Ultra Low Power detects Clear Fluid -  Using an ultra low power technique, this circuit detects when a clear fluid is inside a piece of clear plastic tubing. It can be used for fluid level control and monitoring . . . [Hobby Circuit designed by Dave Johnson P.E.,  10/22/05]

Ultra Low Power LCD Indicator -  This circuit serves as an ultra-low power replacement for multiple LED on-off indicators. It also has the advantage of being easy to read in full daylight. With the parts shown, it is possible to display four bits of information. . . .  [G. Forrest Cook]

Version 4 of a DC Current Indicator -  The circuit below uses some common components to turn on an LED whenever DC current above a certain level is detected. The circuit uses a very popular LM393 dual voltage comparator from National Semiconductor and a common 1N4148 signal diode. The diode . . . [Hobby Circuit designed by David A. Johnson P.E.,  10/16/05]

Version A of 1.5v LED Flasher -  To squeeze even more energy from a alkaline battery cell, this circuit adds two transistors to a circuit similar to the above design to boost the efficiency. A small 1.5volt alkaline N cell should flash the LED for a full year. It too uses a "charge pump" technique to provide a LED the needed voltage . . . [Circuit by David Johnson P.E.,  06/03/00]

VISUAL AC MAINS VOLTAGE INDICATOR -  You should not be surprised if someone tells you that the mains voltage fluctuation could be anywhere from 160 volts to 270 volts. Although majority of our electrical and electronics appliances have some kind of voltage stabilization internally built-in, more than 90 per cent of the faults in these appliances occur due to these power fluctuations. . . .  [© EFY Enterprises Pvt. Ltd. All rights reserved.]

Voltage Indicator -  The solidstate voltmeter circuit described here indicates the mains voltage with a resolution that is comparable to that of a general-purpose analogue voltmeter. The status of the mains voltage is available in the form of an LED bar graph. . . 

Voltage indicator transitions between colours -  03/02/15  EDN Design Idea   Using a two-colour LED, this Design Idea indicates power-on and voltage level.. . .  [EDN Design Idea]

Water Flowing in Pipe Indicator -  The vibrations associated with water flowing through a pipe are picked up by an inexpensive piezoelectric wafer.   The signal from the wafer is first boosted by a micropower transistor Amplifier and then fed to an ultra low power voltage comparator.    When the vibration signal has sufficient Amplitude, a FET transistor switch is activated…[Circuit designed by Dave Johnson P. E. , 6/06/2009] . . . [Circuit by David Johnson P.E.,  07/26/09]

Water Level Alarm -  A circuit that offers visual indication of fluid level in a vessel, with a switchable audible alarm. Example uses would be to monitor the level of water in a bath or cold storage tank. . . .  [Andy Collison's website]

Water Level Indicator with alarm -  This circuit not only indicates the amount of water present in the overhead tank but also gives an alarm when the tank is full. The circuit uses the widely available CD4066, bilateral switch CMOS IC to indicate the water level through LEDs. When the water is empty the wires in the tank are open circuited and the 180K resistors pulls the switch. . .  [Radioland.nt.au]

Water-Level Indicator -  Here is a simple water-level indicator for overhead tanks that uses three LEDs (LED1, LED2, and LED3) to indicate minimum, middle, and maximum water levels in the tank. . . .  [© EFY Enterprises Pvt. Ltd. All rights reserved.]

Water-Tank OverFlow Indicator -  Water is a vital but scarce natural resource. To prevent water wastage, this water-tank overflow indicator comes in handy. It gives audio as well visual alarm whenever the water tank overflows. . . .  [© EFY Enterprises Pvt. Ltd. All rights reserved.]

Wireless LED Flasher -  There are times when you would like to flash a LED through a thick plastic case without wires. The circuit below performs this feat. It can be powered from a supply ranging from 3v to 6v. It draws about 300ua of average current at 5v . . . [Circuit by David A. Johnson P.E.,  01/09/11]

Wireless RFID Smart Key Detector -  Many cars now use a key with an imbedded RFID microchip.   The microchip is read when the key is inserted into the car’s ignition.   The keys containing a microchip often look the same as most other keys. People duplicating car keys will often not know of the microchip.   This circuit will light a LED if it detects a key containing a 125KHz RFID chip.  . . . [Hobby Circuit designed by David Johnson P.E.,  07/09/06]

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