The circuit explained here employs a rather different approach for implementing a touch operated switch action. Here instead of the resistance, the warmth of the finger is used for sensing and operating the circuit's output.
Irrespective of the ambient conditions, our hands and fingers mostly exhibits some extra warmth or increase in the level of the temperature compared to the atmospheric levels.
Using Finger Warmth for the Triggering
This feature of our body has been exploited here for making this thermally activated touch switch circuit.
The proposed thermo-touch operated switch circuit has its own distinct advantages in contrast to the normal "touch resistance" based switches.
This design is not prone to humid areas, or wet conditions where normally a resistance based switch would falter and generate erratic results.
Using 1N4148 as the Sensor and IC 741 as the Comparator Driver
The circuit utilizes the ubiquitous 1N4148 diodes whose forward voltage drop alters by about 2 mV in response to a rise of 1 degree Celsius temperature over it.
Looking at the circuit diagram, when the diodes D3 and D4 are touched with the finger, the voltage at point A drops rapidly in comparison to point B, sufficient to make the output of the IC 741 change state.
The IC 741 has been configured as a comparator, and it compares the forward voltage drop of the diodes with respect to the reference voltage clamped at point B.
The output generates a TTL or a CMOS compatible logic pulse at point C, which can be easily used for triggering a flip flop circuit and an intended load.
P1 and P2 are the presets which may used for setting and optimizing the circuits response or the sensitivity.
R1, R4 = 10K
R2, R3 = 56K
R5 = 1K
R6 = 1M,
P1 = 10K preset,
P2 = 1K preset
C1 = 104/ disc
T1 = BC547
IC1 = 741
D1----D4 = 1N4148