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Last Updated on: Monday, August 17, 2015 04:25 AM

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More  Latch Circuits   
Light Flasher Circuits   Relay Circuits

Universal Flasher Circuit Using a Latching Relay
designed David Johnson, P.E.

Latching relays are nifty devices.  Most contain two separate coils.  When a voltage is applied to one coil the relay latches in one state and stays in that state until voltage is applied to the unlatch coil.  Since the latching and unlatching pulses only need to last about 25 milliseconds, it is possible to control a sizeable amount of power using little energy.
There are many applications where you would like something to be cycled on and off.  Perhaps you want to flash an advertising sign on and off or maybe you want to flash some Christmas trees lights.  This kind of flashing need is a perfect application for a latching relay.  The hobby circuit below controls a latching relay from a simple 9v battery.  The time between flashes can be adjusted from 0.25 seconds to 5 seconds.
The electronic circuit is powered by a 9v battery.  It uses a single 74C14 (CD40106) CMOS hex Schmitt trigger inverter logic IC.  One inverter is configured as a square wave generating oscillator.  I included a 1M variable resistor in the oscillator circuit so the frequency could be adjusted from about 0.1Hz to about 2Hz.  Other on/off times can also be achieved by changing the value of C2.

The output of the oscillator is split into two 25ms pulse generator circuits.  One circuit pulses the latch relay coil, when the oscillator output swings from zero volts to +9v, while the other circuit generates a pulse for the unlatch coil, when the oscillator output swings from +9v to zero volts.  Two n-channel FETs drive the two different coils.  The relay selected is rated for 10 Amps of current.  This should be heavy enough for most flashing applications.  The relay contacts are wired into the device to be flashed on and off.  The contacts can handle AC or DC loads.

Click on Drawing Below to view PDF version of Schematic

Circuit Circuit Universal Flasher Using a Latching Relay


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