Nostalgic TTL Frequency Meter

Each of us is a little (or even more) nostalgic. There is a nostalgia for the years when electronic measuring devices(old today)were made.Made with integrated circuits produced in the 80s (the TTL family appeared at the beginning of the 70s), this frequency meter is a pleasant reminder of those times.In addition, we will recover materials and get a measuring device that can prove to be useful.

TTL integrated circuits are recovered from old computers, instrumentation devices or others from those times. The display digits are recovered from old TVs, old analog satellite receivers or others.

The only more modern components are the 5V/3A switching power supply and the 230V to 12V/1.5A adapter. But even these can be replaced with old components from the 70s-80s years.

The frequency meter is physically made on 2 boards: P1 and P2.

It starts from a 4MHz oscillator (time base) made with quartz crystal Z1 and U21 type 7400 gates (P1).

From C4 (10-40pF) the frequency of the 4 MHz oscillator is fine-tuned during the calibration operation.

The signal thus obtained is divided by 4 using U22, type 7474. We thus obtain a 1MHz signal, which is applied to the counter-divider made with U23...U28.At its outputs, TTL signals of frequency 1HZ(1s)... 10 Hz(100ms)...100Hz(10ms) are obtained.Through the switch SW1 (Time Base) one of these frequencies is chosen and applied to the integrated circuit U30 which divides it by 2.

Thus, if we choose the 1s signal via SW1, at the output of U30, pin9, we will have a signal with a 50% fill factor, 1s in logical 1, 1s in logical 0.This signal is applied to pin 1 of U29, which is the main counting gate. The signal whose frequency we want to measure is applied to pin 2 of U29, through J1.

The signal on pin 8 U30 (in antiphase with that on pin 9) is applied to the input U31, pin2. This is a double retriggerable monostable. This is the measurement automation circuit: after each input pulses count, U31 will give a short pulse to load the memories on the P2 board, followed by a short pulse to reset the counters, from the same P2. We will find these pulses at J2. Also here we find the voltages (selectable through SW1) that light up the decimal points of the digits with the LED on P2.

Also on P1 is the signaling circuit for exceeding the measurement range, made with U30, U29, U21 and Q2.

And also on P1 is the switching source that powers the whole device, 5V/2...3A stepdown.The input voltage is 9-25V applied to J2-Sup.

At J3 there are:

• +5V supply voltage for P2 supply
• Time base LED flashing voltage(on P2) through Q1(on P1).
• Flashing voltage override LED(on P2) through Q2 (on P1).
• Counter pulses, which are sent to P2 to be counted by the main counter.
• The Count Over impulse coming from P2 to P1 to trigger the operation of the Over circuit.

On the P2 board there are 3 strings of integrated circuits and the 6 LED displays.

U1-U6 type 7490, decade counter, connected in cascade, constitute the counter of the pulses in the time unit, which come from P1.

U7-U12 type 7495, constitute the memory of the number of pulses stored in counters U1-U6. Although the 7495 is a shift register, its memory capacity and parallel transfer are used here.

U13-18, are 7-segment BCD decoders and drivers. The information stored in the 7495 memories is decoded to be able to control the 7-segment LED digits.

U19, U20, type CST-534 red, Common Anode are the LED digits on which the information is displayed. In 1 capsule there are 3 independent digits with decimal point. The current through them is limited to approx. 10 mA from resistors R1-R42.

During a measurement, the pulses to be counted come from P1 on pin 14 to U1 and are counted by the chain U1-U6. Followed by a pulse on pin 8 to the 6 circuits 7495, which loads the memories. The information from their outputs is decoded and applied to the LED digits.

A short pulse follows on pin 2,3 at 7490, which resets the counters. Then the cycle starts again.

LED D1 flashes in the rhythm of the time base, and D2 signals that the measurement range has been exceeded.

TTL integrated circuits (only the numerical code):

7490-12 pcs.

74123-1 pc.

7474-2 pcs.

7400-1 pc.

7404-1 pc.

7495-6 pcs.

7447 (7446)- 6 pcs.

LED display typ CST-534 .56 inch, red or equivalent 2 pcs.

LED round, 5mm, 1red+1green

Transistors BC171 or equivalent 2 pcs.

Quartz crystal 4MHz , HC49 case- 1 pc.

1N4148 diode-2 pcs.

