Design of automatic thermostat for fish tank

This fish tank automatic thermostat can be used by fish aquarium enthusiasts to feed tropical fish. The circuit is simple and the effect is good.
Working principle

The circuit is shown in Figure 1 (click here to download the schematic diagram). When the water temperature is lower than the set temperature (usually 23-27℃ for tropical fish), the Iceo of 3AX31 (as a thermistor) becomes smaller, and its resistance value increases. Point a is low level, and after IC1A inverts, it outputs high potential, VT2 is turned on, relay j1 is attracted, and heat converter R4 works. On the contrary, point a is high level, VT2 is turned off, j1 is released, and R4 is powered off.
Component selection and production
See the table below for the component list.

serial number	name	model	quantity
R1	resistance	820	1
R2	resistance	1.2M	1
R3	resistance	1.5K	1
RL	Soldering iron core (heat converter)	20-100W	1
RP	Trimmer resistor	680K	1
C	Electrolytic Capacitors	220u/25V	1
VD1－VD5	Rectifier diode	IN4001	5
VT1	Transistor	3AX31	1
VT2	Transistor	9013	1
IC1A	Hexa-Inverter	CD4069、CD4049	1
J	Electromagnetic relay	JRX-13F 12v	1
T	Power Transformer	10V 2-5W	1
S	Toggle switch		1

Thermistors use germanium triodes 3AX31, 3AX34, etc. with good temperature sensitivity curves, and inverters use CD4049 or CD4069, etc. The power transformer uses a small transformer of 10V, 2-5W.

The production of temperature sensing head and heat converter is shown in Figure 2. Find a small piece of suitable organic glass and punch a hole (see the schematic diagram for the half-sunk hole) so that the transistor 3AX31 can fit in. Put the transistor with shortened pins and connected leads into the hole, and then fill the small hole with epoxy resin. It can be used after the epoxy resin is completely dry. The heat converter uses a 20-100W soldering iron core (the power depends on the volume of the fish tank), and the test tube is 20cm long and 2cm in inner diameter. First connect the soldering iron core lead, put it into the test tube, then put in the stone crystal sand with good thermal conductivity, and finally seal the test tube mouth with a rubber stopper (it is best to seal it with epoxy resin).
First place the temperature sensor, heat converter and thermometer in water, adjust RP, observe the thermometer, and put it into use when the temperature is controlled at 23-27℃.