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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. Automatic hand sanitiser dispenser We wanted an automatic sanitiser dispenser as it is not ideal to press down on the dispenser with possibly contaminated hands. While we were able to source some sanitiser (with difficulty!), we could not find an automatic dispenser, so I decided to make one. The basic parts were mostly salvaged from empty hand wash bottles – pipes, spray nozzles and a big plastic bottle. To make it automatic, we used a small DC-powered submersible liquid pump. After making a small hole in the cover of the bottle, we inserted the spray nozzle which we salvaged from a liquid hand wash dispenser. Next, we attached the pump and rolled up the rubber pipe up to the shoulder of the nozzle. The pump is submerged into the liquid sanitising agent. The nozzle mouth is connected to the pump discharge. The nozzle mouth may be plugged slightly with small objects to reduce the rate at which the liquid is dispensed. The goal is to run the pump for a short period when somebody brings their hands within 15cm of the nozzle. We choose the HC-SR04 based siliconchip.com.au ultrasonic distance-measuring sensor because I had quite a few of them on hand. Instead of using a full Arduino board, I deployed just the ATmega328 chip. This has a 16MHz crystal and load caps connected so it can be programmed directly with the code from an Arduino board. It’s powered from a 5V supply derived from a 12V battery. The Arduino code constantly triggers the ultrasonic sensor; if it detects an object nearby, it brings its digital outputs at pins 18 and 19 high until the object moves away. When pin 18 goes high, it forwardbiases the base-emitter junction of NPN transistor Q1 via a 150W current-limiting resistor. This sinks current from the negative terminal of the motor, which also runs from the 5V supply. Diode D1 absorbs the motor’s back-EMF spikes. If you use a pump that needs more voltage, you can use the same arrangement but just connect its positive terminal straight to the 12V battery (or use a 9V battery, if you have a 9V pump). The BD139 can supply at least 1A in the configuration shown here, so should be adequate to drive any small pump. Australia’s electronics magazine The proximity limit of 15cm can be adjusted in the sketch code, which can be downloaded from siliconchip. com.au/Shop/6/5679, along with the required Arduino libraries. You can program the chip on an Arduino Uno board and then transfer it over to a socket on the project circuit board. Bera Somnath, Vindhyanagar, India. ($100) November 2020  61