#588 74HC14 Schmitt Oscillator

About the 74HC14 Hex Inverter with Schmitt Trigger Inputs, and testing its behaviour as a relaxation oscillator square wave generator.

Build

Notes

The 74HC14 is a high-speed CMOS hex Schmitt-trigger inverter. It consists of six independent inverters with Schmitt-trigger inputs, meaning each gate converts an input signal into a clean digital output while providing hysteresis, which helps eliminate noise and improve signal integrity. This makes it particularly useful for signal conditioning, waveform shaping, and debounce circuits. It operates within a voltage range of 2V to 6V, with low power consumption and fast switching characteristics, making it ideal for various digital applications.

Key Specifications:

Supply Voltage: 2.0V to 6.0V
Input Hysteresis (4.5V supply): 0.9V
HIGH output voltage (4.5V supply, Iout = 4mA): 4.2V typical (3.98V min)
LOW output voltage (4.5V supply, Iout = 4mA): 0.2V typical (0.26V max)
Propagation delay (4.5V supply): 13-25ns

Test Circuit: The Schmitt Oscillator

Note: For comparison, I’ve covered the same circuit with 74LS family chips in LEAP#582 74LS14 Schmitt Oscillator

An inverter Schmitt oscillator circuit is a simple and widely used configuration for generating square wave signals, typically built using a Schmitt-trigger inverter (like the 74LS14) along with a resistor and capacitor. The circuit works by exploiting the hysteresis property of the Schmitt-trigger, which causes the inverter to switch states at different voltage thresholds. The capacitor charges and discharges through the resistor, creating a time delay that determines the oscillation frequency. When the capacitor voltage reaches the upper threshold, the inverter switches to a low output, causing the capacitor to discharge; when it falls to the lower threshold, the inverter switches back to a high output, repeating the cycle. This results in a continuous square wave output whose frequency is determined by the RC time constant (f = 1 / (1.2 * R * C)). The circuit is valued for its simplicity, reliability, and ability to produce stable oscillations, making it useful in applications like clock generation, tone generation, and timing circuits.

Circuit Design

schematic

bb_build

Test Results

The 74HC14 produces a square wave quite close to 50% duty cycle, unlike the 74LS variant. It is also able to be used with a wider range of R and C values.

Note: in these tests, capacitors >=1µF are electrolytic, and < 1µF are monolithic ceramic.

R1	C1	Calc Frequency	Measured Frequency	+duty%	Waveform Quality
220Ω	10nF	378.8 kHz	589.4 kHz	46.5%	poor, significant ringing
220Ω	100nF	37.88 kHz	68.49 kHz	46.7%	good
1kΩ	20pF	41.67 MHz	39.6 MHz	50%	bad, more like a sine wave
1kΩ	100pF	8.333 MHz	10.3 MHz	45.4%	poor, significant distortion
1kΩ	330pF	2.525 MHz	4.02 MHz	44.5%	poor, significant distortion
1kΩ	1nF	833.3 kHz	1.63 MHz	43.4%	ok, significant ringing
1kΩ	10nF	83.33 kHz	140.8 kHz	43.7%	good
1kΩ	33nF	25.25 kHz	40.58 kHz	43.9%	good
1kΩ	100nF	8.333 kHz	15.21 kHz	44%	good
1kΩ	330nF	2.53 kHz	4.41 kHz	44%	good
1kΩ	1µF	833.3 Hz	2.49 kHz	45.8%	very good
1kΩ	10µF	83.33 Hz	145 Hz	44%	very good
1kΩ	100µF	8.333 Hz	13.9 Hz	44.2%	good
1kΩ	1000µF	0.833 Hz	1.8 Hz	48.2%	poor, square but a little unstable
1kΩ	2200µF	0.378 Hz	0.7 Hz	41.8%	poor, square but unstable
2.2kΩ	10nF	37.88 kHz	65.78 kHz	43.4%	good
2.2kΩ	100nF	3.788 kHz	7.59 kHz	43.6%	good
10kΩ	10nF	8.333 kHz	65.78 kHz	43.4%	good, a little ringing
10kΩ	100nF	833 Hz	1.72 kHz	43.5%	good, a little ringing
100kΩ	10nF	833 Hz	1.44 kHz	43.2%	good
100kΩ	100nF	83.3 Hz	169.6 Hz	43.3%	good but a bit unstable