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CIRCUIT NOTEBOOK Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number. Automated tyre inflator/deflator ing winter, then as the ambient temperature increases going into summer, they can become overinflated, which could lead to the tyre popping in extreme cases or suffering excess wear. This circuit takes the guesswork out of the equation; rather than letting the air out, then re-inflating them to the correct pressure, it does it all for you. The main components are a microcontroller, 0-100PSI pressure sensor, solenoid driver, motor driver and LCD screen. Pushbuttons S1 & S2 are used to set the desired pressure, which is shown on the LCD screen. It then checks the tyre pressure and if it is higher than the set pressure, actuates the pressure release valve solenoid until the tyre pressure drops to the set pressure. Alternatively, if the tyre pressure is too low, the motor driver is activated to power up a 12V compressor which then raises the pressure until it reaches the set point. The motor is then switched off. The prototype was built on a Curiosity development board. ► The 0-100PSI pressure sensor is connected directly to the pipes near the hose connection point, although an onboard sensor can be used instead. ► siliconchip.com.au Australia’s electronics magazine ► I decided to build my own digital tyre inflator, mainly because pretty much all the affordable ones I found online could only increase the tyre pressure, not decrease it. Sometimes you just want to dial in a pressure value (in PSI, kilopascals or bar) and let the device do all the work for you. It runs off a 12V battery, so it could even be kept in your car and used on the road. Keep in mind that if you keep your tyres inflated to the right pressure dur- The final build uses a custom PCB design and a significant heatsink. This all mounts closely to the motor, pipes and valves. December 2020  85 In either case, when the tyre pressure reaches the target, the piezo buzzer sounds to let you know. In terms of the air plumbing (not shown), the following are all connected together via threaded pipes and joiners: the air hose, 12V compressor, pressure sensor and pressure release valve. Make sure all the pipes and connectors are gas-tight; use yellow Teflon tape on each set of screw threads. The air release valve is not shown on the circuit; it is a readily available type and connects to the terminal marked “VALVE” near the bottom. 86 Silicon Chip The output from the absolute pressure sensor at upper left is 0.5V at 0 PSI, 2.5V at 50PSI and 4.5V at 100PSI. This signal is reduced via a resistive divider to be within the 0-3.3V range that the PIC24 micro can handle, then filtered and buffered by IC3a, part of an MCP6004 quad rail-to-rail op amp. It is then fed into one of the PIC24’s analog input pins, AN6 (pin 25). The output from an optional second pressure sensor (eg, ambient) goes to AN7 (pin 24), while the output of an analog temperature sensor (IC4) is similarly fed to AN8 (pin 23). Australia’s electronics magazine This could be used to compensate the pressure setting for variations in ambient temperature, but you would need to change the software. Analog input AN9 (pin 26) is used to sense the battery voltage, for the under-voltage cutout to protect the battery. The air release valve is powered by Mosfet Q4, which is controlled via an opto-isolator for simplicity. It is driven from one output of MCP23S08 I/O expander IC2, as many of IC1’s pins are occupied driving the display. Similarly, the motor is driven by a parallel pair of high-current Mosfets siliconchip.com.au (Q2 & Q3), which are in turn driven by Mosfet Q1. It is controlled by another opto-isolator, this time driven from the RA0 output of IC1 (pin 2). There is no need to reverse the motor, so this part of the circuit acts as a switch. The I/O expander (IC2) also handles sensing when pushbuttons S1 and S2 are pressed, along with controlling the air release valve, piezo buzzer, some of the low-speed LCD signal lines plus indicator LED2. Circuit power comes from a 12V battery which must be able to supply a siliconchip.com.au substantial amount of current to drive the compressor motor. Power for the rest of the circuit goes via protective fuse F1 and reverse polarity protection diode D1, with the piezo and pressure relief valve running off that 12V rail (it’s also fed to IC3d for battery voltage sensing). The rest of the circuitry runs off 5V or 3.3V, derived from the 12V supply by low-dropout linear regulators REG1 & REG2. An in-circuit serial programming (ICSP) header is provided to allow microcontroller IC1 to be re-proAustralia’s electronics magazine grammed in-circuit, while the COM1 serial port header is provided for debugging purposes. The original prototype was built by hanging the various modules off a PIC24FJ256GA7 Curiosity development board from Microchip, while my final version uses a custom-designed PCB. The PIC24 firmware files (source code and HEX file) are available for download from siliconchip.com.au/ Shop/6/5637 Tom Croft, Sunnybank Hills, Qld. ($150) December 2020  87