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The circuitry of the aircon controls two things; the main switch of the aircon for on and off and the compressor for temperature adjustments. For these, a relay is used, which ideally would be short-circuited when a voltage difference is applied at the input.
As seen in the diagram, the control of the main switch is simple, making use of a 2N2222. The computer is connected to the base of the transistor. When it sends a signal of 5 volts to either turn the main switch on or off, the emitter and the collector becomes short and the resistor will have the 12 volts difference. This is parallel to the relay and hence, the relay gains the voltage difference and is activated.
The compressor makes use of this circuit also, except that it has an additional circuit of a 3-minute timer. This is used to prevent the compressor from overloading, which happens when turning the compressor on and off with little time interval.
The 3-minute timer circuit's main component is the binary counter LS160. The binary counter has 4 parallel input and output. The output pins show the current state of the counter. The input pins, on the other hand, give the current state that the counter assumes whenever it is reset. In this circuit, the parallel inputs are not used since every after a reset, the counter should start counting from zero. It is seen that the most significant digit of the output pins is anded to the computer input. Therefore, after 8 counts and with a computer input signal, the transistor, and hence the relay, is activated to turn the compressor on or off.
Now, an ne555 timer feeds in to the clock of the LS160 every after 23 seconds. So, after 8 counts, the delay time is more or less 3 minutes. Also, another ne555 timer is used to reset the binary counter LS160. This is done every after a signal is sent by the computer so that after which, the LS160 counts again from zero.
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