Fish tank thermostat circuit

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

The circuit is shown in Figure 1. When the water temperature is lower than the set temperature (usually 23-27°C for tropical fish), the Iceo of 3AX31 (used as a thermal sensor) becomes smaller, and the resistance value it presents increases. Point a is low level, and after being inverted by IC1A Output high potential, VT2 is turned on, relay j1 is closed, and heat exchanger R4 works. On the contrary, point a is high level, VT2 is cut off, j1 is released, and R4 is powered off. The component selection and production component list is shown in the table below. No. Name Model Quantity R1 Resistor 820 1 R2 Resistor 1.2M 1 R3 Resistor 1.5K 1 RL Soldering iron core (heat exchanger) 20-100W 1 RP Trimming resistor 680K 1 C Electrolytic capacitor 220u/25V 1 VD1-VD5 Rectifier diode IN4001 5 VT1 Transistor 3AX31 1 VT2 Transistor 9013 1 IC1A Six-phase inverter CD4069, CD4049 1 J Electromagnetic relay JRX-13F 12ｽv 1 T Power transformer 10V 2-5W 1 S Toggle switch 1 Thermal sensor should be selected with a better temperature change curve Germanium transistors 3AX31, 3AX34, etc., inverters such as CD4049 or CD4069. The power transformer is a small 10V, 2-5W transformer.  　　

The production of the temperature sensing head and heat exchanger is shown in Figure 2. Find a small piece of suitable plexiglass and drill a hole (see the schematic diagram for a semi-countersunk hole) so that the transistor 3AX31 can fit in. Put the transistor with the pins shortened and the leads connected into the hole, and then fill it with epoxy resin. small holes and can be used after the epoxy resin is completely dry. The heat exchanger 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 inner diameter. First connect the soldering iron core leads and put them into the test tube, then put in crystal sand with good thermal conductivity, and finally seal the mouth of the test tube with a rubber stopper (it is best to seal it with epoxy resin). First place the temperature sensing head, heat exchanger and thermometer in the water, adjust the RP, observe the thermometer, and control the temperature at 23-27°C before putting it into use.