Issue with a touch switch

Question

I made a touch switch in PCB based in the following schematic

When touching the plate with a finger the BJT turns on and the 555 trigger pin goes LOW activating the pin 3 (OUTPUT). The time the output is on depends of the 100K resistor and the 10u capacitor

Below you can see the schematic and the PCB

Changes in the original circuit

• I’ve used VCC=9v instead of VCC=6V
• To level out any fluctuations in the power supply voltage that might affect the operation of the timer. I’ve added a 10 nF capacitor to pin 5
• Probably because of the noise generated by my power supply the circuit was turning on randomly so I’ve added C3 to prevent this (The value of C3 was discovered by trial and error).

With the changes described above circuit works well with a power supply with 9V.

The issue appears when I change my power supply by a 9V battery.

• When I use a 9V battery the circuit only works well if I’m not isolated from the earth (touching the floor with a bare foot)
• If I’m isolated from the earth (using shoes) the circuit usually stops working after 3 or 4 tries and only works again if I touch the earth.

When I connect the GND of the circuit to the GND of the oscilloscope the circuit works fine (Probably because the GND of the oscilloscope is connected to earth).

How can I improve the circuit to prevent this behavior when using a battery?

Answer 1

The touch plate is the input to a differential of 0V. If 0V is floating with some tiny capacitance to earth then the input impedance, although high, shunts the finger capacitance induced voltage. In this circuit with Rc=1M and 6V , Ib must be greater than 5uA/hFE positive current.

The better solution to this might be a better touch pad which includes both input and Vbat with interleaved strips of conductors . Then the isolation from earth ground is irrelevant.

Basically you need 2 contacts between input and any Vbat within spec of 555 that are both touched anywhere on the touchpad of interleaved tracks.

Proof of concept is by finger capacitance or high resistance .

Answer 2

Firstly, the circuitry is not a good example of a touch sensor.

The NPN TR, 2222A, need to see current on the base pin, in order to pull 1 Mega ohm down, ib = ((9V / 1M) / beta), beta may be about 60 to 120 (I did not check), which is higher than ((6V / 1M) / beta). A finger has to draw that much of current from somewhere and supply it to the 2222A base.

First possible solution; You can bias the input by a large resistor (to say 910 Mega ohm) pulling the TR base to the supply, or similar, so a finger can add some noise to trigger on the TR. It is not any reliable due to the high impedance, parasitics, noise susceptibility, along with the uncertainty of the current through the finger

The second solution; Use a device designed for touch sensor applications: DigiKey -> search "touch sensor" -> Interface, sensor, touch sensor.