The post explains an effective capacitive touch sensor circuit which may be used for detecting intruders near a priced object such as your vehicle. The idea was requested by Mr. Max Payne.
The Circuit Request
Please Post a Capacitive/Body/Touch Sensitive Circuit can be applied on bicycle. Such device seen on car security system, When somebody come closer to car or a simple touch would trigger the alarm for 5 seconds.
How this type of alarm works, the alarm only trigger when somebody come closer (say 30cm) what type of sensor they use?
Circuit Image Courtesy: Elektor Electronics
The Circuit Design
The capacitive touch sensor circuit may be understood with the help of the following description:
IC1 is basically wired as an astable, but without incorporating a real capacitor. Here a capacitive plate is introduced and takes the position of the capacitor required for the astable operation.
It must be noted that larger capacitive plate will produce better and much reliable response from the circuit.
Since the circuit is intended to work as a vehicle body touch alert security system, the body itself could be used as the capacitive plate, and it being huge by volume would suit the application quite well.
Once the capacitive touch sensor plate is integrated, the IC555 comes into a standby position for the astable actions.
On detecting a "ground" element at a close proximity, which could be the hand of a human, the required capacitance is developed across pin2/6 and ground of the IC.
The above results in an instantaneous development of frequency as the IC starts oscillating in its astable mode.
The astable signal is acquired at pin3 of the IC which is appropriately "integrated" with the help of R3, R4, R5 along with C3----C5.
The "integrated" result is fed to an opamp stage rigged as a comparator.
The comparator formed around IC2 responds to this change from IC1 and translates it into a triggering voltage, operating T1 and the corresponding relay.
The relay may be wired with a siren or a horn for the required alarming.
However it's seen practically that IC1 produces a peak positive to negative voltage pulse at the instant when a caapcitive ground is detected near the plate.
IC2 solely responds to this sudden rise in the peak voltage for the required triggering.
If the capacitive body continues to be at the close proximity of the plate, the peak frequency voltage at pin3 vanishes to a a level which may be undetectable by IC2, rendering it inactive, meaning the relay stays active only at the instant the capacitive element is brought or removed near the plate surface.
P1,P2 may be adjusted for acquring maximum sensitivity from the capacitive plate
For obtaining a latching action, the output of IC2 may be further integrated to a flip flop circuit, making the capacitive touch sensor circuit extremely accurate and responsive