Quantum Research Group
Charge-Transfer touch sensor
Original report: Michael Horan
Projects a proximity field through the air or any insulator to detect near-proximity or touch, specifically designed for human interfaces (panels, appliances, toys, or lighting controls).
8-pin PDIP Integrated Circuit (IC)
- Power (2.45V – 5.25V)
- Out (To PIC)
- Option 1 (See Strap Option)
- Option 2 (See Strap Option)
- Gain (Changes Sensitivity)
- Electrode Connect 1
- Electrode Connect 2
- A 100nF (0.1 uF) capacitor must be placed between Vdd & Vss (yes, power and ground)
for the IC to work properly.
- A capacitor (Cs) (10 – 500 nF) should be placed between Sns1 & Sns2.
- A resistor (1 – 50K?) should be placed between Sns2 & the electrode (sensor).
- Gain should be connected to power or ground (see Gain).
How it works
The IC continuously discharges ‘bursts’ storing voltage in Cs. As the electrode is touched, voltage dumps from CS, which is recognized by the IC, sending a message through its ‘OUT’ pin. Utilizes HeartBeat™ Output, a built-in feature that constantly checks for the ‘health’ of the IC (this feature can be sampled by using a pull-down resistor on out & feeding the resulting pulse to a counter).
The IC requires three (3) successive detections to create an output. This prevents an accidental trigger by stray energy.
If a detection exceeds the timer setting, the timer forces a full recalibration. This allows for full functionality even if the conditions change (e.g., if something falls on the electrode).
These are read only on power up.
Note: Option pins should never be floating
DC Mode Output – active-low on detection, or until Max On-Duration expires
Toggle Mode Output – on/off mode (for power loads), fixed at 10s
Gain affects the sensor’s sensitivity. Connected to power (Vdd), sensitivity will increase. Sensitivity can also be increased by using a larger electrode or a larger capacitor (Cs). Likewise, sensitivity will decrease when gain is connected to ground (Vss), using a smaller electrode or a smaller capacitor (Cs).Remember, though, that increasing Cs decreases response time but increases resolution.
Experimentation will be helpful.
There is no restriction to the design of the electrode, provided you use common sense.
Kirchoff’s Current Law should be considered when creating your electrode. It states that a sensor’s field current must complete a loop, returning to its source for the capacitance to be sensed. Remember that the human body naturally has several picofarads of ‘free space’ capacitance to the local environment, which is two times more than needed. A field can be shaped by limiting it with ground wire (a grounded wire will prevent the
capacitance field from extending beyond it).
What does “>>” mean (as in "Cs >> Cx")???
It means Cs is an order of magnitude or more greater than Cx"