DZ5V6 diode glace case- 1 pc.

Resistors:

27r-1pc.

910r...1K- 3pcs.

240r-43pc.

330r=2 pcs.

15K-2pcs.

Condensators:

trimmer 10-40pF-1 pc.

100uF/6V-1pc.

220uF/16V-2pcs.

2200uF/16V1pc.

10nF/16V-2pcs.

0,1uF/16V-2pcs.

0,47uF/16V (for decoupling)-14 pcs.

Others:

SW1- Linear switch, 3 sections, with retention,18 pins, 2X3cm-1pc.

Step down: switching power supply, 5v/2...3A, 2X4cm-1pc.

J2,J3:connectors 10pins.2,54mm, male-2pcs.

J4,J5:connectors 10pins.2,54mm,female-2pcs.

J1: connector 2pins.2,54mm, male, horizontal mounting-1pc.

J2-sup: connector BarrelJack horizontal-1pc.

PCBs-2pcs, done as in the Step3.

Mechanical points for connecting the two plates, as in Photos 3, 4.

230V to 12V/1.5A adapter.-1pc.

Optionally, cables for connecting the 2 boards, as in the Main Photo.-2 pcs.

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 Photos 1, 2

For their practical realization, this link must be accessed:

https://drive.google.com/drive/folders/1URvq44foSM6bIgWB29JthAjxlsl6-hXh?usp=share_link

Here is the project for the 2 boards, P1 and P2, using the Sprint Layout program, a program that you must have and use.

It is possible to order PCBs from the factory, but in our case it is more advantageous to make PCBs yourself using one of the methods available to you, according to the project given at this address.

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

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

Update! Because there were requests to build the device even from those who do not have the Sprint Layout program active, I uploaded the Gerber files for the two PCBs, at the above address. This allows ordering the PCBs at the factory, and further the frequency meter can be made according to the instructions here.

Once we have obtained the PCBs we can proceed with the assembly of the boards.

Planting the components on the PCBs is done according to the PCB plans in the link above, in Sprint Layout. We can help by Photos 3,4.

We use tin and tin soldering tools.

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

The 5V/2...3A power supply and D5 (DZ5V6) will be mounted last. This will be done only after previously adjusting (separately, on the workbench) the output voltage of the power supply to 5V (digital multimeter). It is very important not to apply a voltage greater than 5.25V to the TTL integrated circuits, because they will be destroyed!

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

On P1

It is being checked the presence of the signal of approx. 4MHz at pin6 of U21.

Next we check the 1Hz, 0.1Hz and 0.01Hz signals at pin11 U28, U27, U26.

Depending on the position of SW1, one of these signals will be found at pin11 of U30.

This signal, divided by 2 and with a 50% fill factor, will be found on pins 8,9 of U30.

At pins 5 and 9 of U 31 there must be short pulses for loading the memories, respectively reset the counters.

The P1 board can be checked independently, without being connected to P2.

On P2

To check P2, it will be connected to P1 through the 2 cables, as in Photo6.

Applying a TTL signal to the input of the frequency meter (J1 connector), in the frequency range accepted by it, we must read the frequency of this signal directly on the 6-digit display.

Applying a TTL signal to the input of the frequency meter (J1 connector), in the frequency range accepted by it, we must read the frequency of this signal directly on the 6-digit display.

With the oscilloscope, check the signals from outputs U1...U6 (pin11),then on outputs U8...U18.

In the case of PCBs made by yourself, attention will be paid to interrupted paths, or short circuits between them.

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

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 U21. We will put both devices under voltage and after a time of at least 15 min. (necessary thermal stabilization) we will adjust from C4 the frequency of the time base as accurately as possible to the value of 4 MHz. Now adjustments can be considered done.

It remains to do the general assembly of the device.

If it will be used with the display on the front panel of a device and with the time base selection buttons (SW1) in horizontal position, the device will be used as in Photo 6.

If it will be used as a stand-alone device, it will be assembled using the mechanical parts like in Photos 3,4.

The final result can be seen in Photos 5, 7.

As it is, the device measures frequencies of TTL signals in the 5Hz.....30MHz 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.

Signal formers can be used followed by frequency multiplication, which 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. 1.5A.

I personally used a 230Vac. mains adapter at 12V/1.5A of which the device consumes less than 1A.

The gauge dimensions are: 16X10X3 cm.

And that's it!