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Project Notes

#207 FrequencyCounter

Test a CMOS frequency counter circuit with a 100Hz - 5MHz range

Notes

I found this circuit published in Electronics magazine (Sep 16 1976).

It’s a classic demonstration of the CD4026 “bucket-brigade” and CD4047 astable oscillator.

Sample/Display Time configuration

The frequency counter is governed by a CD4047 oscillator. Since this offers a clean 50% duty cycle, it is ideal for flipping the circuit between two modes:

  • sampling period
  • display period

The frequency is tuned with 10nF and a resistor network comprising 2 x 220kΩ and a 20kΩ pot. Thus the frequency limits are expected to be between about 49Hz and 51Hz.

At 50Hz, the cycle time is 20ms. At 50% duty cycle, that’s 10ms for sampling and 10ms for display.

So given the frequency count displayed is based on a 10ms sample, the reading is therefore in tenths of a kHz. The decimal point is enabled on the second digit in the display so the result may be easily read as kHz.

Here are some traces of the trigger signal:

  • CH1: oscillator output Q COMPLEMENT
  • CH2: reset trigger signal

trigger_oscillator

Close-up on the trigger signal:

trigger_edge

7-Segment CD4026 Display

The counter is based on the classic CD4026 “bucket-brigade” configuration

Input Modulation

A signal may be directly fed into the CD4026 chain. That works fine as long as the signal is suitable.

With a 2V p-p sine wave directly connected with a 10nF decoupling capacitor, the counter works up to 3MHz or so.

To get a little more headroom, I was able to extend the frequency range to just over 5MHz with a NE5534P op amp configured as an adjustable comparator.

Performance

At 5kHz, the NE5534P op amp procudes a clear square wave input to the counter.

  • CH1: input signal
  • CH2: non-inverting input
  • CH3: op amp output/input to counter

counter_input_5kHz

At 5MHz, the NE5534P op amp output is quite distorted, however it is still good enough to drive the counter.

  • CH1: input signal
  • CH2: non-inverting input
  • CH3: op amp output/input to counter

counter_input_5MHz

Construction

Breadboard

The Schematic

The Build

Credits and References

About LEAP#207 CMOSOscillators
Project Source on GitHub Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is my personal collection of electronics projects, usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting (IMHO!).

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Feel free to borrow liberally, and if you spot any issues do let me know. See the individual projects for credits where due. There are even now a few projects contributed by others - send your own over in a pull request if you would also like to add to this collection.