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In another thread, we drifted off into light sensitivity of parts.
One time I was servicing a dollar bill acceptor - a currency validator for a change machine - and had pretty much gotten the repair done. You insert a dollar bill into the slot, and if the system recognizes it as a legitimate piece of currency, it pulls a solenoid and starts a motor to pull the bill into inventory, and it then outputs a "vend" signal - a pulse - that triggers the rest of the system to dispense coins once the bill has cleared. I have a soft spot for these change machines, because they were the topic of the first traiining lecture/demonstration I ever wrote.
The thing would sit there on my bench and accept bills and output vends all I wanted. I tilted the unit on its face to access an adjustment and CLAMP, it cycles through a vend pulse. No bill required. Of course in the change machine no one will be putting it on its face, but it still seemed like a problem.
I went nuts tring to find why this thing would vend when put on its face, but worked normal sitting upright. Looked for loose conductors that might sway, loose solder joints that might do who knows what.
I ultimately discovered that there was a light sensitive component on the small control board in the thing, and whenever I tipped it forward, the bright light over my bench triggered this transistor into conduction, which initiated the vend pulse cycle.
The same lesson is learned over and over, and I am no exception, things do not happen in isolation, you can't ever forget the environment a system lives in. The problem was not in the unit itself, it was in the environment.
One time I was servicing a dollar bill acceptor - a currency validator for a change machine - and had pretty much gotten the repair done. You insert a dollar bill into the slot, and if the system recognizes it as a legitimate piece of currency, it pulls a solenoid and starts a motor to pull the bill into inventory, and it then outputs a "vend" signal - a pulse - that triggers the rest of the system to dispense coins once the bill has cleared. I have a soft spot for these change machines, because they were the topic of the first traiining lecture/demonstration I ever wrote.
The thing would sit there on my bench and accept bills and output vends all I wanted. I tilted the unit on its face to access an adjustment and CLAMP, it cycles through a vend pulse. No bill required. Of course in the change machine no one will be putting it on its face, but it still seemed like a problem.
I went nuts tring to find why this thing would vend when put on its face, but worked normal sitting upright. Looked for loose conductors that might sway, loose solder joints that might do who knows what.
I ultimately discovered that there was a light sensitive component on the small control board in the thing, and whenever I tipped it forward, the bright light over my bench triggered this transistor into conduction, which initiated the vend pulse cycle.
The same lesson is learned over and over, and I am no exception, things do not happen in isolation, you can't ever forget the environment a system lives in. The problem was not in the unit itself, it was in the environment.
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