Pretty Sure a Noise Problem

[Tobias]

I made this deal that will allow me to monitor ignition timing on an engine. The device has two inputs, Tach and TDC. The Tach signal is a square wave that goes hi when the coil fires for each spark plug. The TDC signal from a variable reluctance sensor. The TDC signal goes to a component that turns the sine wave into a square wave.

The tach turns on the output of a flipflop and then the TDC turns off the output of the flipflop. So if the ignition timing is 45 degrees, the output will be high 50% and low 50%. I then take this PWM with varying duty cycle through a RC circuit to turn this square wave into a 0-5v signal. 45 degrees is not 2.5 volts.

It works pretty good on the test bench however I am picking up noise along the circuit on the race car. I have access to a dyno next week where I can really test the hell out of this thing. Any suggestions based on the attached schematic to help eliminate the noise? I am going to bring my scope and solder wire to pins along the circuit to figure out the problem area.

[Tobias]

Forgot attachment

[mackrackit]

If it is a noise problem add a muffler... or some more capacitors.
With out seeing the board layout it is hard to say where but try some across the VDD and VSS as near as possible to each device.

[Archangel]

I made this deal that will allow me to monitor ignition timing on an engine. The device has two inputs, Tach and TDC. The Tach signal is a square wave that goes hi when the coil fires for each spark plug. The TDC signal from a variable reluctance sensor. The TDC signal goes to a component that turns the sine wave into a square wave.

The tach turns on the output of a flipflop and then the TDC turns off the output of the flipflop. So if the ignition timing is 45 degrees, the output will be high 50% and low 50%. I then take this PWM with varying duty cycle through a RC circuit to turn this square wave into a 0-5v signal. 45 degrees is not 2.5 volts.

It works pretty good on the test bench however I am picking up noise along the circuit on the race car. I have access to a dyno next week where I can really test the hell out of this thing. Any suggestions based on the attached schematic to help eliminate the noise? I am going to bring my scope and solder wire to pins along the circuit to figure out the problem area.
Race Cars tend to have solid copper ignition wires and Very High Energy Ignition coils . . . . I would recommend you shield everything, install a choke inline with the power input and use ferrite beads close to the IC's, Use shield braid on your sensor wires and ground the shields only at the chassis of your "deal" which should be enclosed in a metal box. Had a buddy who raced some midget cars who had a problem with RF from his transmitter, killed the Ign. when he keyed the mic, this fixed his problem.