CD4011 touch switch circuit

1. Introduction to touch switch circuit
This circuit controls the on or off of the circuit by touching a metal contact button. The circuit of this work has no connection with other circuits and can be used to control the switch of battery-powered electronic products. The circuit diagram is shown in the figure below.

2. Circuit Principle　
The circuit consists of pulse generator, pulse delay, contact button, pulse inversion, CD4011 phase comparison, pulse shaping, switch key and analog switch. The circuit working flow diagram is shown below.

The pulse generator is composed of NOT gates 1, 2, and 3 of 4069, which generates a pulse with a frequency of 25k1"1z. This pulse signal is divided into two paths and transmitted to gates 4, 5, and gate 6 respectively through the pulse delay circuit. The two circuits are respectively connected with resistor R3, capacitor C2 and resistor R4, capacitor C3; these two resistors and capacitors are equal, and the pulse signal can be synchronously delayed by 5μs. The contact button is a metal piece the size of a thumbtack, which is connected to the upper one of the two pulse signals. If a person does not touch the contact button, the two pulse signals have the same frequency and the same phase. After passing through the pulse inversion circuit, they become pulses of the same frequency with a phase difference of 180. Such two pulses are respectively input to the CD4011 phase comparison circuit, that is, the two input ends of gate 7.

When the touch button is not touched, the two pulse signals input to CD4011 gate 7 have the same frequency and opposite phases; that is, the logical values ​​of their inputs are always one is 0 and the other is 1; at this time, CD4011 gate 7 outputs a high level, no pulse; medium signal output; gate 8 outputs a low level, and the touch switch circuit does not work.
When a person touches the touch button with a finger, due to the human body induction, it is equivalent to connecting a capacitor to this RC delay circuit, resulting in an increase in the capacitance of this circuit and a longer signal delay time. The pulse phase of this circuit will lag behind the phase of the other pulse. In this way, the phase difference of the two pulse signals input to CD4011 gate 7 is no longer exactly 180 degrees. . In a pulse cycle. When one input end of gate 7 is high, the other input end is also high in a very narrow time interval. In this way, in each pulse cycle, such a time period appears; during this time period, both input ends of gate 7 are high; so gate 7 outputs a low level.
After inversion by gate 8, a high level pulse is obtained. Since this period is a very small part of a pulse cycle, the pulse duty cycle output by gate 8 is very small.
In order to use the narrow pulse signal output by gate 8 to control the switch circuit, the narrow pulse signal needs to be shaped. The narrow pulse output by gate 8 is shaped by diode D, resistor R6 and capacitor C4, and output to the switch key circuit by gates 9 and 10.
The switch key circuit is essentially a 2-frequency division circuit, which consists of half of the dual D flip-flop CD4013. The control signal divided by CD4013 is output to the driving transistor 9013, which drives the speaker and the light-emitting diode to simulate the state of the switch.

3. Circuit adjustment points
The size of the capacitors C2 and C3 of the pulse delay circuit should be the same as the capacitance generated by human body induction. In this work, 50pF is used. The frequency of the pulse generator is closely related to the time constant of the pulse delay circuit. It is recommended that the time constant of the delay circuit is 1/8 of the pulse period. For example, in this example, R3×C2=5μs, and the pulse period is 2.2R2×C1=40μs.
Because the upper circuit in the pulse delay circuit is connected to a contact button. And an additional non-gate is used in the pulse inversion circuit, resulting in an increase in the distributed capacitance of the circuit; therefore, in the lower circuit, a resistor R5 is connected to make the distributed capacitance of the two circuits roughly equal.
The shaped control signal is connected to the input end of CD4013 through the filter circuit R7 and C5. After experiments, it is known that this pair of resistors and capacitors are indispensable, and they play a vital role in the stable operation of CD4013.
The circuit should be adjusted according to the working condition of the circuit. Since the main circuit works at super-audio frequency, it is difficult to measure. In order to solve this problem, we can infer whether the working state of the circuit is normal from the macro measurement results. The method is to use a voltmeter to measure the voltage at the input of gate 9. This voltage reflects the working state of the pulse delay circuit and the CD4011 phase comparison circuit. When the contact button is not touched, the voltage at the input of gate 9 is as small as possible; taking the power supply voltage of 6V as an example, the voltage below 0.5V is normal; when the contact button is touched with a finger, the voltage should be above 5V.

4. Dual D flip-flop CD4013
The pin wiring diagram of CD4013 is shown in the figure below. The switch key circuit used in the circuit uses the upper half of CD4013 to form a 2-frequency divider; the lower half is not used, all idle input terminals are grounded, and idle output terminals are left floating.