Nostalgic CMOS Frequency Meter

The CMOS series of integrated circuits appeared shortly after the TTL series. This means that at the beginning of the 70s it was available on the market in an accessible way for any user. Hence the nostalgia of those who used these circuits and who remember those years. Today, these integrated circuits are considered outdated, but building a device that uses them can bring back pleasant memories, can bring back into use components that would otherwise constitute electronic waste, and last but not least, a useful device could be obtained in the electronics technician's laboratory.

The HCMOS series appeared on the market a little later and had, in addition to CMOS (low consumption and higher degree of integration compared to TTL), an increased working speed. The latter can be used at over 60 MHz.

The device uses 3X HCMOS integrated circuits, the others being CMOS. The two families interface directly, without problems. In this way, a high working speed is obtained, with low consumption. Single supply voltage of 5V. Fewer integrated circuits are used than in the case of TTL, resulting in a more compact device.

Integrated circuits can be recovered from old computer systems, old instrumentation devices and others. LED displays can be recovered from old instrumentation boards, analog satellite receivers or others.

All the components that make up this device are in current production and can be purchased from websites specialized in sale of electronic components, at convenient prices.

We start from a 4MHz oscillator, made with Z1 and U1. A CMOS circuit does not work at this frequency and powered at 5V. This is the reason for choosing an HCMOS circuit as an oscillator. Also for reasons of working frequency, I also chose U2 from the HCMOS series. It divides by 4, at its output having a signal with a frequency of 1MHZ.

The other integrated circuits are CMOS, with the exception of U8, which is HCMOS, also chosen for reasons of working at higher frequencies.

The 1MHz signal is applied to pin 2 of IC U3, CD4518. This IC contains two frequency dividers by 10, so that at its output (pin14) a 10KHz signal is obtained.

There are two more stagess of these, made with U4, U5.

A 100HZ signal (0.01s) is obtained at pin 14 of U4. At pin 7 of U5 we will have a 10Hz signal (0.1s). These two signals will give the frequencies of the ”Time Base”, in this case in number of 2, selectable with Sw1. The second section of U5 is not used.

The "Time Base" signal selected by Sw1 is divided by 2 by the first section of U6, obtaining a signal with a fill factor of 50% (0,1s+0,1s or 0,01s+0,01s).

During the first period of time, the input pulses are counted, and at the end of this period, the monostable made with the first two gates of U7 is activated, which then activates the second monostable made with the next two gates of U7.

At the outputs of these two monostables there are the 2 short pulses for loading thecount memory, followed by the counter reset pulse.

Then the cycle of counting, memorizing and resetting starts again.

The CMOS level signal, whose frequency we want to measure is applied to pins W1, W2 (Fin). They pass through the corresponding gate from U1, which is open for a duration given by "Time Base" (0,1s or 0,01s).

These pulses are applied to pin1 to U8, which is a double divider with 10 made in HCMOS technology.

The use of this type of integrated circuit allows the measurement of frequencies above 60 MHz.

The next two integrated circuits, U11 and U14 are also double dividers by 10 and together with U8 is the main counter of the frequency meter.

U9, U10, U12, U13, U15, U16 have the role of storing the pulses counted by the main counter, decoding the obtained numbers and being the drivers for common cathode LED displays (U17, U18, U19), through 220 ohms current limiting resistors.

Q1, Q3, U6 is the signal circuit for exceeding the measurement range, which is signaled by D2 blinking.

D1 lights up flashing in the rhythm of the time base, through Q2.

CMOS ICs

U3,U4,U5,U11,U14...CD4518

U6...CD4027

U7...CD4011

U9,U10,U12,U13,U15,U16...CD4511

HCMOS ICs

U1...74HC00

U2....74HC74

U8...74HC390

Other components

U17,U18,U19...LED display typ SEC 5244, .56 inch,common cathode red, or equivalent

Q1,Q2,Q3.. BC171

Z1...Quartz 4MHz HC49 case

D1,D4...1N4148

D2,D3...LED 5mm red, green

D5...PL6V2

R1,R2...10Mo

R3,R10,R11...330r

R4,R5,R7,R8,R9,R12,R55,R56...15K

R57...33r

R6...4K7

R13 to R54 (42pcs.)...220r current limiting resistors through the display LEDs at 12...14 mA

All resistors are at 0.125W.

