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I'm trying to make a 12v timer that, when a switch is turned on that will run a 12v pump, the timer will wait 15 seconds and then power on a 12v solinoid. When the same switch is turned off, the solinoid will stay energised for 30 seconds. I was tinking maybe a op amp and npn transister but can't get the second delay. Any help would be appreciated.<p>Thanks<p>Bob<p>[ November 11, 2003: Message edited by: Bob Quale ]</p>
Power the solonoid thru two diodes; the first diode gets power from the 15 second delay, the second diode gets power from a 30 second source. You need 2 555 timers for the 15 second delay, one for the delay another to produce a pulse that turns on an SCR or latching relay that will power the solonoid for an extended time. A third 555 timer will provide the 30 second power to the solonoid (you will need a buffer if the current required is more than 200 mA). Turning off power to the 15 second delay circuit will initiate the 30 second timer.
I thought that the 555 timers didn't like to have a steady on state at the trigger as I would be doing when I turn the switch on. The switch may be on for any lenght of time as long as it has the 15sec delay and 30sec end.<p>Thanks Bob
The 555 triggers on the negative edge of the input pulse, so you will need to invert the positive signal from power on. Use a coupling capacitor so the signal does not stay low.
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