Electronic Design

MOSFET Circuit Forms Reliable Switch Debouncer

Mechanical switches can be noisy, generating multiple transient pulses that may cause faulty operation of fast digital circuits. In this design, a pair of MOSFETs are used in a monostable mode to quench the switch-bounce pulses. The switch appears to the digital circuit as an ideal switch with no operating delay.

The circuit consists of two MOSFETs connected through positive feedback to form a one-shot timer (see the figure). The circuit operates as follows: When the switch closes, the gate of Q2 is pulled down, driving Q2 "on." This, in turn, drives Q1 "on." Q1 shunts the switch, effectively providing a momentary switch bypass until the switch settles. Then the circuit times out and Q1 and Q2 go "off," and the circuit is ready for the next cycle.

The drain of Q2 can be used as a pulsed logic output, appearing when the switch closes. The switch debounce time (t) can be set to suit the switch requirements:

τ = -R1 x C x ln(VT(Q2)/VDD)

where VT(Q2) < VDD, and VT(Q2) = Q2's threshold voltage (VTH on the data sheet). With the values shown, the debounce time is approximately 10 ms.

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