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Frequency Divider using 7490 Decade Counter

EFY TestedHere is a low-cost frequency divider using 7490 decade counter circuit for generating different square-wave signals. The circuit is built around a 10MHz crystal oscillator, hex inverter IC 7404 and seven decade counter ICs 7490.

IC 7490 is a 4-bit, ripple-type decade counter. It consists of four master/slave flip-flops, which are internally connected to form a divide-by-two section and a divide-by-five section.

Each section has a separate clock input to change the output states of the counter on a high-to-low clock transition. The output states do not change simultaneously due to the internal ripple delays.

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Therefore the decoded output signals are subject to decoding spikes and should not be used for clocks or strobes.

Frequency Divider Circuit using 7490 Decade Counter

Fig. 1: Circuit of frequency divider using 7490 decade counter
Fig. 1: Circuit of frequency divider using 7490 decade counter

Since the output of the divide-by-two section is not internally connected to the succeeding stages, IC 7490 can be operated in various counting modes.

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In this circuit, ICs 7490 are configured as divide-by-10 counters. The power supply to the circuit is regulated by IC 7805 (IC1). LED1 indicates power on/off to the circuit.

A 10MHz clock pulse is generated by the crystal and the associated circuit consisting of IC2 (7404). This clock pulse is fed to pin 1 of IC3 (IC 7490), which divides it by 10 to give a 1MHz clock pulse at its output pin 12. The 1MHz clock pulse is fed to the input of the next stage and so on up to IC9.

Fig. 2: Proposed control panel for the frequency divider using 7490 decade counter
Fig. 2: Proposed control panel for the frequency divider using 7490 decade counter

Thus at all the seven counter stages, we get unique output pulses (1 MHz, 100 kHz, 10 kHz, 1 kHz, 100Hz, 10Hz and 1 Hz, respectively). These output pulses are selected by rotary switch S2 and fed to an output jack. The blinking/flash rate of LED2 indicates the output frequency.

However, you can identify output frequencies of 1 Hz and 10 Hz only. Above 10 Hz, the LED blinks so fast that it’s not possible to estimate the frequency.

Assemble the circuit on a general-purpose PCB and enclose it in a cabinet.

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