This circuit does two things - assume a 12V car light bulb is attached to J802.
1. Launch Key Enabled is 3.3V - the green LED glows to indicates continuity through J802 and Q804
2. and Launch Rocket is 3.3V - approximately 3 A flows through J802 and Q804 to actually light the light bulb.

It has been working so far - multiple tests over multiple days. A simulation of the continuity circuit (Q802/Q804) works and matches what I have measured when the circuit is working.

All of a sudden, it stopped working. The green LED does not light with Launch Key Enabled at 3.3V and the 3A current does not flow when Launch Rocket is 3.3V. The gate is at 3.3V and the other voltages are correct. The relay clicks when the the Launch Rocket goes high.

However, if I take a voltmeter probe and touch ground with the black lead and touch the base of Q802 with the other lead, the circuit operates as expected. It isn't a mechanical thing. Just pushing on the base of Q802 does not activate the circuit. One probe (red or black) has to be on ground and the other probe has to touch the base of Q802 (or the end of R809 attached to the base) to activate the circuit. The voltmeter can be on or off. One of the leads has to be to ground. Putting a lead on the +5V or 12V to Igniter Circuit does not work.

The circuit is not built on a push in proto-board, but with soldered wires on a breadboard. I have checked the wiring (and continuity) and it is good. There are no obvious solder bridges. The solder joints look nice and shiny.

What is the appropriate sacrifice to the EE gods - goats or chickens for a situation like this?

Should I try moving Q802, R810, D801, D803, R812, and R809 to another spot on the breadboard? Maybe there is a crack in the board around those parts causing an intermittent connection?

Any other suggestions on how to solve this mystery are greatly appreciated!

 

I usually suspect some sort of circuit oscillation when odd things like an alternate path for energy from measurement leads makes things work. I don't see any bypass/decoupling capacitors on power rails/points or near active devices on your circuit schematic. If they are missing, that might be the cause of your strangeness.

The decoupling capacitor…is it really necessary?



This is with the assumption that otherwise the circuit is correct and nothing is broken.

 

I usually suspect some sort of circuit oscillation when odd things like an alternate path for energy from measurement leads makes things work. I don't see any bypass/decoupling capacitors on power rails/points or near active devices on your circuit schematic. If they are missing, that might be the cause of your strangeness.

The decoupling capacitor…is it really necessary?

View attachment 343629

Makes sense. I can add a 0.1uF cap on the power supply side of U801. C303 is there to get rid of switching spikes based on the simulations. The only other chip on the board is a 555 timer (not shown in the schematic), and it has decoupling caps. Where else would you put a decoupling cap?

 

Any other suggestions on how to solve this mystery are greatly appreciated!

Use the volt meter to gets some readings on Q802 and I can work this through with you, if you have the time.

 

Use the volt meter to gets some readings on Q802 and I can work this through with you, if you have the time.

What readings do you want?

 

Readings on all three pins of Q802 when the Launch Key Enable is High.

 

Readings on all three pins of Q802 when the Launch Key Enable is High.

Note: making measurements changes the state of the system.
12V to Igniter Circuit 12.2V
Emitter 10.2V (LED OFF)
Base 9.3V (LED ON)
Collector 0.03V (LED OFF)
Vbc 7.3V (LED ON) - making this measurement turned on the LED

 

OK, now read the Drain voltage on Q804 with the Launch Key Enable both Low and High.

 

OK, now read the Drain voltage on Q804 with the Launch Key Enable both Low and High.

Low - 0.002V
High - 0.03V
LED Off in both cases

 

That indicates an open circuit between the drain connection of Q804 and R809.

 

Low - 0.002V
High - 0.03V
LED Off in both cases

If Launch Key Enable is OFF (0.047V), and I touch the Q802 base, the green light comes on!

 

That indicates an open circuit between the drain connection of Q804 and R809.

Maybe the relay died?

 

If Launch Key Enable is OFF (0.047V), and I touch the Q802 base, the green light comes on!

Probably because the impedance of the meter is low enough to turn Q802 on.
Relay contact, open circuit on J802 and/or bad connection somewhere.

 

Probably because the impedance of the meter is low enough to turn Q802 on.
Relay contact, open circuit on J802 and/or bad connection somewhere.

Thanks so much for the help! I can't tear it apart now, but will report back tomorrow if replacing the relay/checking the wiring fixes the problem.

 

Question, does the "Launch Rocket" go High on both Q801 and Q805 at the same time?

 

Question, does the "Launch Rocket" go High on both Q801 and Q805 at the same time?

Yes. Same signal.

 

Yes. Same signal.

Could that have killed the relay?

 

Possible.
I would suggest activating the relay maybe 50ms before Q801 to prevent arcring.

 

Possible.
I would suggest activating the relay maybe 50ms before Q801 to prevent arcring.

Is there a simple hardware delay circuit, or should I do it in code and use two GPIO lines?

 

Makes sense. I can add a 0.1uF cap on the power supply side of U801. C303 is there to get rid of switching spikes based on the simulations. The only other chip on the board is a 555 timer (not shown in the schematic), and it has decoupling caps. Where else would you put a decoupling cap?

Even if it's not the cause of this, they should be in the circuit. A 01.uf and 10uf at least.