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Touch Activated Battery Powered Latching Relayseptember 12, 2011


Wily was just finishing his first cup of coffee when he got a call from a home remodeling company.  They needed a custom touch activated switch for one of their clients.  Wily sipped his coffee as the caller described the switch. 

The unit needed to be fairly small.  It would be wired into the underside of some kitchen cabinets.  A wire from the touch switch would be connected to a strip of copper tape mounted just under the lip of the cabinets and would serve as the touch sensor.  When touched, they wanted a relay to switch 120vac power to a string of fluorescent or LED lights, mounted under the cabinets.  With each touch of the strip, the lights would toggle between an on and off mode.  The total light power was to be less than 100 watts.  Wily gave this request some thought and decided he would take on the project.

Under Cabinet Light Copper Tape
From his past experience Wily knew of a nice touch activated switch available from Faraday switches.  This tiny device has a single transistor output with a latching function.  The unit could be powered by any DC supply ranging from 3v to 28v.
How should Wily switch the AC power to the lights? 

He could use a mechanical relay, a solid state relay or a latching relay.  Standard relays require power to hold the relay contacts closed.  Solid state relays are more efficient but they too need 10ma to 20ma of current to maintain an on condition. 

A latching relay is very efficient since it can be pulsed on and off using short current pulses.  Wily weighed all the options and decided on a latching relay.  He picked a small device made by Panasonic with 10 Amp relay contacts.

FS3X Faraday
Capacitance Switch

Wily’s next chore was to decide how to power the relay and touch module.  The Panasonic relay had dual 5v coils with a coil resistance of 100 ohms and needed a pulse lasting only 25 milliseconds to either latch or unlatch the relay.  The Faraday Switch required only 4 microamps of current.  Together the two units required very little power.  Wily reviewed his options.  He looked at small AC to DC power supplies.  He considered charging up a super capacitor by tapping into the AC line for power.  In the end he decided to power the relay drive circuit and the capacitance touch switch module from a common 9v battery.  The power demands of the system were so small that the battery should last for many years.  Wily also liked that the battery kept the touch module and power supply completely isolated from the AC line.

Panasonic DK1A-L2-5V Latching Relay

Wily decided to split the system into two parts.  He would mount the relay inside a plastic junction box.  Four wires leading from that box would connect to a second box containing the touch switch module, some simple relay driving circuits and a 9v battery.
Touch Control System Serpac SR221 Box

The complete circuit is shown below.

November 2011

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Good Idea
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