Pop Bumper Troubles

While testing out my new test playfield with just the mechanical switches, one of pop bumpers got stuck.  As the pop bumper was stuck, the switch remained closed, and this kept the solenoid powered.  The switch generated a lot of heat melting the pendulum from the pop bumper skirt until it melted.  As it melted it became short enough it could not trigger the switch anymore and finally released.  This is not good.  This is a bit of a safety issue.  What happens when you are playing the game and you do not notice.

To solve this problem, I determined I needed a single pulse that would not fire again until the switch had been pressed again.  So even if the switch is held down, the pulse is sent only once.  The lower logic level voltage (5V) will not heat up the switch as it will be current limited by a resistor rather than just the solenoid on 35V.

To produce this single pulse I used the following:

74AC08 - AND Gate
74HC14 - Hex Inverting Schmitt Trigger
1uF Capacitor
100KOhm Trim Pot
470 Ohm Resistor
A button or switch

Single Pulse Generator and my first Eagle CAD drawing.

Up until this point, I had not bothered trying to figure out EagleCAD.  There is a freeware version of EagleCAD.  The main limitations are in the size of the PCB board that you can make and the amount of PCB layers (only 2).  Unless you are designing motherboards, you probably won't need 6 layers in your PCB.  You can download EagleCAD here.  The other free CAD software out there is KiCAD.  However, it is not used as much as Eagle so many of the schematics out there for download are for EagleCAD.  KiCAD does allow for larger projects.  For me, the free EagleCAD is all I need.  If I need more, for $125 you can get a full featured, not for profit version.

Back to the schematic.  The output of this circuit goes to the MOSFET or whatever is being used to trigger the MOSFET.  When the switch is open the logic at the switch is LOW.  The schmitt trigger inverts this to HIGH.  So the AND gate sees HIGH on pin 1 and LOW on pin 2.  When the switch is closed this is reversed but there is a delay on pin 1.  The delay is caused by the resistor and capacitor combination.  The length of the delay is how long both pins are HIGH and thus the output is HIGH.  This can be determined by the equation time constant t = RC.   It usually takes 3 to 5 time constants for the voltage to drop to zero.  This likely does not matter much because after one time constant it is likely that the voltage has already dropped below the logic HIGH level.  For this circuit the time constant is 0.1 seconds when the trim pot is at 100 KOhm.  I decided to use a trim pot because all the solenoids are different and probably require different timings to fully function.