I had a very annoying time tracking down a problem with my poke detector circuit. For some reason, it recently started acting up. Now, the circuit is basically just a phototransistor feeding into an AND logic gate. The weird behavior was that occasionally the output of the logic would get lots of noise on it and there would be strong crosstalk between different channels, i.e. different pokes.
After poking around with a scope for a while and re-reading documentation on the AND gate, I figured out the problem: the phototransistor did not activate fully in the "no poke" condition and this caused a small voltage drop across the transistor. This voltage drop was right about 0.8 volts, the TTL low level (TTL levels are 0.8V-=LOW and 2V+=HIGH). Apparently if the voltage is between LOW and HIGH, strange things happen.
I fixed the problem by installing a higher value resistor prior to the phototransistor. This served to drop more volts before the phototransistor. Now the reading is only 0.3 volts when there is no poke and the system is working just fine.
After poking around with a scope for a while and re-reading documentation on the AND gate, I figured out the problem: the phototransistor did not activate fully in the "no poke" condition and this caused a small voltage drop across the transistor. This voltage drop was right about 0.8 volts, the TTL low level (TTL levels are 0.8V-=LOW and 2V+=HIGH). Apparently if the voltage is between LOW and HIGH, strange things happen.
I fixed the problem by installing a higher value resistor prior to the phototransistor. This served to drop more volts before the phototransistor. Now the reading is only 0.3 volts when there is no poke and the system is working just fine.
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