May 8, 1997

Circuit simulates contact bounce

Shyam Sunder Tiwari, IGCAR, Kalpakkam, India

The circuit in Figure 1a simulates contact bounce of electromechanically and mechanically actuated electrical contacts. The circuit generates contact-bounce pulses for contact closing and opening. You can easily program the contact-bounce period and frequency through variable resistors. This circuit is useful for testing debouncing-hardware circuits and debouncing-software programs.

Figure 1b shows the input and output waveforms. The input signal can be low-active or high-active TTL, or the circuit can generate the input by connecting R₃ and C₃ to IC_1A in Figure 1a. If you don't use an external input signal, then the input remains an open circuit. Thus, you can use IC_1A either as a buffer for external signals or as an oscillator with R₃ and C₃. The output of IC_1A connects to the delay generator comprising R₁ and C₁. Varying R₁ varies the bounce period.

IC_2A compares the delayed and direct outputs of IC_1A. The output of this comparator controls gated-oscillator IC_3A. R₂ and C₂ set the oscillator frequency. The number of oscillator pulses within the bounce period varies with frequency. Hence, to accommodate the same number of pulses at different frequencies, you need to tune together both R₁ and R₂. XOR gate IC_2B compares the output of the gated oscillator with the output of IC_1A to generate the desired output. The output is a modified signal with embedded bounce pulses.

The circuit works well with a 5V dc power source. In addition to HCT logic, other CMOS ICs in the HC, ACT, and C families are suitable. However, LS and S series of bipolar ICs may not work well with high values of resistors and low values of capacitors. (DI #2018)

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