|2. Capacitance Change Sensor Circuits
More often than not, the touch button or plate is located some distance from the excitation circuit. After many experiments, I settled on the simple transistor circuit shown in figure 1. This circuit has many advantages. The circuit is simple enough that it can be housed in a very small package. Using surface mounted components the circuit requires a circuit board less than 0.5 cm x 0.5 cm in size. It has good static
discharge and noise immunity. It also draws negligible power when it is a standby mode and it can be positioned up to 1000 feet from the excitation
signal. In addition, the circuit only requires two unshielded wires. Usually, inexpensive telephone cable will work fine. Finally, by making the base emitter resistor variable, the minimum capacitance sensitivity of the circuit can be adjusted over a wide range.
The transistor acts as a current amplifier with a minimum capacitance threshold.
When the total capacitance between the transistor base and an earth ground exceeds a certain level, the transistor begins turning on, forming a switch between its emitter and collector
When the transistor begins turning on, its collector terminal begins tracking the excitation signal that is connected to
the transistor emitter. The diode connected to the transistor collector converts the pulsating DC signal to a direct current voltage, which is routed
back to an interface circuit. The interface circuit and the exciter circuits are usually located near the exciter circuit but do not have to be.
The transistor circuit works best for capacitance changes in excess of 25pf with changes greater than 50pf as a preference.
The output signal of the sensor circuit is a DC level, swinging from zero volts
to several volts, when a touch sensor circuit is activated. However, if the distance between the sensor circuit and the circuit used to detect an
activated switch is great, the unshielded wires will often collect a lot of unwanted AC power line noise signals. I highly recommend adding an
interface circuit to process the DC level swing from the sensor circuit, before sending the signal to a computer system or to a power switch circuit. To remove the AC noise components, a
passive RC filter is recommended at the front end of the interface circuit. The filter circuit not only filters unwanted AC line noise, but also does
a fine job of preventing damage to the interface circuit from electrostatic discharge. The output of the filter circuit can be routed to an N-channel FET or to Schmitt trigger circuit.
The Schmitt trigger circuit does a fine job of converting the slow voltage swing from the sensor to a fast clean logic voltage shift. The Schmitt
trigger action also requires a consistent minimum input voltage level, which helps to prevent false switch action.
The circuit shown in figure 1 shows both examples
of an FET and a Schmitt trigger circuit. I personally prefer the Schmitt trigger circuit but have also used the FET circuit when the distance between the touch switch and the
interface circuit are short..
The output of the interface circuit can be used to operate both solid state and
mechanical relays. It can also be fed to logic inputs of a computer system. I have also used the simple logic circuits to produce a sequence of
switch outputs. A touch sequence can also be used to turn on and off various loads according to the logic circuit. As an example, the first touch of a button might turn on one light. The second might turn on two lights, a third touch might turn on
three lights and a forth touch might turn off all lights.