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.
Parts List
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
Irin Annmaria says
can you please explain how to design this circuit
c00e6752-9f70-11e3-b67b-000bcdcb5194 says
circuit doesnt seem to work. the output stays on (ex. LED) is on all the time even after touching d3 and d4
Swagatam says
you can replace D3/D4 with LM35 IC for getting confirmed results, or try the first circuit from the following article, which is a tested one:
https://homemade-circuits.com/2013/01/how-to-charge-25-nos-li-ion-cells-from.html
Ted Weissgerber says
P stands for potentiometer or center tapped variable resistor or trimmer. They can be preset.
Swagatam says
it's a preset.
Jason says
will it be triggered if something cold touches the diodes?
siree says
thank you sir.
sir we touch wit a finger as shown shown in ur article. but, the finger doent hav give same voltage everytime. i dont uderstnd under wat conditions of the finger does this node A drops the voltage compared to node B…. plz explin me…..
siree says
helo sir..
can u plz explin me about preset..
what is the main applications for this circuit?
i hav some circuit named skin response meter….
is this similar to that?