In many applications where water is involved we need to measure water level. There are many ways of measuring water including floaters, capacitance, resistance based meters. More advanced are optical, pressure based methods. Some of them depend on properties of liquid while other are quite universal. If you want reliable method for most cases, the you should stick with non contact methods that doesn’t involve touching devices to liquid. One of great methods involve ultrasound. A simple and cheap rangefinder can do the trick pretty well. PeterHaban have constructed simple water level mater based on well known HC-SR04 Ultrasonic rangefinder module that is attached to Arduino Uno.
Ultrasonic module simply measures sound wave propagation time that consists of froward and reflected parts. By knowing time and sound speed in the air it is possible to measure the distance to the object – in this particular case to surface of water. Measurement resolution can reach below centimeter which is really great for most cases. Peter used this level meter on his rainwater harvesting tank. He added this sensor to wireless sensor node where he collects temperature, light level and now water level. All data is then sent to MySQL database for convenient storage and graphing.
Heart rate or pulse is one of important diagnostic and monitoring parameters in human body. If you are doing exercises you may also need to monitor heart rate in order to determine its effectiveness. Ohoilett shares pretty simple circuit that may be used to detect pulse. The idea here lies on simple method which is based on measuring passing light through the finger. In biomedical engineering this is called photoplethysmography. During one heart period blood is pumped through the body, so capillary volume increases and decreases periodically. By shining light through the finger we can detect its variation and so heart rate.
In order to have reliable data wee need to condition incoming signal to Arduino input level. So there are couple operational amplifiers used where first one takes current signal from photodiode and converts it to voltage. This stage is called trans-impedance amplifier. Then signal is passed through high pass filter which is made of RC circuit. And last stage is gain stage, which amplifies signal to Arduino input level.
All is left is to detect signal peaks within software and represent it in one or another way.
SPD005G is a bridge gauge type pressure sensor which measures pressure with respect to atmospheric pressure. Its measuring limits are from 0 to 14.5psi. Interfacing such sensor require some conditioning circuit that is build out of three operational amplifiers.
Differential signal from sensor enters two amplifiers that are used as buffers for next standing differential amplifier. Differential amplifier simply gains differential voltage which then enters to Arduino A0 input. Arduino converts input voltage in to mV that is displayed on LCD. If you need to display value in Pa or psi, then you probably need to do calibration with known reference.
Husham wanted simple portable temperature logger that would work outside reliably. He’s built electronic part out of several building blocks where control is given to Arduino Nano. Precise time is kept on RTC module based on DS1307. Temperature is measured with LM35 sensor. Data then is logged in to SD card which also is interfaced by using SD card module. Current temperature and time is also displayed on LCD screen.
Special care is taken on building enclosure and adding basic control. So instead of using regular buttons he decided to add couple Hall effect sensors that would act as switches when magnet is brought close to area. One Hall sensor is used to turn LCD backlight and another for resetting microcontroller. Device is powered with couple Li-Ion batteries. Since it only logs data to SD card, eventually you need to take SD card out in order to load data to computer. Probably wireless interface like Bluetooth could make things much easier.