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Circuits designed by David Johnson, P.E. Last Updated on:
Thursday, December 01, 2011 05:33 AM The contents &
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High DC Current
Indicator
A while back, someone requested a simple method to indicate when 20 to 50 amps of DC
current was flowing through a wire. The circuit below works well and is about as
simple as you can get. A sensitive reed relay from
www.Jameco.com acts as the current sensor. The magnetic field produced by such a
current is enough to close the relay contacts, if one or two turns are wound around
the relay....
Lead-acid Battery Low Voltage Indicator - Lead-acid batteries should not be
discharged below a certain voltage. Discharging a cell below about 1.8v and can
often damage it, which will result in a lower amp-hour capacity. Large battery
banks are critical in off-grid energy storage systems. One bad cell in a series
wired string will weaken the whole bank. The circuits below are designed to be
connected across either a single 2 volt cell or a 6v three cell battery bank. It
will alert the user of a weak cell by turning on a red LED when the cell voltages drop
below 1.8v. The circuit takes advantage of a low voltage LMP2231 op amp from
Nation Semiconductor and a LM385 voltage reference in the first circuit. In that circuit
the red LED must have a forward voltage of about 1.6v at 5ma, in order to operate
properly. In the second three cell bank circuit, a TLV3701 voltage comparator is
used. The circuits are simple enough that they could be placed on a postage stamp
size circuit board.
Moving Air Indicator
Many months ago a Discover Circuits reader wanted a method to indicate when a fan was
blowing air. He didn’t want to route any electrical wires to a sensor near the
fan. Instead he wanted all the electronics for both the fan and other circuits
mounted some distance away from the fan. I scratched my head a bit....
Power Loss Indicator - Dave Campbell designed this clever circuit. It
uses two neon lamps to indicate if power to a 120vac appliance had been interrupted.
(added 12/04) ….
Remote LED Indicator Light - There are times when you would like to transmit a
signal from one LED indicator light to second LED at another location. The circuit
below works well for this application. It takes advantage of the fact that the
internal infrared LED inside an opto-isolator has a lower voltage drop than the visible
LED being tapped into. Using a darlington type opto-isolator also means very
little current needs to be diverted to the isolator....
RS232 Data Bus Status Indicator
This circuit indicates TXD and RXD activity. The thresholds are set at plus 3
volts and minus 3 volts.
RS232 Serial Interface Status Indicator
I designed this circuit many years ago and have used it several times since. It
uses a single logic IC to indicate the TXD and RXD line activity of a typical RS-232
serial data communications link. The thresholds are set at plus 3 volts and minus
3 volts. Two dual color LEDs are used as indicator lights.
Remote LED Indicator Light
There are times when you would like to transmit a signal from one LED indicator light to
second LED at another location. The circuit below works well for this application.
It takes advantage of the fact that the internal infrared LED inside an opto-isolator
has a lower voltage drop than the visible LED being tapped into. Using a
darlington type opto-isolator also means very little current needs to be diverted to the
isolator.....
