I've made a circuit as shown in the diagram (the transistors are actually phototransistors where indicated) for a pinewood derby finish gate. So far it is working quite well, with the voltage on the four output wires (going to the bottom) registering as 5v when illuminated from above. When the light is blocked overhead (presumably by a derby car), the voltage drops to about 0.1-0.2 volts.
My question is, will my Uno see this as high and never registering a low signal, or is this voltage too low to register as a high signal? At what voltage does the Uno decide high or low? Also if the signal will still be read as high, is there a diode or something I can use to get the voltage to 0?
Well it can vary a little between AVR chip types and operating voltage but the 328P chip as used on a Uno board:
A low is anything from -0.5 vdc to .3 X Vcc ( -.5 vdc to +1.5vdc on a 5 volt board)
A high is anything from .6 X Vcc to Vcc + .5vdc (3.0vdc to 5.5vdc on a 5 volt board.)
Those are the limits guaranteed by the manufacturer - it means that you can rely on each pin switching somewhere between
1.5 and 3.0V, but it does no mean that they will switch reliably for slowly varying voltages in that range - logic inputs typically
require a fast transition from LOW to HIGH and from HIGH to LOW - the time spent between 1.5 and 3.0V should be very
short (typically a few us at most).
If you have a slowly varying input (your phototransistor sounds like a case of this) it is wise to use a schmitt-trigger input
(such as on a 74HC14 inverter chip) to generate fast transitions from a slow moving edge - and also using a significant
amount of hysteresis to prevent noisy signals causing multiple transitions.
The ATmega microcontrollers do have, I believe, a small amount of hysteresis on the input pins which will reduce the
risk of multiple edges, and you can always use software debouncing to get round the problem (by ignoring short-term
oscillations / multiple edges)
sparticus1701:
My question is, will my Uno see this as high and never registering a low signal, or is this voltage too low to register as a high signal? At what voltage does the Uno decide high or low? Also if the signal will still be read as high, is there a diode or something I can use to get the voltage to 0?
Look in the datasheet under "Pin Threshold and Hysteresis"
