Simple Frequency Meter Circuits – Analogue Designs

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.

Switch Range

• 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.

PCB Layout

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.

IC 555 Analogue Frequency Meter

The figure below exhibits the 555 IC arranged like a linear-scale analog frequency meter having a full scale sensitivity of 1 kHz. The circuit's power is received through a stabilized 6 V supply.

The input signals for this analogue frequency meter can be in the form of pulses or square -wave signals with peak-to-peak limits of 2 volts or higher.

Transistor Q1 amplifies this pulsed input signal sufficiently high to trigger the pin#2 IC 555. The output of the IC at pin#3 is connected with the 1 mA full-scale deflection moving-coil meter M1. Diode D1 works like an offset cancel stage with the help of multiplier resistor R5.

Whenever the IC 555 which is configured as a monostable multivibrator get triggered by an input pulse, it creates a pulse having a fixed duration and amplitude. When every single pulse includes a peak voltage of 6 volts and a 1ms period, and it triggers the IC pin#2 with a frequency of 500 Hz, a high logic 500 milliseconds is created at pin#3, in each 1000 milliseconds.

Furthermore, the average value of output from the IC 555 assessed over this time interval can be calculated as
500 milliseconds/1000 milliseconds x 6 volts = 3 volts or half of 6 volts.

Likewise, in case the input frequency is 250 Hz, and a high pulse of 250 milliseconds in each 1000 millisecond period is created. As a result, the avergae output voltage from the IC now equals 250 milliseconds/ 1000 milliseconds x 6 volts = 1.5 volts or one quarter of 6 volts.

This shows that, the circuit's average value of output voltage, tested within a realistic overall quantity of pulses, is directly proportionate to the repeating frequency of the monostable multivibrator. We can get only mean or average measurements from moving-coil meters. In the circuit diagram a 1 mA meter can be seen connected in series with multiplier resistor R5. This resistor R5 adjusts meter's sensitivity at approximately 3.4 volts full-scale deflection. The meter is hooked up to offer the mean output value of the multivibrator and its display is instantly proportional to the input frequency.

Using the part values as indicated in the analogue frequency meter diagram, it is configured to produce full-scale deflection at 1 kHz. To set up the circuit, at first, a 1 kHz square wave frequency is applied to the indicated output, and full scale-adjust potentiometer R7 (it regulates the pulse length) is adjusted and fixed to provide a full-scale measurement on the meter.