A thermal touch switch circuit is a device which detects the temperature from finger touch and toggles an output load through a relay switch.
It is possible to make a heat-operated thermal switch that does not depend on skin capacitance and resistance effects. In fact, the concept exploits the negative temperature coefficient characteristics of silicon diodes for the detection.
How the Circuit Works
In this thermal touch switch circuit, 741 acts as a comparator of temperature of the silicone diodes. The diode is considered to have a positive feedback. The initial voltage at each diode is assumed to be same while the output op-amp voltage is kept lower.
R4 and R5 are two resistors in the circuit that act as potentiometers. Due to potentiometer effect the voltage at non-inverting input stays lower than the inverting input.
When D1’s temperature goes high due to touching, the voltage drop at the non-inverting input goes down along with the increase of D1’s forward voltage. When D1’s voltage falls below the non-inverting input, the op-amp output increases.
This non-inverting input goes further up by positive impact of R4 and R5.
So, even when D1’s temperature goes down, the temperature at D2 stays high in comparison to D1.
To bring the original position of the circuit back, the diode D2 has to be touched. Such a touch on D2 leads to a sudden fall of voltage at the non-inverting input of the op amp.
When the temperature of non-inverting input goes below that of inverting input, the op-amp output will go down.
The circuit of a thermal touch switch can be used for different relays and other circuits. For example, adding two LEDs to signal the state of the situation of the circuit.
For brighter displays, the LEDs should be driven straight out of the op-amp. The use of two transistor buffers and higher currents are required for this too.
The circuit made in above mentioned method must be left nulled. This is done to adjust the differences in diode forward voltages and for the offsetting of the voltage of 741. This is done by pressing the switch S1, which cuts off the positive feedback loop and by fine tuning P1 until op-amp output Voltage gets half supply (4.5 V).
To use the circuit as a solar thermostat, a diode is affixed on the solar panel and another in the hot water resource.
The required temperature between these two diodes is acquired by adjusting P1. As soon as the required temperature difference is obtained, the circuit gets opened. Usually, a temperature difference of 25 to 30˚C between two diodes starts the thermostat.