Working process of the electronic circuit diagram of the aquarium temperature probe

This is the electronic circuit diagram of an aquarium temperature probe that is capable of monitoring the water temperature and indicating the rise in temperature through visual and audio indicators. This circuit uses a diode 1N34 as a temperature sensing probe. The resistance of the diode will vary depending on the temperature in its vicinity.

Environmental factors including light and temperature can affect fish farming. Water temperature has a profound effect as fish cannot reproduce above or below critical temperature limits. Temperatures between 24°C and 33°C have been found to be optimal for inducing spawning in fish. This specific temperature range is also necessary for the healthy growth of nursery fry (young fish). Increased water temperatures due to sunlight can adversely affect the fish rearing process.

How the circuit works:

The diode 1N34 senses the temperature of the water in the aquarium. Typically, a diode can produce a voltage of about 600mV when a potential difference is applied to its terminals. The diode produces an output voltage of 2mV for every degree Celsius increase in temperature. That is, 10mV at 5C, rising to 70mV when the temperature is 35°C. This component is used in the circuit to sense the temperature change of the aquarium water.

Since the output of the diode sensor is too low, a high gain inverting DC amplifier is used to amplify the voltage. CA3140 (IC1) is a CMOS version of an operational amplifier that can operate with outputs as low as zero voltage. The highest output of IC1 is 2.25V lower than the input voltage at pin 7. With resistors R4 and VR2, the change in the diode voltage can be amplified to the required level. Resistor R1 limits the current flowing through diode D1, and preset VR1 (1 kilo-ohm) sets the input voltage at pin 3. IC3 (7805) provides a stable voltage of 5 volts to the input of IC1, making the input voltage stable for accurate measurement of temperature.

The output of IC1 is fed to the display driver LM3915 (IC2) through preset VR3 (50 kilo-ohms). With careful adjustment, the tap of VR3 can provide a voltage of 0-400 millivolts to the input of IC2. The high-sensitivity input of IC2 can accept as little as 50mV if the reference voltage at its pin 7 is adjusted using a variable resistor. To increase the sensitivity of IC2, one end of preset VR4 is connected to the "reference voltage end" pin 7 and its tap is connected to the "high side" pin 6 of the internal resistor chain.

When about 70mV is provided to the input of IC2 by adjusting preset VR3, LED1 (green) lights up indicating a temperature of about 35°C, which is the crossover point. When the input receives 100mV, LED2 (red) lights up to indicate about 50°C. Finally, if the input receives 130mV corresponding to a temperature of 65°C, the buzzer starts beeping.

In short, when the water temperature is below 35°C (normal), the LED and buzzer remain in standby state. For every 30mV increase in input (corresponding to a 15°C increase in temperature), the LED and buzzer will activate.

Pin 16 of IC2 drives the piezo buzzer through transistor T1. When pin 16 of IC2 goes low, T1 turns on and the buzzer beeps. Resistor R7 keeps the base of transistor T1 high to avoid false alarms. IC4 provides a stable 9VDC for the circuit.

Build the circuit on a common PCB and mount in a suitable box. The glass signal diode D1 is immersed in the water to sense the water temperature. Its leads should be painted with enamel paint to avoid short circuit in water. Alternatively, enclose the diode in a small glass tube or test tube with enough internal space to mount the diode. Use wax or other methods to make the sensor assembly waterproof to ensure that it will not be short-circuited by water.

Be careful when calibrating and setting up the circuit. With a 5VDC supply to diode D1 and an ambient temperature of about 35°C, D1 produces about 70mV. Adjust VR3 until the voltage in its tap increases to 70mV so that the input (pin 5) of IC2 receives 70mV, corresponding to the diode output voltage at 35°C. At this stage, the green LED1 should turn on. If not, adjust VR4 until LED1 just lights up. Immerse the diode in temperature-regulated hot water (35?C) and adjust VR3 and VR4 until the green LED1 lights up. Increase the water temperature to 50°C by adding hot water. Now the red LED2 will glow. In this position, the voltage on pin 6 of IC1 will be around 100mV. When the water temperature increases further to 65°C, the buzzer starts beeping.