> You have some of your reasoning reversed regarding comparator behaviour. > Comparators are typically: +in > -in --> output transistor off --> output TRUE / > HIGH with pull-up R. >
I have double-checked the datasheet (http://www.ti.com/lit/ds/symlink/lm339.pdf). It says "The output NPN sinks current when the positive input voltage is higher than the negative input voltage and the offset voltage". My interpretation is therefore the opposite of what you say above. Is it possible that different versions of the LM339 work differently? Regards Rob > The 176 outputs are connected to the comparator -inputs. The two 0V -ins are > thus below the +ins at 0.36V and their output transistor will thus be off / not > conducting to GND. > > Of course that doesn't solve the problem, the 3rd, oscillating, -in should be > toggling its output transistor and the overall output should be toggling rather > than steady +5. > Is power good at the 339 supply pins (including gnd)? > Is the oscillating 176 output high long enough (pulse width when high) for the > frequency response of the 339?
