LM555 for Air Cannon 2011-08-27

The idea is to have two monostables timers (LM555) for triggering the air cannon.

Timer1 (IC1) is triggered by a switch (S1) to ground. It will start and activate the relay via T1. When timed out its output will go negative triggering the second timer (IC2) which is a non-retriggerable monostable. With its output going high T2 will conduct and pull the collector towards ground, thus disabling IC1 by resetting it until timer2 times out.

The circuit might be triggered upon power up, causing it to reset IC1 until IC2 has timed out. If this is an issue you might use a power up reset circuit for IC2. If 10nF for C5 won't suffice use 100nF for coupling.

Schematic

View attachment AIR-CANNON-SCH.gif

PCB

View attachment AIR-CANNON-BRD.gif

The PCB layout measures 2.3375X2.550".

PCB (Revised)



A revised PCB layout. Its dimensions are now: 2.400X2.545".

Timer2 (IC2) should not be triggered upon power up since the trigger input is pulled high by R6 (4.7K)

The layout has a full ground fill.

If it is not desired erase the rectangular ground trace first. Then rip up the two traces marked red in file DUAL-TIMER-RIP and perform "Ratsnest" to eliminate air wires. They are necessary for a full ground fill only.

If the ground fill is desired perform "Ratsnest" or start the autorouter typing "Auto;" The autorouter will fill the ground plane with the selected parameters stored with the PCB layout design.

Also attached are the Eagle files. Schematic and layout were created using Eagle version 3.55 and should with no problems be loaded into any later version. Ignore messages about "restring", since the default settings vary between those versions and 3.55.

Here's another design using the dual timer chip NE556.

Boncuk