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DiscoverCircuits.com -- Hobby Corner
"Hobby Circuits for
your Hobby Projects"
Last Updated on:
06/19/2009 02:15:47 PM
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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. |
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Click on Schematic below to view PDF
version of this Circuit |
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