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Pressure based Hit Switch


This versatile hit switch is the electronics equivalent of a conventional switch. It can be used to control the switching of a variety of electronic devices.
The circuit of the hit switch uses a piezoelectric diaphragm as the hit sensor. A piezoelectric material develops electric polarization when strained by an applied stress. The hit sensor makes use of this property.
When you hit or knock the piezo element (hit plate) with your fingertip, a small voltage developed by the piezo element is amplified by transistor T1. The combination of transistor T1 and bridge rectifier comprising diodes D1 through D4 act as voltage-control switch. The inverter gates of IC1 together with associated components from a bistable switch. IC1 is a CMOS hex inverter. Out of six available inverter gates, only three are used here. IC1 operates at any voltage between 3V and 15V and offers a high immunity against noise. The recommended operating range for this IC is -550C to 1250CC. This device is intended for all general-purpose inverter applications.
Initially, the input of gate N1 is low, while the input of gate N2 is high. Triggering the voltage control switch by hitting the sensor pulls the input of gate N1 to the high level and cause the bistable to toggle. The capacitor gets charged via resistor R1 and the circuit change its state. This latch continuous until the bistable switch gets the next triggering input.
Every time the hit plate receive hit, the voltage control switch triggers the bistable circuit. That means every subsequent hit at the sensor will toggle the state of the switch. The red LED (LED1) connected at the output of gate N3 indicates ‘on/off’ position off the switch. Relay RL1 is activated by the hit switch to control the connected load.

circuit diagram of hit switch

PARTS LIST
Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)
R1 = 56 KΩ
R2 = 15 KΩ
R3 = 1 KΩ
R4 = 10 KΩ
Capacitor
C1 = 0.1 µF
Semiconductors
IC1 = CD4069
T1 = BC547
T2 = 558
D1 – D4 = 1N4148
D5 = 1N4007
LED1 = Red LED
Miscellaneous
RL1 = 12V, 200Ω 1C/O Relay
Hit plate

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