Hello dknguyen,
The frist part of the circuit is not shown on the schematic as this is working correctly, but I'll describe it anyway. Basically an IR LED is mounted opposite a IR sensor and between the two there's a wheel that turns with holes bored around the perimeter. The light pulses through to send signals to trigger a comparator, and this then negative edge triggers a monostable 555. The output pulse from the 555 is sent to two LN2917 speed switches that are used as tachometers to monitor the wheel speed. One is set up to trigger as soon as the wheel starts to turn and the other can be adjusted to trigger at various preset speeds.
The output signal from the LN2917 is +5 volts and drops to near ground when the speed switch toggles. This signal didn't activate the analog switch so I tried using a timer (a 556 double timer for the two signals) set up in bistable configuration to act as a flip-flop. It seemed to work OK using a single 555 on the breadboard, but when the whole thing was assembled with the 556 the circuit didn't function correctly. The LEDs were very irratic and eventually I found that the solution was to touch the 556 with my finger and everything would work correctly! The wiring is as shown on the diagram. I've changed the ICs and I can't see and other errors. Any suggestions?
Vielle568