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How to by pass a tactile button switch

Former Member

     Sorry if this is way to basic for this forum, but I need some help.

 

I am living in a part of the world where we need to run stand-alone electronic air filters to keep the air clean for personal living spaces.   These devices have 3 tactile button switches that select low/med/high settings that need to be depressed to activate the unit after power has been applied to the appliance.  Now, we lose power about 4-8 times a day here, and it is necessary to go back around to each of the 4 units and press the "high" button on the control panel each time to turn them back on.

 

  I have taken a look at the control board for the unit and thought that simply putting a jumper across the tactile button switch would close the circuit so that it would permanently run whenever the power came back on.  However, this doesnt work, as it only triggers the switch when the power is already on and then the short is made across the contacts.

 

  Any ideas how could I possibly get the same effect as pushing the button every time the appliance has the power cycle?  I dont want to completely rework the control board, as this appliance is not my personal property.

 

Nick

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  • Nick, Connecting long wires to the pushbutton contacts might cause interference. Especially when the contacts are scanned by a processor. To prevent those problems, the best way is to connect a relay contact across the contacts of the pushbutton on each unit. You need a  relay for each unit, installed close to the pushbutton contacts. The relay coils can be energized by a timing circuit, providing a pulse some time after the mains power is back. (Control circuits sometimes need some time to boot before they read the buttons.) If pushing the ”HIGH” button has no influence when the system is already on, you could make a simple timing circuit that creates a random pulse every 2 minutes. If the contacts may remain closed, you can place a large electrolytic capacitor over the relay coil and a series resistor to the supply. This way the contact will be closed after a delay, made by the resistor and the capacitor. Using a single timing circuit for all units you have to install wires between the units. Another solution is to install a timing circuit for each unit. Be aware that during maintenance and cleaning pushing the ”OFF”-button is not enough to remove to remove power from the unit when this modification is made. Johan

  • in reply to jvdberg@ieee.org

    Thanks Johan!  That definetly has gotten me rolling again.  I suppose I was looking for a simpler solution, but your expalanation of the problem makes sense and i realize i was over simplifiying the issue.  I have some relays about, but will need to sketch this out a little and see about getting it all together.  Thanks for the help!

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