right from 10 Hz to a maximum of 100 kHz, depending on the setting of the selector switch S1. Each of the 20 k preset settings associated with S1 a can be individually adjusted for getting other ranges of frequency full scale deflection on the meter, as desired.
The overall consumption of this frequency meter circuit is only 10 mA.
The values of R1 and C1 decides the full scale deflection on the relevant meters used, and could be selected depending on the meter employed in the circuit. The values could be fixed accordingly with the help of the following table:
How the circuit Works
Referring to the circuit diagram of the simple frequency meter, 3 BJTs at the input side work like voltage amplifier for amplifying the low voltage frequency into a 5 V rectangular waves, to feed the input of the IC SN74121
The IC SN74121 is a monostable multivibrator with Schmitt-trigger inputs, which allows the input frequency to be processed into a correctly dimensioned one-shot pulses, whose average value directly depends on the frequency of the input signal.
The diodes and R1, C1 network at the output pin of the IC work like an integrator for converting the vibrating output of the monostable into a reasonably stable DC whose value is directly proportional to the frequency of the input signal.
Hence, as the input frequency rises, the value of the output voltage also rises proportionately, which is interpreted by a corresponding deflection on the meter, and provides a direct reading of the frequency.
The R/C components associated with the S1 selector switch determines the monostable one-shot ON/OFF timing, and this in turn decides the range for which the timing becomes most suitable, to ensure a matching range on the meter and minimum vibration on the meter needle.
- a = 10 Hz to 100 Hz
- b = 100 Hz to 1 kHz
- c = 1 khz to 10 kHz
- d = 10 kHz to 100 kHz
Multi-range Accurate Frequency Meter Circuit
An improved version of the first Frequency Meter circuit diagram is displayed in the above figure. The TR1 input transistor is a junction-gate FET followed by a voltage limiter. The concept allows the instrument with a large input impedance (of one megohm range) and safety against overload.
Switch bank S1 b simply holds the positive ME1 meter terminal "grounded" for the 6 range configurations designated on S1 a and thus supplies the discharge path for the corresponding range condenser as outlined in the remarks to Fig. 1. That being said, at seventh place, the meter and a preset resistance, VR1, are switched around the D7 reference diode of Zener.
This preset is tweaked during setting up to provide a meter full scale deflection which is then accurately calibrated for that specific reference level. This is important since Zener diodes on their own offer a 5% tolerance. When fixed, this calibration is finally governed from a dashboard panel potentiometer VR2 which provides the control for all frequency ranges.
The highest amplitude of the input frequency placed on the f.e.t. gate is restricted to approximately ± 2.7V through the Zener diodes D1 and D2, collectively with resistor R1.
In the event the input signal is higher than this value in both polarity, the respective Zener will grounds the excess voltage stabilizing it to 2.7 V. Capacitor C1 facilitates certain high frequency compensation.
The FET is configured like a source-follower and the source load R4 works as an in-phase mode of the input frequency. Transistor TR2 functions like a straightforward squaring amplifier whose output causes the transistor TR3 to switch on and of as per the explanation previously provided.
The charging capacitors for every single 6 frequency ranges are determined with the switch bank S1a. These capacitors must be extremely stable and high grade such as a tantalum.
Although indicated as solitary capacitors in the diagram, these could be made up using a couple of paralleled parts. Capacitor C5, for instance, is built using a 39n and an 8n2, a overall capacity of 47n2, while C10 consists of a 100p and a 5-65p trimmer.
The PCB track design and the component overlay for the above shown frequency meter circuit is shown in the following figures
Simple frequency Meter Using IC 555
The next analogue frequency measuring device is probably the simplest yet features a reasonably accurate frequency reading on the attached meter.
The meter could be the specified moving coil type or a digital meter set on a 5 V DC range
The IC 555 is wired as a standard monostable circuit, whose output ON time is fixed through the R3, C2 components.
For each positive half cycle of the input frequency, the monostable turns ON for the specific amount of time as determined by the R3/C2 elements.
The parts R7, R8, C4, C5 at the output of the IC work like stabilizer or integrator to enable the ON/OFF monostable pulses to be reasonably stable DC for the meter to read it without vibrations.
This also allows the output to produce an average continuous Dc which is directly proportional to the frequency rate of the input pulses fed at the base of T1.
However, the preset R3 must be properly adjusted for different ranges of frequencies such that the meter needle is fairly stable and an increase or decrease of the input frequency causes a proportionate amount of deflection over that specific range.