Here is a digital frequency comparator for oscillators with a 7-segment display and a light-emitting diode that shows the result (LED). When an oscillator's frequency count is less than "8," the LED that goes with it stays off. When the number "8" is reached, the LED turns on and the 7-segment display shows "8."

This demo circuit compares the frequencies of two NE555 timers set up as astable free-running oscillators. The digital frequency comparator part of the circuit has two 74LS90 decade counter ICs (IC2 and IC6), two 74LS47 7-segment display driver ICs (IC3 and IC7), a 74LS74 set/reset flip-flop (IC4), a 74LS00 NAND gate (IC8), and two 7-segment displays (DIS1 and DIS2). The counters get their frequencies from the oscillators that are built around the timers and are not stable but can run on their own. How a circuit works When the power is turned on to the circuit, resistor R1 and potmeter VR1 start to charge timing capacitor C1. As soon as the voltage of the capacitor reaches 2/3Vcc, the internal comparator of IC1 turns on the flip-flop, and the capacitor starts to discharge through VR1 towards ground. When the voltage of the capacitor reaches 1/3Vcc, the lower comparator of IC1 is triggered, and the capacitor starts charging again. Charge and discharge happen again and again. This means that the capacitor charges and drains between 2/3 and 1/3 of the power supply every so often (Vcc). The output of NE555 is high when capacitor C1 is being charged and low when it is being drained. The second oscillator, IC5, works the same way. The potentiometer lets you change the frequency of the oscillator (VR1 or VR2). Through the DPDT switch, the output pins (pin 3) of the oscillators (IC1 and IC5) are linked to the decade counters (IC2 and IC6). IC2 and IC6 both keep track of the first eight cycles. IC 74LS90 is a 4-bit ripple decade counter. It has a counter to divide by two and a counter to divide by five. Each part has its own input for the clock. The P output (pin

12) of the divide-by-two section is connected to the CP1 input of the divide- by-five section (CP0). When the clock pulse hits the divide-by-two section, it changes into a divide-by-ten counter. A high pulse at pins 2 and 3 resets the 74LS90 decade counter. At first, resistor R2 pulls down pins 2 and 3 so they don't move. The outputs of IC2, P through S, are connected to the inputs of IC3, A through D. The clock pulse is also sent from pin 11 (S) of IC2 to pin 3 of IC4(A). The number of items is shown on the seven-segment display.

Display The 7-segment decoder/driver (74LS47) takes four binary-coded decimals (8421), makes their complements internally, and decodes the data using seven AND/OR gates with open-collector outputs to drive the display segments directly. In the "on" state, each segment-driver output can sink 40mA of current. The ripple-blanking input (RBI), blanking input (BI), ripple-blanking output (RBO), and lamp test are turned off by connecting pins 3, 4, and 5 of the display driver to Vcc (LT). With the help of current- limiting resistors R3 through R9, IC3 sends segment data to the 7-segment display (each 220 ohms). The reset pin (RST) of NE555 is controlled by IC4, which is called IC4. It is a dual D-type flip-flop that has direct clear and set inputs and outputs that are the opposite of each other. On the rising edge of the clock pulse, the data from the inputs is sent to the outputs. The flip-flops work in toggle mode because the Q output is connected to the D data input. Circuit application

At first, resistor R10 pulls pins 1 and 13 of the flip-flops high to set them. When the NAND gate N2 of IC8 sends a low pulse to the reset pin of any flip-flop, the flip-flop is reset and the Q output goes high. When the flip- flop gets a clock pulse, the Q and Q outputs go high and low, respectively. This turns on the LED. When IC4's output is low, the oscillators start over. With the help of NAND gates N3 and N4, the reset signal is made. When the S2 switch is pressed, both the oscillators and the counters start to work. When any of the counters reaches "8," the corresponding display shows "8" and an LED lights up. This means that the oscillator is moving faster. Now, both counters stop counting because the output of the flip-flop goes low, resetting both astable oscillators. If the frequencies of both astable oscillators are the same, both displays will show the number "8," and LED1 and LED2 will both light up.