Dave's Indicator Circuits:  Page 1,    Page 2,    Page 3

• FLASHING LED POWER INDICATOR DRAWS LOW CURRENT
  This circuit flashes a power indicator LED to keep the average current low.

• 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. The turns do not have to be tight. I was able to close the contacts with two turns about 2 inches in diameter with 20 amps. A simple LED and resistor can be used to indicate the current by tapping into the automotive +12v supply.

• 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....

• 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.