When bathing a baby, it’s important that the water is neither too cold nor too hot. You test the water by dipping your fingers in the bath tub before putting the baby into it. Here is a simple circuit that indicates the water temperature, helping you to use water that is just right for your baby. Limit points for high and low temperatures can be set to any level. So this circuit can also be used to ensure optimal temperature for a room, greenhouse, pool, geyser, fish-tank or freezer. xD !!!

The circuit requires 9V for operation, which you can get either from six AA-size or AAA-size cells in a battery holder or from a 9V PP3 battery.

Assemble the circuit on a general purpose PCB and enclose in a suitable cabinet. The circuit board is small enough to fit inside the miniature plastic enclosure of a handheld gadget. If you want to build it as a probe-type instrument, drill a hole at one end of the enclosure and glue a short length of stiff plastic tubing (fish-tank aerating tube or a scrap of model-maker’s polystyrene tube). Extend the thermistor out from the circuit using two insulated wires. Glue the thermistor to the end of the tube. Apply plenty of glue to the thermistor so that it dries to form a waterproof seal.

For low-temperature setting, put the thermistor in a glass of tepid water. Adjust VR1 until LED1 goes off For  high-temperature setting, put the thermistor in a glass filled with hot water. Adjust VR2 until LED2 goes off. Test your settings by immersing the probe in water within the acceptable temperature range. In this case both the LEDs should glow. Fig. 2 shows the author’s prototype.