When the user finger bumps into connects the electric circuit public earth an electronic contact, or, because the finger enters time the 60Hz alternating voltage which the neighbor power line radiation in the electrostatic field produces, the ordinary touching switch design can examine the resistance value to drop. But, if the nearby does not have the power line, the equipment from the battery power supply (for example in auto plant), or has not connected the electric circuit public earth time the direct-current electronic contact, how should manage?
Shown in Figure 1 electric circuit’s principle of work is examines the electric capacity which the finger contact electronic contact produces to increase. Although one kind of simple design possibly needs a complex electric circuit, but shown in Figure 1 the electric circuit is actually one inexpensive method, uses the part to be very few.
In Figure 1, IC1A takes one kind of operating frequency approximately is 150 kHz square wave-form oscillators uses. This oscillator’s output by alternating current coupling to potentiometer R2, R2 hypothesis actuation level, thus establishes touches the board sensitivity. Adds to the N trench square shape signal several volts negative displacements to tie the field effect to manage Q1 the electronics grid, repeatedly will urge Q1 to enter the shutdown condition from the conducting state. The Q1 drain electrode presents a 0V~12V amplitude of vibration the square-wave signal. The peak examination electric circuit which by D1, R7 and C4 are composed produces enough big DC voltage, forces IC1B the output for the logical low level.
However, when some people bump into touches the board, earths the capacitance value which the ‘OR’ circuit public earth increases to reduce on the FET electronics grid’s exchange slaving voltage, Q1 continues the breakover. Adds to D1 the square-wave voltage to drop. On the C4 loss of voltage to the logical threshold value below, therefore the IC1B output becomes the high level. You may through adjust R2 to establish the sensitivity, and compensates the FET pinch-off voltage the change. For in order to fresh or remembers past times, you may also use the 12AX7 dual triode half to make the oscillator, the other half replace Q1. Selects the anode resistance to be possible to use the 12V plate supply.
