1.0 Introduction
Human touch detectors are basically electronic systems that raise an appropriate alarm when it detects a human touch. The alarm signal establishes a threshold above which intrusion is declared.
The operating parameters could be changed by adjusting weighting factors under varying conditions for sensor and varying alarm threshold.
The human touch detector could be operated based on any of these three principles:
- Charge detection.
- Oscillation.
- Referencing to ground.
1.1 Background Of The Project
This project work is directed towards the field of electronic touch sensor circuits. Touch sensors are devices which measure the parameters of contact between the sensor and an object (in this case a human touch).
Prior art circuits exist that sense the touch of a person and correspondingly cause some action to occur, such as switching power to a connected load. They are essentially composed of two types.
- Charge detection circuit.
- Oscillation circuits.
Charge detection circuits operate by basing a capacitor to a certain voltage level and by sensing the momentary increase in charge stored on the capacitor when a human touches a metallic touch pad connected to the charge storage capacitor. This technique has been used in industrial applications for many years to measure liquid level, humidity and material composition
At the heart of any capacitive sensing system is a set of conductors which interact with the electric field. The tissue of the human body is field with conductive electrolytes covered by a layer of skin, a lossy dielectric. It is the conductive property of fingers that make capacitive touch sensing possible.
A simple parallel plate capacitor has two conductors separated by a di-electric layer. Most of the energy in this system is concentrated directly between the plates some of the energy spills over into the outside the plates, and the electric field lines associated with this effect are called fringing fields.
A finger can be sensed because it can hold a charge and this occurs if the finger is floating or grounded. The electrical environment changes when a human hand invades the fringe field with a portion of the electric filed being shunted to ground instead of terminating at the second conductor. The resultant decrease in capacitance on the order of femtofarads as compared to picofarads for the bulk of the electric field-is detected by the converter.
These circuits suffer from a number of problems. Firstly, the input touch sensing circuit must be biased to a precise voltage level which will not trip the logic circuit, but which is close enough to the trip level so that when a person touches the pad, the incremental increase on the detection capacitor caused by the transfer of static charge from the person's body causes the circuit to triqqer. Because of this basing techniques, these type of circuits are quite sensitive to variation in component values, fluctuations in AC line voltage and environmental conditions.
Another type of touch sensor circuit is the oscillator input circuit. In this type of circuit a person's touch on the touch pad causes a change in frequency of a reference oscillator of a change in the amplitude of pulse from the oscillator, which when compared to a reference signal or voltage causes a trigger signal to be generated.
These type of circuits are superior to the charge detection circuits in terms of reliability (i.e. not missing true touch or generating a false signal), but are much more complex and expensive to design due to the requirement of the oscillator and the comparator circuitry.
1.2 Objective Of The Project
The primary objective of this project is to provide a device which can overcome the problems noted above and satisfy the needs in this field for a high-reliability, inexpensive touch sensor circuit. According to the disclosed touch sensor circuit, a sense input sense a person's touch by detecting a change in capacitance when someone touches the sensor and generates a sense signal which drives a logic circuit that detects the sense signal and causes a power switch to apply power to a load. This project includes:
- A sense input circuit.
- A logic trigger circuit.
- A power switch.
- A power supply circuit.
These elements are configured to operate as described with the description of the preferred embodiment.
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