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Motors produce very high spikes.
This is a good suggestion - try this 100K pull down resistor Colin shows in the second pic. It will help eliminate a possible source of false triggers.**broken link removed**
The 8.2 ohm resistor is going to keep the anode of the diode clamped pretty firmly to ground. But after it triggers the first time, the other side of the junction has nothing pulling below +.7v - the diode is reverse-biased, the B-E drop is +.7V. So, it doesn't really shut off cleanly, just kinda bleeeeeeeds dowwwnnnn through the base. The transistor probably has a gain of 100, with the 100k resistor and the 10V supply that means it only takes 1 micro-amp of current to trigger it. If hunting for something noise sensitive, you could make a worse guess than this one.The overall resistance will be a fraction under 8.2ohms. Can you explain?