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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
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