Bill Bowden
This circuit was adapted from the "Toggle Switch Debounced Pushbutton" by John Lundgren. It is useful where the load needs to be switched on from one location and switched off from another. Any number of momentary (N/O) switches or push buttons can be connected in parallel.
The combination (10K, 10uF and diode) on the left side of the schematic insures the circuit powers up with the load turned off and the NPN transistor conducting. These components can be omitted if the initial power-on condition is not an issue.
When a switch is closed, the 1uF cap voltage is connected to the junction of the 220 ohm and 33K resistors causing the circuit to change state. When the switch is opened, the cap charges or discharges to the new level through the 1M resistor, and the circuit is ready to toggle again in about 1 second. It takes a little time for the cap to move to the new level, either +V or ground.
The (0.1uF) capacitor at the transistor base was added to supress noise that might cause false triggering if the switches are located far away from the circuit. The circuit was tested using a 12 volt, 25 watt automotive lamp, and IRFZ44. Other MOSFETs can probably be used.