C1...trimmer capacitor 10-40pF

C2...47pF/25V

C3,C4,C5...10nF/25V

C6...10uF/16V  

C701 to C716 (16 pcs.)...0,47uF/16V. It is a notation for the decoupling capacitors (not shown on the Schematic Diagram) that are mounted as short as possible on the supply pins of each IC, on the back of the PCB.

C8...22uF/16V

J1...connector BarrelJack horizontal

SW1...vertical switch, 2X3 pins, with retention

W1,W2...connectors 1pin, male

PCB...done as in the Step3.

230V to 5V/1A adapter

Tools and Measuring Instruments:

Tools for soldering tin and tin.

Cutter pliers.

Screwdriver.

Digital multimeter.

Oscilloscope 10MHz.

Digital frequency meter,well calibrated. Any kind.

TTL pulse generator. Any kind.

PCBs can be seen in Photo 1.

For their practical realization, this link must be accessed:

https://drive.google.com/drive/folders/12Zksc0w258sKMuQx8QKkxvCUtAxCdWr4?usp=share_link

Here is the project for board, using the ExpressPCB program,program that is freely available on the Internet.

ExpressPCB allows the execution of the PCB in your own regime, through one of the methods available to you.

1.6 mm thick sticlotextolite is used. Without metallized holes, the transitions from one side to the other are made with nonizolated wire.

In this case PCBs is made by yourself and attention will be paid to interrupted paths, or short circuits between them

At the end, the tracks are covered with a thin layer of tin.

If we have obtained the PCB, we can proceed with the assembly of the board.

Since we are talking about reused integrated circuits, it is good to check them before mounting on the PCBs.

Planting the components on the PCB is done according to the PCB plans in the link above, in ExpressPCB. We can help by Photos 2,3.

We use tin and tin soldering tools.

Where appropriate, we will use cutter pliers to shorten component terminals.

It is very important to not apply a voltage greater than 6V to the board, because the HCMOS circuits can't take it anymore and will be destroyed!

The oscilloscope will be used to check the functioning of the circuits.

It is being checked the presence of the signal of approx. 4MHz at pin6 of U1.

Next we check the 100Hz (0,01s) at pin14 U4, and 10Hz (0,1s) at pin10 U5.

Depending on the position of SW1, one of these signals will be found at pin13 of U6.

This signal, divided by 2 and with a 50% fill factor, will be found on pins 1,2 of U7.

The first 2 gates of U7 form a monostable that gives at the end of each counting cycle a short pulse that loads the memories in CD4511 with the numerical information that will be displayed. This pulse will be visualized with the oscilloscope at pin4 U7. The pulse from pin10 U7 will also be visualized, which reset the main counter and which is an even shorter pulse, due to the derivation with C4.

The overall operation is checked with a pulse generator, whose signal is applied to the Fin input (W1,W2). The frequency of the input signal is clearly displayed on the LED display. If the measuring range is exceeded, D2 will flash. D3 flashes in the rhythm of the time base.

If everything is OK, we will calibrate the time base.We will use a calibrated frequency meter, of good quality, whose input we will connect to pin 6 of U1. We will put both devices under voltage and after a time of at least 15 min. (necessary for thermal stabilization) we will adjust from C1 the frequency of the oscillator as accurately as possible to the value of 4MHz. Now adjustments can be considered done.

Signal formers have been added to the device in Photo2, as in Photo3. The first works in the range 100KHz-65MHz, the second between 30Mhz and 1050MHz. There are 2 separate inputs, the outputs of the formers being switched to the input of the frequency meter. The signal level is in the range of 30mV...3V. About these in a future article.

As it is, the device measures frequencies of CMOS,TTL signals in the 5Hz.....65MHz range.

The frequency will be displayed in KHz.

To measure the frequencies of signals of another format and another level, signal formers will be used.

If these formers are followed by specialized frequency dividers, the range of measured frequencies can be extended upwards.

If signal formers will be used followed by frequency multiplication, this allows the downward extension of the measured frequency range.

All these will be the subject of another(s) Instructables.

Consumption on the +5V branch is approx. 0,45A(most of the consumption being given by the LED display).

I personally used a 230Vac. mains adapter at 5V/1A .

The gauge dimensions are: 158X108X20 mm.

And that's it!